MICRO-TUBULAR AND MICRO-MONOLITHIC SOLID OXIDE FUEL … · for their financial support under...

1

MICRO-TUBULAR AND MICRO-MONOLITHIC SOLID

OXIDE FUEL CELLS FOR ENERGY AND ENVIRONMENT

Submitted by:

Mohamad Fairus Bin Rabuni

Department of Chemical Engineering

Imperial College London

A Thesis Submitted for the Degree of Doctor of Philosophy and

the Diploma of Imperial College London

1

DECLARATION OF ORIGINALITY

I hereby declare that this thesis and the work reported herein was composed by and originated

entirely from me. Information derived from the published and unpublished work of others has

been acknowledged in the text and the relevant references are included in this thesis.


Department of Chemical Engineering,


January 2019

2

COPYRIGHT DECLARATION

The copyright of this thesis rests with the author. Unless otherwise indicated, its contents are

licensed under a Creative Commons Attribution-Non-Commercial 4.0 International Licence

(CC BY-NC).

Under this licence, you may copy and redistribute the material in any medium or format. You

may also create and distribute modified versions of the work. This is on the condition that: you

credit the author and do not use it, or any derivative works, for a commercial purpose.

When reusing or sharing this work, ensure you make the licence terms clear to others by naming

the licence and linking to the licence text. Where a work has been adapted, you should indicate

that the work has been changed and describe those changes.

Please seek permission from the copyright holder for uses of this work that are not included in

this licence or permitted under UK Copyright Law.


Department of Chemical Engineering,


January 2019

3

ABSTRACT

Micro-tubular solid oxide fuel cells (MT-SOFCs) have interesting features and presents several

advantages and competitiveness such as better thermal shock resistance, higher power density

and portable characteristics. Further enhancement in terms of cell performance remains as a

challenge to be competitive with the planar design made from similar cell materials. In this

work, improvement of the anode micro-structure prepared via a phase-inversion assisted

process has been studied by employing common SOFC materials including nickel-yttria

stabilised zirconia (Ni-YSZ). A new anode design with longer micro-channels and larger pore

entrance made using solvent-based bore fluid gave better electrochemical performance

compared to the conventional single-channel anode design. Such new design, when utilising

YSZ material could be made into a suitable electrolyte scaffold for incorporating anode

materials, copper-ceria (Cu-CeO2). MT-SOFC with Cu-based anode was tested for direct

methane (CH4) utilisation.

Notwithstanding the great potential of MT-SOFC, problems such as low mechanical robustness

of the small micro-tube must be addressed. A multi-channel design has been proposed whereby

NiO-YSZ anode substrate with various number of channels have been prepared and developed

into complete single micro-monolithic cells. Their properties were compared and eventually

tested for electrochemical performances. Micro-monolithic design has a better mechanical

property, up to 4-8 times compared to the single-channel counterpart. Electrochemical test

showed that 7-channel cell achieved about 120 % increment in terms of power density than the

conventional single-channel design. The flexibility in the operation of solid oxide

electrochemical reactors allows the use of such devices as a fuel cell and electrolyser. Such

unique characteristics have been investigated for fuel cell operation with hydrogen (H2) and

carbon dioxide (CO2) electrolysis using a novel 6-channel micro-monolithic cell in which

excellent performances have been demonstrated.

4

ACKNOWLEDGEMENT

First and foremost, I would like to sincerely thank my supervisor, Professor Kang Li for giving

me the opportunity to join his research team as a PhD candidate in the field of solid oxide fuel

cells. A special acknowledgement goes to postdoctoral research fellow, Dr Tao Li who I could

not thank enough throughout my time here. As I am not familiar with such topic prior to joining

this field of research and with their supervision, support, trust, and thoughtful knowledge are

definitely the most important aspects for the completion of my study.

I would also like to thank all the current and previous members of Prof. Li’s group; Dr. Bo

Wang, Dr. Kang Huang, Nur Izwanne Mahyon, Yunsi Chi, Marc Plunkett, Vatsal Shah, Dr.

Jengyi Chong, Dr. Farah Aba, Dr. Ji Jing and others. Thank you for the help with lab training,

testing equipment and result discussion throughout my time in Imperial College London.

I would like to acknowledge the research funding provided by the UK EPSRC. Also, I am

thankful to my employers, University of Malaya (UM) and the Ministry of Education Malaysia,

for their financial support under SLAB/SLAI fellowship during my study.

And last but not the least, I would like to grant my special appreciation to my family for all the

help and support throughout my study. Without their unconditional love and inspiration, it

would have been impossible for me to face the challenges I have encountered during the

completion of my study.

5

TABLE OF CONTENT

Page

Abstract 3

Acknowledgement 4

Table of content 5

List of Figures 9

List of Tables 14

Abbreviations 15

Nomenclature 16

Chapter 1 Introduction 18

1.1 Background 18

1.2 Research objectives 23

1.3 Thesis arrangement 25

Chapter 2 Literature Review 27

2.1 SOFC principle and thermodynamics 27

2.2 SOFC components and materials 32

2.2.1 Anode 33

2.2.2 Electrolyte 36

2.2.3 Cathode 38

2.2.4 Interconnect 40

2.3 SOFC geometries classification 41

2.3.1 Conventional planar and tubular design 41

2.3.2 Micro-tubular design 43

2.3.3 Multi-channel anode design 51

2.4 Fabrication of precursor of micro-tubes and micro-monoliths 52

2.4.1 Preparation of spinning suspension 52

2.4.2 Extrusion of hollow fibre precursors 52

2.4.3 Sintering process 55

2.4.4 Co-sintering process 57

2.5 Hydrocarbon-fuelled SOFCs 58

6

2.5.1 Direct hydrocarbon utilisation 59

2.5.2 Anode for hydrocarbon-fuelled SOFCs 60

2.6 Solid oxide electrochemical reactor (SOER) 63

2.6.1 Steam (H2O) electrolysis 66

2.6.2 CO2 electrolysis 68

2.6.3 Steam and CO2 co-electrolysis 69

2.7 Conclusion 71

2.8 References 72

Chapter 3 Fabrication of micro-structured NiO-YSZ anode for micro-tubular

solid oxide fuel cell (MT-SOFC)

87

3.1 Introduction 88

3.2 Experimental 90

3.2.1 Materials 90

3.2.2 Fabrication of micro-tube 90

3.2.3 Fabrication of a complete cell and reactor assembly 93

3.2.4 Characterisations 95

3.2.5 Electrochemical performance test 96

3.3 Results and discussion 97

3.3.1 Morphology 97

3.3.2 Bending strength, electrolyte gas-tightness property

and porosity

100

3.3.3 Electrochemical performances 102

3.4 Conclusion 109

3.5 References 109

Chapter 4 Electrode design for direct-methane micro-tubular solid oxide fuel

cell (MT-SOFC)

113

4.1 Introduction 114

4.2 Experimental 116

4.2.1 Materials 116

4.2.2 Fabrication of micro-tube 116

4.2.3 Fabrication of a complete fuel cell 117

7


4.2.5 Cell sealing and reactor assembly 119


4.3.1 Micro-structure of micro-tube 121

4.3.2 Gas-tightness and mechanical property 123

4.3.3 Vacuum-assisted co-impregnation process 123


4.4 Conclusion 137

4.5 References 137

Chapter 5 Fabrication of novel micro-monolithic anodes for solid oxide fuel cell 142



5.2.1 Materials 145

5.2.2 Fabrication of anode substrate 145


5.2.4 Fabrication of a single cell and reactor assembly 148



5.3.1 Morphology 150

5.3.2 Gas permeation and mechanical strength 153


5.4 Conclusion 162

5.5 References 163

Chapter 6 Micro-monolithic SOFC for extended application 166



6.2.1 Materials 168

6.2.2 Fabrication of fuel electrode 169


6.2.4 Fabrication of a complete single cell 172

6.2.5 Reactor assembly and sealing 173

8



6.3.1 Morphology 174

6.3.2 Gas-tightness and mechanical property 177


6.4 Conclusion 193

6.5 References 196

Chapter 7 Conclusion and future works 200

7.1 General conclusion 200

7.2 Future works 206

7.2.1 Hydrocarbon-fuelled system 206

7.2.2 Cell with dual-layer electrolyte 206

7.2.3 New materials for the development of MT-SOFC 207

7.2.4 Long-term durability test 207

7.3 References 207

List of publications 208

Appendices 210

9

LIST OF FIGURES

Page

Figure 1.1 Basic configuration of a fuel cell. 18

Figure 1.2 Overall structure of the thesis. 25

Figure 2.1: Operating principle of typical solid oxide fuel cell; (a) oxide ion (O

Operating principle of typical solid oxide fuel cell; (a) oxide ion

(O2−) conducting electrolyte and (b) proton conducting electrolyte)

conducting electrolyte and (b) proton conducting electrolyte.

27

Figure 2.2: Schematic of typical SOFC current-voltage curve showing the

operating cell voltage (V) and voltage losses (η) as a function of the

operating current density (j).

32

Figure 2.3: Schematic of (a) a single cell of SOFC and (b) arrangement for a

stack of SOFC. Reproduced with permission from [28].

33

Figure 2.4: TPB regions for different anode material; (a) pure electronic

conductor, (b) mixed ionic and electronic conductor and (c) ionic-

electronic composite. Reproduced with permission from [21].

34

Figure 2.5 Conductivity of different mole % of yttria in YSZ in air at 1000 °C.

Reproduced with permission from [26].

36

Figure 2.6 The lattice structure of perovskites. Reproduced with permission

from [28].

39

Figure 2.7 Schematic diagram of a typical design of SOFC. Reproduced with

permission from [38].

41

Figure 2.8 Schematic diagram of the plastic mass ram extrusion process.


44

Figure 2.9 The paste co-extrusion process. Reproduced with permission from

[53].

45

Figure 2.10 Typical steps involve during the fabrication of a complete single

cell.

46

Figure 2.11 Schematic diagram showing the dope suspension and bore fluid are

extruded simultaneously through the concentric orifices.

53

Figure 2.12 Example of the micro-structure of ceramic micro-tube. 54

Figure 2.13 Progress of ceramic micro-structure during the sintering process. 56

10

Figure 2.14 Operating principle of reversible SOFC-SOEC process.


63

Figure 2.15 Total energy demand (∆H), electric energy demand (∆G) and heat

demand (T∆S) as a function of temperature for (a) steam

electrolysis and (b) CO2 electrolysis. Reproduced with permission

from [121].

65

Figure 3.1 Set-up for degassing process. 91

Figure 3.2 (a) Schematic diagram of the fabrication process of anode micro-

tubes; (b) photographic image of spinneret from the bottom and

detailed dimensions; (c) as-prepared micro-tubular SOFCs.

Reproduced from Ref. [21] with permission from the Royal Society

of Chemistry.

93

Figure 3.3 Schematic representation of (a) a single cell (NiO-YSZ|YSZ|YSZ-

LSM/LSM) and (b) reactor assembly and sealing for a single MT-

SOFC. (Not to scale)

94

Figure 3.4 Schematic diagram for three-point bending test. 96

Figure 3.5 SEM images of two different anode substrate; (a) NiO-YSZ (BF-P)

and (b) NiO-YSZ (BF-S) where (i), (ii) and (iii) represent the

overall cross-section, magnified cross-section and the inner surface,

respectively.

98

Figure 3.6 SEM images for (a) NiO-YSZ (BF-P) and (c) NiO-YSZ (BF-S)

anode substrate with YSZ electrolyte, respectively where the inserts

show electrolyte layer thickness and (b) and (d) show electrolyte

outer surface for NiO-YSZ (BF-P) and NiO-YSZ (BF-S),

respectively.

100

Figure 3.7 j-V curves and j-P curves for cell developed from (a) NiO-YSZ (BF-

P) substrate and (b) NiO-YSZ (BF-S) substrate. Reproduced from

Ref. [21] with permission from the Royal Society of Chemistry.

103

Figure 3.8 Short-term stability test under 0.7 V at 750 °C for cells derived from

different anode substrates.

104

Figure 3.9 Electrochemical impedance spectra (EIS) for cells from (a) NiO-

YSZ (BF-P) substrate and (b) NiO-YSZ (BF-S) substrate.

106

11

Figure 3.10 Comparisons of the 2D distributions of reactant fluxes, pressure and

concentration overpotential for different designs (Sp: spongy layer;

Ch: channels). Reproduced from Ref. [21] with permission from the

Royal Society of Chemistry.

107

Figure 4.1 Schematic diagram of the steps involved in the overall fabrication

of cell components of MT-SOFC.

117

Figure 4.2 Set-up for co-impregnation process. 118

Figure 4.3 Reactor assembly and sealing. 120

Figure 4.4 SEM images of the sintered YSZ micro-tube: (a) overall cross-

section, (b) magnified cross-section, (c) and (d) cross-section and

magnified cross-section showing the thickness of the skin-layer

region, respectively, (e) and (f) inner and outer surface,

respectively.

122

Figure 4.5 SEM image showing the porous YSZ bulk. 124

Figure 4.6 Percentage mass increase after a single cycle at different

impregnation time.

124

Figure 4.7 Anode loading after 15 cycles of impregnation. 125

Figure 4.8 SEM images of copper (Cu)-impregnated micro-tube; (a) cross-

section and (b) inner surface.

126

Figure 4.9 SEM images of Cu-CeO2-YSZ anode; low-resolution: (a) and (b)

cross-section and the inner surface, respectively and high-

resolution: (c) and (d); inner surface and the magnified inner

surface, respectively, (e) magnified cross-section and (f) Cu-CeO2

coating layer.

127

Figure 4.10 Chemical mapping of the co-impregnated micro-tube: (a) SEM

images, (b) distribution of Cu element and (c) distribution of Ce

element; (i) cross-section and (ii) inner surface.

128

Figure 4.11 Current-voltage curves for system fuelled by (a) dry H2 and (b) dry

CH4.

131

Figure 4.12 The AC impedance spectra of MT-SOFCs under open-circuit

conditions at various operating temperature fuelled by (a) dry H2

and (b) dry CH4.

134

12

Figure 4.13 Current density for operation at 750 °C with dry CH4 under constant

0.7 V.

135

Figure 4.14 SEM-EDS carbon mapping post operation at 750 °C with dry CH4

under 0.7 V for 30 hours; (a) and (d) the SEM images of the mapped

area, (b) and (e) distribution of C element and (c) and (f) sum

spectrum.

136

Figure 5.1 Schematic diagram of (a) spinning set-up and (b) changeable multi-

nozzle spinneret designs.

147

Figure 5.2 SEM images of sintered NiO-YSZ anode substrate; (a)–(d) anodes

with different number of channels, (e)–(g) magnified anode region

showing sponge-like structure and micro-channels.

150

Figure 5.3 Microscope images showing different sponge-region thickness for

(a) 3-channel and (b) 7-channel anode substrate, respectively.

152

Figure 5.4 Nitrogen (N2) gas permeance for anode with a different number of

channels.

153

Figure 5.5 Schematic diagram of the gas diffusion pathway; (left) conventional

single-channel anode and (right) 3-channel anode.

154

Figure 5.6 Fracture force (N) for different types of anode substrates. 156

Figure 5.7 j-V and j-P curves at various temperatures for different micro-

monolithic cells.

157

Figure 5.8 Stability test using 7-channel cell design under constant 0.7 V at

750 °C.

158

Figure 5.9 Effects of number of channels of micro-monolithic SOFCs and

temperature on electrochemical impedance spectra (10−2 to 105 Hz)

at open circuit.

160

Figure 6.1 (a) in-house made spinneret and (b) schematic diagram of the steps

involved in the overall fabrication of cell components of micro-

monolithic SOFC.

171

Figure 6.2 SEM images of cross-sectional NiO-YSZ micro-tube: (a) overall

cross-section, (b) – (c) magnified cross-section of the wall between

channels and outer surface, (e) magnified cross-section of centre

wall, and (d) and (f) cross-section of wall between channels.

174

13

Figure 6.3 SEM images of (a) inner surface and (b) outer surface of the sintered

6-channel NiO-YSZ substrate.

176

Figure 6.4 SEM images of (a) cross section of NiO-YSZ substrate with YSZ

electrolyte and (b) outer surface of YSZ electrolyte.

177

Figure 6.5 j-V and j-P curves for pure H2-fuelled system at different operating

temperatures.

179

Figure 6.6 j-V and j-P curves for three identical cells operated under pure H2-

fuel cell mode at 750 °C.

180

Figure 6.7 Electrical impedance spectra (EIS) showed the effect of temperature

on polarisation in fuel cell mode using pure H2 as a fuel.

182

Figure 6.8 j-V curves for three identical cells operated at 750 °C with CO2-CO

(90-10 vol. %) gas input.

185

Figure 6.9 j-V curves for SOEC-SOFC dual-mode operation with a feed

mixture of CO2-CO (90-10 vol. %) at various operating

temperatures.

186

Figure 6.10 Electrical impedance spectra (EIS) for operation with CO2-CO (90-

10 vol. %) at different temperatures.

188

Figure 6.11 j-V and j-P curves for a system operated with CO2-CO (90-10 vol.

%) gas mixture at different operating temperature in fuel cell mode.

189

Figure 6.12 Short-term stability test under CO2-CO (90-10 vol. %) electrolysis

operation.

190

Figure 6.13 Post-test SEM-EDX analysis for two different fuel electrode

regions, (a) and (b), where (i), (ii) and (iii) represent the SEM

images of the mapped area, carbon (C) element distribution and sum

spectra, respectively.

191

Figure 6.14 SOEC-SOFC cyclic operation for a period of 10 hours. 192

14

LIST OF TABLES

Page

Table 1.1 Features of different types of fuel cells. 20

Table 2.1 Common materials for SOFC components. 33

Table 2.2 Requirements for the anode, electrolyte and cathode for SOFC. 39

Table 2.3 Development of single MT-SOFC prepared from phase-inversion

process.

47

Table 3.1 Parameters for NiO-YSZ anode fabrication. 91

Table 3.2 Power densities of cells with different anode substrate at various

temperature.

104

Table 4.1 Parameter for YSZ micro-tubes fabrication. 117

Table 4.2 SOFCs with Cu–cermet anodes fed with methane (CH4) as fuel. 132

Table 5.1 Composition of the spinning suspension and fabrication and

sintering conditions.

146

Table 5.2 Dimensions of single-channel and multi-channel samples. 151

Table 5.3 Comparison of volumetric power density with reported

honeycomb-SOFCs.

161

Table 6.1 Compositions of spinning suspension, electrolyte ink and oxygen

electrode.

170

Table 6.2 The electrochemical performances of H2-fuel cell with similar cell

materials.

180

Table 6.3 Studies on CO2-CO electrolysis using similar cell materials. 194

15

ABBREVIATIONS

ASR Area specific resistance

BCY Yttrium doped barium cerate

BZCY Barium cerium yttrium zirconate

BZY Barium yttrium zirconate

BZCYYb Yttrium and ytterbium co‐doped barium zirconium cerium oxide

DI De-ionised

DMSO Di-methyl sulfoxide

EDS/EDX Energy dispersive spectroscopy

GDC (or CGO) Gadolinium doped ceria

LSCF Lanthanum strontium cobalt ferrite

LSCM Lanthanum strontium chromium manganite

LSF Lanthanum strontium ferrite

LSGM Lanthanum strontium gadolinium manganite

LSM Lanthanum strontium manganite

LST Lanthanum strontium titanate

MT Micro-tubular

NMP N-methyl-2-pyrrolidone

OCV Open circuit voltage

PEG Polyethylene glycol

PEM Polymer electrolyte membrane

PESf Polyethersulfone

PMMA Polymethyl methacrylate

ScSZ Scandia doped zirconia

SDC Samarium doped ceria

SOER Solid oxide electrochemical reactor

SOEC Solid oxide electrolysis cell

SOFC Solid oxide fuel cell

SSC Samarium strontium cobalt oxide

TEC Thermal expansion coefficient

TPB Triple phase boundary

YSZ Yttria-stabilised zirconia

16

NOMENCLATURE

Symbol Description Unit

A Area of hollow fibre/micro-tubular SOFC cm2 / m2

ASR Area specific resistance cm2

Di Inner diameter of hollow fibre/micro-tubular SOFC m / cm

Do Outer diameter of hollow fibre/micro-tubular SOFC m / cm

F Faraday constant C mol−1

I Current A

L Length of hollow fibre/micro-tubular SOFC m / cm

N Measured load at which fracture occurred N

OCV Open circuit voltage V

P Gas permeability mol m−2 s−1 Pa−1

Pelec Electric power W

Pd, elec Power density/power normalised by the effective area W cm−2

Pmax Maximum power density W cm−2

Pa Atmospheric pressure Pa

Po Initial pressure measured in the test cylinder Pa

Pt Final pressure measured in the test cylinder Pa

Q Gas permeation rate mol s−1

Ri Inner radius of hollow fibre/micro-tubular SOFC m / cm

Ro Outer radius of hollow fibre/micro-tubular SOFC m / cm

R Gas constant J mol−1 K−1

T Temperature °C / K

VC Volume of the test cylinder m3

V Operating cell voltage V

VN Nernst voltage V

Vtn Thermo-neutral voltage V

j Current density A cm−2

jo Exchange current density A cm−2

jL Limiting current density A cm−2

le Electrolyte thickness cm

nH2, inlet Hydrogen molar flow rate provided to the cell mol s−1

17

pH2 Partial hydrogen pressure Pa or atm

pH2O Partial water pressure Pa or atm

pO2 Partial oxygen pressure Pa or atm

t Time s / min / h

Δp Pressure difference Pa

ΔH Reaction enthalpy J mol−1

ΔG Gibbs energy J mol−1

ΔS Reaction entropy J mol−1 K−1

α Charge transfer coefficient -

σ Electrical conductivity S cm−1

σF Bending strength MPa

ε Efficiency -

ηa Activation polarisation V

ηc Concentration/diffusion polarisation V

ηΩ Ohmic polarisation or loss V

ρa Anode resistance Ω cm

ρe Electrolyte resistivity Ω cm

Chapter 1

18

1 CHAPTER 1

Introduction

1.1 Background

According to the Intergovernmental Panel on Climate Change (IPCC) report, total annual

anthropogenic greenhouse gas (GHG) emissions have increased by approximately 10 GtCO2-

equivalent between 2000 and 2010 [1]. This increase came from the energy (47 %), industry

(30 %), transport (11 %) and building (3 %) sectors. Fast-rising energy demand and rapid

growth of the global population continued to be the main factors of increases in the GHG

emissions and have become challenging issues yet to be resolved. Since energy sector

contributes to a significant proportion of the GHG release, it is essential to have a more

sustainable way to fulfil the increasing energy demand through more efficient use of fossil fuel

with minor impact on the environment. In the same time, awareness of the importance of having

alternative energy sources has been growing as the global fossil fuel reserves are limited and

constantly depleting. While the emergence of green energy sources, for instance, wind, solar,

geothermal and hydroelectric energy could reduce dependency on fossil fuels, the major

drawbacks are related to the financial and technical challenges.

Figure 1.1: Basic configuration of a fuel cell.

Chapter 1

19

In recent years, fuel cells have become a promising energy generation device which transform

the chemical energy of fuels into electrical energy directly [2]. The basic configuration of the

fuel cell is shown in Figure 1.1. Compared with typical combustion, the conversion of fuel into

electricity through a chemical process in fuel cells is in theory, more efficient as it is not limited

by the Carnot cycle [2]. In a conventional combustion method, the chemical energy is first

transformed into thermal energy for steam generation that is used to turn the turbine

(mechanical energy). Although it is often described that the conventional methods to have a

relatively lower efficiency, the modern technology such as combined cycle gas turbine could

achieve much higher efficiency, e.g. > 60% [2, 3].

Fuel cells are regarded as a green technology considering their low emissions of contaminants,

including sulfur oxides (SOx) and nitrogen oxides (NOx). There are various types of fuel cells,

which could be categorised according their particular materials, mainly the electrolyte, the

charge of the species being transported, and operating temperature [2, 3], as summarised in

Table 1.1.

Alkaline fuel cells (AFC), polymer electrolyte membrane fuel cells (PEMFC) and phosphoric

acid fuel cells (PAFC) can be classified as “low-temperature” types and require the use of

highly pure hydrogen (H2). This is because the noble metal electrode (i.e. platinum) is sensitive

to contaminants such as carbon monoxide (CO) which could degrade the cell performance. In

addition, if hydrocarbons are to be used as the fuels, extra processors including a reformer and

a gas-cleaning system must be added to first reform hydrocarbons into high-purity H2, leading

to greater complexity and cost. On the other hand, solid oxide fuel cell (SOFC) and molten

carbonate fuel cell (MCFC) are identified as “high-temperature fuel cells” which are suitable

for co-generation and combined cycle systems. It is noteworthy that extensive and

comprehensive reviews of the different types of fuel cells, their properties and materials are

available in the literature [2-6] and are beyond the scope of this study.

Chapter 1

20

Table 1.1: Features of different types of fuel cells.

SOFC MCFC AFC PAFC PEMFC

Anode/cathode/catalyst Cermet or

perovskites

Stainless steel-

supported alloy

Noble metals

supported on carbon

Noble metals

supported on carbon

Noble metals

supported on carbon

Electrolyte Ceramic Liquid alkali

carbonate

KOH aqueous

solution

Concentrated liquid

H3PO4

Solid polymer

membrane

Electrolyte support / LiAlO2 Asbestos SiC /

Charge carrier O2−/H+ CO32− OH− H+ H+

Operating temperature

(°C)

600-1000 600-700 ~70 160-220 60-80

Operating pressure

(*atm)

1 1-3 1-10 1-8 1-5

Fuel H2, CO, CH4 H2, CO, CH4 H2 H2, External

reformate

H2, CH3OH

*atm = 1.01×105 Pa; CO32− = carbonate ion; LiAlO2 = lithium aluminate; KOH = potassium hydroxide; OH− = hydroxide ion; H3PO4 = phosphoric

acid; SiC = silicon carbide; CH3OH = methanol;

Chapter 1

21

The high operating temperatures for SOFC, ranging from 600 to 1000 °C affords the flexibility

in fuel intake [2, 7]. Various fuels such as H2, biogas, landfill gas and fossil fuels can be applied.

Furthermore, at high temperatures, an SOFC can facilitate internal reforming, which means

that hydrocarbons and reforming agents can be fed directly into the anode compartment with

no requirement for an external reformer. This high operating temperature also activates

electrochemical oxidation of hydrocarbon fuels in the presence of catalysts. Additionally, the

heat produced could be reintegrated for even higher overall system efficiencies. For example,

the surplus heat can be used in an integrated gas turbine system allowing a co-production of

heat and power, a process known as combined heat and power (CHP) [8]. This heat recovery

system, together with the energy generation contributes to an overall system efficiency up to

60-85 % [5, 9, 10].

Another interesting feature of SOFC is that its solid electrolyte could resolve problems such as

corrosion and electrode wetting which are commonly encountered in other types of fuel cells

such as PEMFC. In addition, it is quiet since there is no vibration or moving parts inside the

system. Geometrically, planar and tubular are the two common designs of SOFC. More recent

micro-tubular design (MT-SOFC) with reduced geometric scale presents additional

technological advantages. These include better resistance to thermal shock, higher volumetric

power density compared to larger tubular design and quick start-up and shutdown [11, 12].

This relatively new design will be further discussed in section 2.3.2.

Hydrogen (H2) is a standard fuel for the fuel cell operation. However, there are several

challenges related to its use as fuel. One is that the H2 generation consumes a considerable

amount of energy. At present, the primary sources for producing H2 are 48% from natural gas,

30% from the refinery and 18% from coal [13]. About 20-30 % of the fuel value of

hydrocarbons is lost during the hydrogen production [14]. Hydrogen is also well-known to be

difficult to handle and store. Other major concerns include the costs of plant construction and

Chapter 1

22

development of the infrastructure. It is predicted that the financial requirements for worldwide

pipeline-based distribution stations could reach $20 trillion by 2030 [15].

In standard SOFCs, since O2− ions are the species that travel through the electrolyte, any

combustible fuel can theoretically be utilised. As the numerous hurdles associated with H2 have

yet to be solved, the SOFC running on hydrocarbons is desirable. The fuel is fed to the anode

compartment where it will go through an oxidation process. The primary concern when using

hydrocarbon is the formation of carbon, which could block the pores and reduce the catalytic

properties of the anode via catalyst encapsulation. From the literature, this problem is inherent

to cells with Ni-cermet anode. Consequently, different approaches to realise successful

hydrocarbon-fuelled systems have been investigated [14, 15]. These include the modification

of Ni-based anodes either by surface decoration, alloying or with the addition of extra catalytic

layer, deposited onto the anode. Another strategy is replacing Ni with other carbon-resisting

materials to improve the anode resistance towards coking. Controlling the feed gas composition

by adding reforming agent such as steam or carbon dioxide (CO2) in order to allow internal

reforming process could also assist in minimising carbon formation.

To date, there are several companies that produce SOFC commercially. Ceres Power (UK) is

among the global front-runner in SOFC field offering fuel cell based micro-combined heat and

power (micro-CHP) product. Their technology presents cost-effective, advanced generation

fuel cell technology for use in distributed power products that lower operating costs, reduce

pollutants emissions with greater efficiency. The unique patented SteelCell® technology

presents the economic advantage and robust technology which could produce power using

various fuels including natural gas and hydrogen at high efficiency. The cell is made via

standard processing equipment and common materials such as steel as a support and ceria-

based electrolyte, indicating that it can be mass produced at a reasonable price.

Chapter 1

23

Despite having the advantage of fuel flexibility, hydrogen (H2) remains as a major fuel for

SOFC operation and obtaining more efficient system with improved performance has become

key research target. In this light, improvement of the electrode design, particularly the anode,

is crucial to achieve a high-performance SOFC. This work is intended to improve the design

of anode for the development of micro-tubular cell fabricated from phase-inversion assisted

process. The enhancement of micro-tubular cell is not limited to getting better electrochemical

performance but is also aimed to improve its mechanical robustness by applying a multi-

channel design.

1.2 Research objectives

The general objective of this study is to develop micro-structured micro-tubes for micro-tubular

solid oxide fuel cells (MT-SOFCs) through a phase-inversion assisted extrusion technique.

Flexibility in structural design and morphology tailoring has been explored to obtain various

anode structure for achieving high-performance cell. Different types of anode substrate has

been developed via such method. The ability for SOFC to be operated using hydrocarbon is

one major advantage. Thus, part of this work has been directed on designing the electrode (Ni-

free anode) in order to be suitable for direct use of methane (CH4) as a fuel.

In addition, realising the necessity to increase the mechanical robustness of the micro-tubes,

multi-channel design has been proposed. The fabrication of micro-tubes with different numbers

of channel has been studied and tested for their electrochemical perfomances in H2-fuelled

system. The last part of this work explored the possibility of using similar cell for both fuel cell

mode and carbon dioxide (CO2) electrolysis. Such study would be beneficial to provide the

insight that shows SOFC’s potential applications for energy and environment. In pursuit of this

broad objective, four specific aims have been identified:

Chapter 1

24

1. To fabricate anode micro-tube with improved morphology for MT-SOFC.

Two distinct nickel oxide-yttria stabilised zirconia (NiO-YSZ) anode substrate is fabricated

through phase-inversion process. A new substrate design made via wet spinning process using

solvent-based bore fluid in order to acquire a highly asymmetric structure consisting long

micro-channels with open entrances. This new design for a single channel NiO-YSZ micro-

tube is developed into a complete cell and eventually tested and compared with the

conventional single channel design.

2. To develop MT-SOFC with Cu-based anode (Ni-free) for direct methane (CH4) utilisation.

Using Ni-free anode for direct CH4 utilisation has been suggested to avoid severe carbon

formation during SOFC operation. The yttria-stabilised zirconia (YSZ) electrolyte scaffold

with particular structure is first developed through wet spinning process using solvent-based

bore fluid. Such structure allows the use of copper-ceria (Cu-CeO2) anode to be incorporated

into the YSZ scaffold. The complete single cells are tested for its electrochemical performance

with dry CH4.

3. To fabricate multi-channel micro-tubes for MT-SOFC.

Applying multi-channel or so-called micro-monolithic design, has been proposed to fabricate

novel and highly robust anodes for SOFC. Micro-monolithic NiO-YSZ anode substrate with

various number of channels are fabricated using in-house-made spinneret and developed into

a complete single cell. Such substrate with different number of channels are characterised and

compared with a single channel counterpart for their electrochemical performances.

4. To investigate other potential application of SOFC.

SOFC is known to have the flexibility in operation by allowing the reversible operation using

the same cell to work under fuel cell or electrolysis modes. This study aimed to develop high-

Chapter 1

25

performance SOFC for both hydrogen (H2) fuel cell and carbon dioxide (CO2) electrolysis. A

novel micro-monolithic (6-channel) anode is fabricated and developed into a complete cell and

tested for its H2-fuelled operation and CO2 electrolysis.

1.3 Thesis arrangement

The thesis consists of seven chapters in total, including an introduction, literature review, four

chapters, one each for each section of research, and finally a chapter of conclusion and

recommendations for future work. The overall thesis arrangement is shown in Figure 1.2.

Figure 1.2: Overall structure of the thesis.

Chapter 1 briefly introduces the research background and main objectives of this study. A

systematic literature review in Chapter 2 provides insight into the principles of SOFC

operation, covering the information on different cell components, SOFC design, description on

Chapter 1

26

hydrocarbon-fuelled operation, the fabrication process of micro-tubes and progress of MT-

SOFC.

Chapters 3 to 6 address different aspects on the development of micro-tubular SOFC using

phase-inversion assisted process. Chapter 3 describes the development single-channel NiO-

YSZ (anode) micro-tube with different micro-structure which ultimately developed into a

micro-tubular (MT)-SOFC. Chapter 4 reports on the use of Ni-free anode in which micro-

structured MT-SOFC for direct methane (CH4) utilisation using a cell with copper (Cu)-based

anode has been developed and applied. Inspired by the new single-channel NiO-YSZ anode

design which comprised of long micro-channels with open entrances as reported in Chapter 3,

the prospect of designing an electrolyte scaffold suitable for incorporating anode materials that

is resistant to carbon formation is investigated. The completed single cell was studied for direct

CH4 operations.

Chapter 5 addresses the fabrication of multi-channel NiO-YSZ anode substrate as an approach

to obtain more robust SOFC with improved performance. Various types of anodes with

different number of channels were fabricated and developed into a single micro-monolithic

cell. All cells were tested and compared for their electrochemical performances in hydrogen

(H2)-fuelled system. Chapter 6 reports on the extended application of micro-monolithic SOFC

for reversible operation; fuel cell and electrolyser. A study on carbon dioxide (CO2) splitting

into carbon monoxide (CO) has been conducted with such unique NiO-YSZ (fuel)-electrode

structure, in which high quality performance has been demonstrated. Chapter 7 concludes the

progress achieved in micro-tubular and micro-monolithic SOFCs and some propositions for

future works are given.

Chapter 2

27

2 CHAPTER 2

Literature review

2.1 SOFC principle and thermodynamics

A solid oxide fuel cell (SOFC) is an electrochemical reactor with the capability of converting

chemical energy into electrical energy directly with a greater efficiency than other types of fuel

cell [1-3].

Figure 2.1: Operating principle of typical solid oxide fuel cell; (a) oxide ion (O2−) conducting

electrolyte and (b) proton conducting electrolyte.

Figure 2.1 shows the standard operational principle of the SOFC system with hydrogen (H2) as

a fuel for both oxide conducting electrolyte and proton conducting electrolyte [16]. There are

two main reactions: the reduction of oxygen at the cathode and the oxidation of fuel at the

anode. For a cell with oxide conducting electrolyte, the oxygen (O2−) ions travel from the

cathode to anode through the dense electrolyte. At the anode, they react with hydrogen (H+)

ions and water vapour is generated. For proton conducting electrolyte, the hydrogen undergoes

oxidation reaction where it is oxidised to form electrons and H+ ions. The H+ ions travelled

from the anode to the cathode and react with oxygen molecules to form water vapour. In both

cell types, the generated electrons move via an external circuit, generating electricity. The

Chapter 2

28

electrons reach the cathode and react with oxygen. In principle, a fuel cell could continuously

produce energy with a continuous supply of fuel and oxidant. Hydrogen (H2) is the standard

fuel for SOFCs due to its high electrochemical reactivity while oxygen (O2) is applied as

oxidant.

Operating principles of a fuel cell including SOFC has been well described in the literature [1-

5, 7, 16, 17]. The thermodynamic efficiency (ε) of a fuel cell is the ratio of the electrical energy

output to the chemical energy of the reaction. The energy output/generated is the Gibbs energy

(∆GT) of the combustion reaction of the fuel at the working temperature (T). The chemical

energy of the reaction is represented by enthalpy (∆HT) or heat of combustion of the fuel at the

working temperature. Therefore, efficiency can be represented as below:

𝜀 =∆𝐺𝑇

∆𝐻𝑇× 100% =

∆𝐻𝑇 − 𝑇∆𝑆𝑇

∆𝐻𝑇× 100% = 1 −

𝑇∆𝑆𝑇

∆𝐻𝑇× 100%

(2.1)

Where (∆GT) is the Gibbs energy of the fuel combustion reaction, (∆HT) is enthalpy or heat of

combustion of the fuel, and ΔST is the reaction’s entropy at the working temperature (T).

For an H2-fuelled system, equations 2.2–2.4 represent the reactions at both electrodes and the

overall reaction [17]:

Anode:

Cathode:

Overall reaction:

2H2 → 4H+ + 4e−

O2 + 4e− → 2O2−

2H2 + O2 → 2H2O

(2.2)

(2.3)

(2.4)

For a cell operating on hydrocarbons (e.g. alkanes, CnH2n+2), the overall reaction becomes:

CnH2n+2 + (3n + 1)O2− → nCO2 + (n + 1)H2O + (6n + 2)e− (2.5)

The maximum voltage of each fuel cell can be represented by the Nernst voltage (VN). With an

ideal gas assumption and H2 as fuel, this can be calculated by:

Chapter 2

29

𝑉𝑁 = 𝐸0 + 𝑅𝑇

2𝐹𝑙𝑛 [

𝑝𝐻2√𝑝𝑂2

𝑝𝐻2𝑂]

(2.6)

where E0 is the standard potential, R is gas constant, F is Faraday’s constant, 96 485 C mol−1

and 𝑝𝐻2, 𝑝𝑂2

and 𝑝𝐻2𝑂 are partial pressure of hydrogen, oxygen and steam, respectively.

For hydrocarbon fuel (e.g. alkanes, CnH2n+2), the Nernst voltage (VN) can be measured using:

𝑉𝑁 = 𝐸0 + 𝑅𝑇

2(3𝑛 + 1)𝐹𝑙𝑛 [

(𝑝𝐶𝑛𝐻2𝑛+2)(𝑝𝑂2

)3𝑛+1

2

(𝑝𝐻2𝑂)𝑛+1( 𝑝𝐶𝑂2)𝑛

]

(2.7)

The real operating voltage (V) is, in fact, lesser than the Nernst voltage attributable to the

voltage losses, also termed as polarisation (η). This can be determined by [4, 17]:

𝑉 = 𝑉𝑁 − 𝜂 (2.8)

In general, the overall polarisation (η) consists of four main categories, which are activation

polarisation (𝜂𝑎), ohmic polarisation (𝜂Ω), concentration polarisation (𝜂𝑐), and crossover losses

are represented by the following:

𝜂 = 𝜂𝑎 + 𝜂Ω + 𝜂𝑐 + 𝜂𝑐𝑟𝑜𝑠𝑠𝑜𝑣𝑒𝑟 (2.9)

Activation polarisation, ηa, is associated with the reaction rate at the electrodes. In chemical

reactions, the required energy for reacting species to overcome the barrier, termed as activation

energy, contributes to activation polarisation. The relationship between activation polarisation

and current density, j can be represented by the Butler-Volmer equation:

𝑗 = 𝑗0 𝑒𝑥𝑝 [𝛼𝜂𝑎𝐹

𝑅𝑇] − 𝑗𝑜𝑒𝑥𝑝 [

(1 − 𝛼)𝜂𝑎𝐹

𝑅𝑇]

(2.10)

where α is the charge transfer coefficient, and jo is the exchange current density. The transfer

coefficient is the fraction of the change in the polarisation, which leads to a change in the

reaction rate constant. The exchange current density (jo) is the forward and reverse electrode

reaction rate at the equilibrium potential. Activation polarisation is usually due to one or more

Chapter 2

30

rate-determining steps in the electrode reactions. The slow step could be the adsorption of

reactant onto the electrodes surface, electron transfer, desorption of product or any other steps

in the reactions. The electrode reaction rate is a function of temperature, pressure and electrode

materials. At high-temperature SOFC operations, the reaction rate is quick; thus, activation

polarisation is trivial.

The ohmic polarisation/resistance, ηΩ, is attributable to the ionic resistance of the electrolyte,

mixed ionic and electronic resistances at the electrodes and contact resistance at the interfaces.

In an SOFC, the main contributor to ohmic polarisation is from the electrolyte, since the ionic

resistivity of electrolyte (ρe) is much higher than the electronic resistivity of the electrodes.

Therefore, electronic resistivity may often be neglected. With the assumption that the contact

resistance is negligible, ohmic polarisation becomes a direct function of the electrolyte

thickness, le:

𝜂𝛺 = 𝑗𝜌𝑒𝑙𝑒 (2.11)

The resistances of SOFC components are expressed by the area specific resistance (ASR) and

the electrolyte resistance per unit area (ASRe):

𝐴𝑆𝑅𝑒 = 𝜌𝑒𝑙𝑒 (2.12)

Therefore, ηΩ could be expressed as:

𝜂𝛺 = 𝑗𝐴𝑆𝑅𝑒 (2.13)

One option to lower the ohmic polarisation is through the fabrication of electrode-supported

cells, allowing a considerable reduction in electrolyte thickness (~5-30 µm) [11, 18]. In

addition, the application of electrolyte materials with better ionic conductivity, such as doped

ceria and lanthanum gallate, could also minimise polarisation.

Concentration polarisation, ηc, also known as diffusion polarisation, can be evident when the

electrode reaction is slowed down by mass transport effects. For instance, this polarisation

arises when the feeding of the reactant and/or removing the reaction products from the

Chapter 2

31

electrode is slower than the discharge of current density, j. When the gas diffusion governs the

reactions at the electrode due to the low concentration of reactant in the feed gases or the

reactant conversion is nearly 100 %, the limiting current density (jL) is reached. This is

characterised by a rapid drop in cell voltage. For an electrode reaction without activation

polarisation, the concentration polarisation is presented as:

𝜂𝑐 =𝑅𝑇

𝑛𝐹𝑙𝑛 (1 −

𝑗

𝑗𝐿)

(2.14)

In ideal situations, where all the polarisation losses are zero, a constant voltage is equal to the

theoretical Nernst potential (VN). However, this is not the case as the measured open circuit

voltage (OCV) will always be lower than the theoretical value. This difference is associated

with the crossover losses due to, first, the gas crossover caused by the inadequate electrolyte

gas-tightness or micro-crack. Second, due to the material not being a perfect electronic

insulator, the internal electrical current may occur.

At any current density (j), the operating voltage (V) of the SOFC can be estimated by

subtracting all the voltage losses from the OCV. The actual voltage of an operating cell is given

by:

𝑉 = 𝑂𝐶𝑉 − 𝜂𝑎 − 𝜂𝛺 − 𝜂𝑐 (2.15)

Figure 2.2 characterises the operating cell voltage (V) as a function of the operating current

density (j) and indicates the region where different types of voltage losses predominate. At a

low current density, activation polarisation (ηa) plays a significant role in the cell voltage

losses, as shown by the rapid drop in cell voltage. With the increase in current density, the

internal resistance polarisation or ohmic loss (ηΩ) dictates, as indicated by the slight voltage

decrease. At higher current densities, the cell resistance is greatly affected by the mass transport

limitations (concentration polarisation, ηc), causing a sharp decline in cell voltage. Open circuit

voltage (OCV) is denoted by a dotted horizontal straight line.

Chapter 2

32

Figure 2.2: Schematic of typical SOFC current-voltage curve showing the operating cell

voltage (V) and voltage losses (η) as a function of the operating current density (j).

Indeed, various factors affect the polarisation, such as properties of selected cell materials and

microstructure. Accordingly, efforts towards minimising these losses to achieve better cell

performance are of high interest.

2.2 SOFC components and materials

A single cell normally comprises a gas-tight electrolyte layer flanked by the two electrodes.

The SOFC units could also be assembled in a series of single cells with interconnect and forms

a stack as shown in Figure 2.3. The SOFC operates at relatively high temperatures, in a range

between 500 and 1000 °C, as these are necessary to obtain sufficient ionic conduction from

ceramic materials. This high temperature, alongside the air and fuel input, creates both a highly

oxidising and reducing atmosphere inside the cells. Each component has specific functions and

individual requirements critical to material selection.

Chapter 2

33

Figure 2.3: Schematic of (a) a single cell of SOFC and (b) arrangement for a stack of SOFC.


It is crucial that the materials used to construct the device are able to withstand the applied

temperatures and remain stable despite the oxidising and reducing nature of the gases. Table

2.1 listed the common materials used for constructing lab-scale SOFC [1-3]. Over the past

decades, various efforts have been made in search of identifying better materials. The following

sections will further discuss the requirements for each component of the SOFC.

Table 2.1: Common materials for SOFC components.

SOFC component Conventional material

Anode Yttria stabilised zirconia (YSZ) and Ni cermet

Electrolyte YSZ (with ~8-10 mol % yttria)

Cathode Lanthanum strontium manganite (LSM)

Current collector Nickel wire (for anode), Silver wire (for cathode)

2.2.1 Anode

The anode is where the fuel oxidation reaction takes place [19]. Fuel is supplied and reacts with

the oxygen (O2−) ions which have travelled from the cathode through the dense electrolyte. An

ideal anode should be able to serve as ionic and electronic conductors for the passage of O2−

Chapter 2

34

ions and electrons and present sufficient reaction sites for the electrocatalytic activity for fuel

oxidation. It also must exhibit high stability in an oxidising atmosphere, have matching thermal

expansion coefficient (TEC) and chemically compatible with the adjacent cell components [18,

20]. During operation, the electrochemical reactions in the anode occur at the region where

oxygen ions, electrons and fuel gas phase, all meet known as a triple phase boundary (TPB). It

is essential to select the material carefully, as it has a considerable influence in extending the

TPB. If the material only acts as an electronic conductor such as graphite, transition metals and

platinum, the effective electrochemical zone (ERZ) is constrained to the interface between

anode and electrolyte (Figure 2.4a). Another type of materials could be that with mixed

conductivities such as perovskites. With these materials, theoretically the ERZ will be extended

to the entire anode (Figure 2.4b). Notwithstanding their potential as future material for anode,

current limiting factors include a relatively higher cost of fabrication and insufficient electronic

conductivity. Alternatively, the metallic phase may be mixed with ceramic component which

form a cermet in order to extend the ERZ, (Figure 2.4c) that could offer both ionic and

electronic conductivities.

Figure 2.4: TPB regions for different anode material; (a) pure electronic conductor, (b) mixed

ionic and electronic conductor and (c) ionic-electronic composite. Reproduced with permission

from [21].

Chapter 2

35

Ni-YSZ cermet has continued to be the most common anode material, where Ni functions as

an electronic conductor and catalyses fuel oxidation, while YSZ provides ionic conductivity

for oxygen ions to travel within the anode. This composite is resistant to Ni coarsening and has

a thermal expansion coefficient (TEC) close to that of the YSZ electrolyte, where TEC of Ni

and YSZ are ~13.3×10−6 K−1 and ~11×10−6 K−1, respectively [16, 21]. This structural type (as

depicted in Figure 2.4c) has larger effective ERZ, making it more efficient than the anode made

of a pure electronic conductor. Other notable features include a better mechanical strength,

minimal interactions with other cell components at high-temperature operation and cost-

effective [22]. Despite the fact that Ni-YSZ or other Ni-cermet based anodes could fulfil most

of the anode requirements, there are several bottlenecks regarding stability issues during

recurring redox cycles, in addition to vulnerability towards carbon formation and sulphur

poisoning when operated with hydrocarbon. Therefore, it is necessary to find alternative anode

materials for a hydrocarbon-fuelled SOFC system.

The literature reports that anodes based on copper (Cu) and ceria (CeO2) to be practical for a

system running on hydrocarbons [14, 23]. Copper is selected as a substitute to Ni because it is

inert to carbon formation while ceria catalyses the fuel oxidation [24]. Ceria possesses mixed

electronic and ionic conductivity and its addition to the anode formulation helps to improve the

catalytic properties of the anode. This mixed ionic-electronic structure also extends the ERZ

throughout the entire anode interfacial region (Figure 2.4b).

Several other candidates for making anode include doped ceria, pyrochlore, spinel, and

perovskite [15, 18, 20, 25]. Among the various types of perovskites, the titanates and chromites

seem to be attractive [19]. La-doped and yttria-doped SrTiO3 displayed satisfactory

performance and the system with LaCrO3 based anodes were found to be relatively stable in

both reducing and oxidising atmospheres [15]. Further discussion on the appropriate anode

materials for hydrocarbon-fuelled SOFCs is provided in section 2.5.

Chapter 2

36

2.2.2 Electrolyte

The electrolyte is another important component of an SOFC in which the operating temperature

depends on the required temperature to attain sufficient ionic conductivity in the electrolyte

[22, 26]. Its primary function is to act as a separation barrier, preventing the mixing between

fuel and oxidant while allowing ions to travel from one side of the electrode to the other. For

oxide ion conducting electrolyte, ions (O2−) will travel from cathode to anode whereas in proton

conducting electrolyte, the H+ ions will travel from anode to cathode. The thermal expansion

of electrolyte must match the neighbouring components to avoid cracking during the sintering

stage. This layer must also be impervious to gas flow, exhibit sufficient ionic conductivity, and

be electronically insulated to prevent short circuit and allow high stability towards oxidation

and reduction reactions.

Figure 2.5: Conductivity of different mole % of yttria in YSZ in air at 1000 °C. Reproduced

with permission from [26].

One of the first studied electrolyte materials was zirconia (ZrO2) doped with 8 mol % yttria

(Y2O3) which is simply known as yttria-stabilised zirconia (8YSZ) [16, 26]. This material has

shown satisfactory ionic conductivity needed for an SOFC operation. YSZ has remained the

Chapter 2

37

most common electrolyte material, as it has appropriate oxide-ion conductivity (0.13 S cm−1 at

1000 °C), displays the anticipated phase stability in harsh cell situations, and is both

commercially available and economical [26]. Figure 2.5 shows variations in the conductivity

of YSZ with different amount of added yttria. Conductivity reaches an optimum value at yttria

concentration of 8 mol % [26]. Beyond that, the conductivity gradually decreases associated

with a reduction in defect mobility and vacancy clustering.

Lowering the operating temperature of SOFC could allow for more choices of electrolyte

materials. For intermediate temperature operation (500-700 °C), doped ceria such as

gadolinium doped ceria (GDC), sometimes denotes as cerium-gadolinium oxide (CGO), has

been suggested [26-28]. This alternate material offers higher ionic conductivity (with reported

value of 0.025 Ω−1 cm−1) than YSZ (<0.005 Ω−1 cm−1) at 600 °C [7, 16]. However, its

mechanical strength has been reported to deteriorate at higher operating temperatures [28].

Also, the likelihood of “current leakage” between the two electrodes could cause a reduction

in open-circuit voltage (OCV). One possible solution is by having a multi-layer electrolyte in

which buffer layer is introduced on top of the other electrolyte (i.e. GDC on top of YSZ

electrolyte) [29]. A cell with dual-layer GDC/YSZ electrolyte presented a higher OCV

compared to a single-layer GDC as the electrolyte. In addition, samarium-doped ceria (SDC)

and lanthanum strontium gallium magnesium oxide (LSGM) have also been reported to exhibit

relatively high ionic conductivity than the YSZ [30]. Unfortunately, their use is restricted for

several reasons, such as stability/compatibility mismatches with the other cell components and

high material costs.

There has been a considerable interest to develop a protonic conductive electrolyte as an

alternative to the O2− ion-conducting electrolyte [6, 15, 16, 18]. Materials such as yttrium-

doped barium zirconate (BZY) and yttrium-doped barium cerate (BCY) can be applied as an

electrolyte [28, 31]. These protons-conducting oxides have been reported to offer improved

Chapter 2

38

conductivities and almost as efficient as the state-of-the-art oxides conducting ceramic [32].

Their feasibility for conducting protons under low operating temperatures opens the attractive

prospect for SOFC operation at much lower temperature compared to the present temperature

range [33]. However, some major drawbacks are the proton-conducting materials are more

expensive and the challenges during the sintering process [6]. Overall, the selection of the most

appropriate electrolyte material for a particular application should be wisely made to ensure

long-term stability and cost-effectiveness.

2.2.3 Cathode

The purpose of a cathode is to facilitate oxygen (O2) reduction, which involves the

electrochemical conversion of O2 into O2− [34, 35]. The O2− travel through the electrolyte and

react with fuel at the anode. The cathode must be structurally porous to permit rapid diffusion

of oxygen, catalytically active for oxygen reduction, sufficient ionic and electronic

conductivities, excellent stability under an oxidising atmosphere, and compatible with the

adjacent components of the cell (i.e. having matching TEC and no chemical reaction). These

stringent requirements limit the selection of suitable materials for making cathodes. In the early

days, noble metals were used and showed satisfactory performance. However, the practical use

for such materials for making cathode is constrained by their expense. As a substitute, porous

perovskite materials have been recommended [16]. They are cheaper, thermally and chemically

stable, and offer excellent catalytic activity for oxygen reduction at high temperatures and

practically good ionic and electronic conductivity.

Chapter 2

39

Figure 2.6: The lattice structure of perovskites. Reproduced with permission from [28].

As presented in Figure 2.6, a perovskite lattice structure, ABO3 contains three elements: large

cations (A), small cations (B) and oxide ions (O2−) with a total valence of +6. The large A-site

cations, for instance, lanthanum (La), strontium (Sr), lead (Pb) and calcium (Ca) have twelve

coordinated oxygen anions. The small B-site cations such as titanium (Ti), chromium (Cr), iron

(Fe), cobalt (Co), zirconium (Zr) and nickel (Ni) are surrounded by six oxygen anions [16, 35].

Some of the common materials are strontium-doped lanthanum manganite (La1−xSrxMnO3, or

LSM), strontium-doped lanthanum cobaltite (La1−xSrxCoO3−δ, or LSC), strontium-doped

lanthanum ferrite (La1−xSrxFeO3−δ, or LSF), and strontium-doped lanthanum cobalt ferrite

(La1−xSrxCo1−yFeyO3−δ, or LSCF) [16, 34]. These materials offer mixed ionic-electronic

conductivities. Although most of the cathode materials are suitable for uses in an oxidising

environment, they are unfortunately susceptible to reduction by fuel. Therefore, they are

inappropriate as an anode material. The essential criteria for the three key components in SOFC

are summarised in the following table:

Table 2.2: Requirements for the anode, electrolyte and cathode for SOFC.

Properties Anode Electrolyte Cathode

Electron conductivity High Poor (Insulator) High

Ionic conductivity High Completely High

Chapter 2

40

Structure Porous Dense, gastight Porous

Function Catalyse fuel

oxidation

Conduct O2− ions (H+

for proton-conducting)

Catalyse oxygen

reduction reaction

Thermal expansion

coefficient (TEC)

Matching with

electrolyte

Matching with both

electrodes

Matching with

electrolyte

Durability Stable at high

temperature and good

endurance to repeated

redox cycle

Stable in extreme

operating conditions

Stable in air at high

temperature and

resistance to cathode

Chemical stability No reaction with other

cell components

No reaction with other

cell components

No reaction with other

cell components

2.2.4 Interconnect

In general, the maximum voltage output of a single cell under general operating conditions is

relatively low, often less than 1 V. Therefore, SOFCs are typically stacked to produce power

of higher voltages. An interconnect must be applied as an electrical connector between each

single cells, connecting the cathode of one individual cell to the anode of the neighbouring cell

in a stack [36]. It also functions as a barrier for these two electrodes, which is required to ensure

separate channels inside the stack for continuous flows of both fuel and oxidant. The materials

requirements for interconnect must exhibit not only an excellent compatibility with other cell

components but also adequate thermal and chemical stability under both oxidising and reducing

atmosphere [36]. Furthermore, the materials must provide high thermal and electronic

conductivity, negligible ionic conductivity, and appropriate mechanical strength.

All these requirements together with extra considerations on cost and manufacturability have

limit the suitable materials to only a handful choices. The potential materials which suit these

requirements could be either the doped rare earth chromite perovskite structure such as YCrO3

or LaCrO3 for a range of operating temperatures from 900 to 1000 °C, or metallic alloys for

Chapter 2

41

lower temperature operation [16, 37]. Since the 1970s, lanthanum chromite (LaCrO3) has been

commonly tested as the interconnect material of high-temperature solid oxide fuel cells due to

the desirable properties such as appropriate electrical conductivity in both reducing and

oxidising conditions, considerable mechanical strength and stability at high operating

temperatures, and adequate compatibility with other cell components [36, 37]. For practical

uses, certain dopants have been tested and studied to tailor and enhance the properties of

LaCrO3. However, it is worth mentioning that this work focuses on the development of a single

cell with better anode or substrate micro-structure for performance improvement. Thus,

examining the interconnect is beyond the scope of this thesis.

2.3 SOFC geometries classification

2.3.1 Conventional planar and tubular design

For over fifty years, numerous studies have been devoted to compact design and cell

performance enhancement involving two common designs, namely planar and tubular, as

shown in Figure 2.7.

Figure 2.7: Schematic diagram of a typical design of SOFC. Reproduced with permission from

[38].

The fabrication of ceramic planar SOFCs is relatively straightforward, using tape casting and

screen printing. Thus, it has a comparatively lower cost. Besides, the planar cells present

Chapter 2

42

greater power densities corresponding to shorter distance for electrons to travel from anode to

cathode [38]. However, this design has certain drawbacks, for instance, less thermal stability,

difficulties in effective high-temperature seals as larger areas requiring gas sealing, prone to

unexpected increase or decrease in operational temperature and challenges during scaling up

from a single cell to bundle/stack design. A crack may occur which is attributable to the non-

uniform stress distribution and temperature gradients during the sealing which demands extra

control on heating and cooling rate. This results in much longer start-up and shut-down times.

In recent years, the development of metal-supported (MS)-SOFCs have been regarded as a

promising substitute to conventional ceramic structures [39, 40]. Such newer design offers

added merits by having better thermal shock resistance and alleviate internal temperature

gradients owing to good thermal conductivity and ductility of the metallic substrate. In

addition, substituting from ceramic to metal supports allows the use of conventional metal

joining and forming techniques which could considerably lower the cost for material and

manufacturing of SOFC stacks. One of the most critical steps during MS-SOFC preparation is

the deposition of a dense electrolyte layer on the pre-fabricated metal substrate. As such

substrate cannot be shrunk, different coating methods including atomic layer deposition (ALD),

pulsed laser deposition (PLD), atmospheric plasma spraying (APS), suspension plasma

spraying (SPS), and sputtering systems, have been attempted for the fabrication of MS-SOFCs

[41]. Nevertheless, these coating techniques, at the moment, is not suitable for preparing larger

cell sizes. Another major drawback is that such processes are rather expensive. Despite

potential advantages offered by MS-SOFCs, processing challenges have restrained the progress

of this cell design. It is worth mentioning that Ceres Power (UK) has successfully

commercialised their unique patented steel-supported cells which resolve most of the problems

encountered by the conventional ceramic SOFC. Efforts to improve performance and long-

Chapter 2

43

term stability are currently ongoing to fully explore the potential of SOFC for energy

generation.

For a tubular SOFC, all components are configured in the shape of the multi-layered cylindrical

tubes. Depending on the position of the particular electrodes, the air and fuel can be fed into

the core or the shell side. In contrast to its planar counterpart, the materials required for gas

sealing at high temperature to separate the fuel from the oxidant can be disregarded, because

the sealing components of the cell could be placed out of the heating area. Furthermore, the

tube design allows appropriate thermal shock resistance and mechanical characteristics.

Nevertheless, the longer current path for the electrons to travel in the inner layer of the tubes

could be a significant drawback. This contributes to a greater ohmic resistance, leading to a

decrease in power densities [11, 38]. Tubular structures can be further categorised as those with

diameters of > 15 mm and those with diameters of < 5 mm, otherwise known as a micro-tubular

cell (MT-SOFC) [11, 12, 42, 43]. In general, the application of MT-SOFC will be similar to

the conventional SOFC. Considering the size of the cells and stacks, the larger tubular SOFC

is suitable for stationary applications whereas the smaller micro-tube is appropriate for portable

uses. The more recent micro-tubular structure has been explored due to a number of benefits it

could offer and will be further deliberated in the subsequent section.

2.3.2 Micro-tubular design

It was not until the early 1990s, with the introduction of a more advanced design, namely the

micro-tubular solid oxide fuel cell (MT-SOFC) by Kendall, did appear to be another

breakthrough in SOFC technology [44, 45]. This smaller tube has diameters in a range of a few

millimetres to the sub-millimetre scale. The reduction in diameter to a level of a micron has

led to an even more complicated production method for such geometry. MT-SOFCs offer

higher volumetric power density, better tolerance to thermal cycling, quicker start-up

Chapter 2

44

capability, flexible sealing between fuel and oxidant streams, and portable characteristics [46-

48].

The possible applications of this type of SOFC for co-generation of heat and power, and for

use in micro hybrid electric vehicles were successfully demonstrated [49]. Besides, the

prospect of direct internal reforming of hydrocarbon fuel with MT-SOFC could reduce the

overall cost and the system becoming lighter and less complex. This design combined with

internal reforming catalyst is simple, allowing fast start-up and can be easily adapted for

different energy demand. These advantages have stimulated further exploration in MT-SOFCs

field. To date, comprehensive information on the progress in MT-SOFCs has been reviewed

by Lawlor et al. [12, 43], focussing on issues of individual cells and stack design along with

discussions on recent research activities.

One of the typical methods to produce micro-tube for MT-SOFC is plastic mass ram extrusion.

Two prominent groups are specialising in this technique. Sammes’s research team is focusing

mainly on the electrolyte and anode-supported MT-SOFCs [42, 50, 51], while Suzuki’s group

from Japan largely prepared anode (micro-tubular) supports coated with electrolyte. Figure 2.8

shows the simplified plastic mass ram extrusion process adopted by Suzuki et al. [52].

Figure 2.8: Schematic diagram of the plastic mass ram extrusion process. Reproduced with


Chapter 2

45

The fabrication of micro-tubes based on multiple coating and sintering steps, however, requires

a complex procedure and a considerable amount of energy. In comparison, a simpler ram co-

extrusion technique has been proposed [53] by combining the rheology of the paste, to be

extruded simultaneously, creating the multi-layer tube shown in Figure 2.9.

Figure 2.9: The paste co-extrusion process. Reproduced with permission from [53].

Alternatively, the fabrication of micro-tubes could be carried out through the phase-inversion-

based extrusion. This relatively new method is analogous to ram extrusion, but with an

additional phase-inversion step which occurs directly after the extrusion process. The main

distinguishing feature is the spinning suspension; liquid form for phase-inversion-based

extrusion, and plastic mass form for ram extrusion. During phase-inversion extrusion, the

solidification of precursors happens resulting from the solvent/non-solvent exchange, whereas

in the plastic ram extrusion process, the tubes are left to dry naturally after extrusion [38].

In essence, the phase-inversion concept introduced in the 1960s was initially employed for

making asymmetric polymeric membranes [54]. A few decades later, our research group, led

by Professor Li, started to apply this concept for making ceramic hollow fibre membranes also

known as ceramic micro-tubes [55]. Since then, there has been a significant progress whereby

Chapter 2

46

single and multi-layer ceramic micro-tubes have been successfully fabricated based on single

or co-extrusion in producing micro-structured ceramic micro-tubes [56-60]. This advanced

technology, combined with the proper sintering process, may produce high-quality micro-tubes

of different materials and various morphology for use in various applications.

Figure 2.10: Typical steps involve during the fabrication of a complete single cell. Figure

reproduced with permission from [61].

One of the potential applications of ceramic micro-tube is for the development of micro-tubular

solid oxide fuel cell (MT-SOFC) [28, 57]. The standard procedure for the preparation of a

complete single cell from the micro-tube is shown in Figure 2.10. The initial step is to produce

the substrate, depending on which component to be used as the support for the cell, i.e. anode

support or electrolyte support, followed by the incorporation of other cell components. Once

completed, the current collection is applied at both the anode and cathode. A complete single

MT-SOFC is then assembled into a complete reactor and sealed with gas-tight sealant, readying

it for an electrochemical test. MT-SOFCs developed from phase-inversion process has been

continuously studied to further improve their electrochemical performance and Table 2.3 lists

the progress of research related to MT-SOFCs.

Chapter 2

47

Table 2.3: Development of single MT-SOFC prepared from phase-inversion process.

Cell

configuration

A/El/C Cell diam.

(mm)

Fuel Temp.

(°C)

OCV

(V)

Pmax

(W cm−2)

Comments Ref.

Electrolyte-

supported

Ni/YSZ/LSCF 1.28 H2 800 ~1.2 0.018 • Proof of concept study: YSZ micro-tube functioning as

electrolyte and scaffold for anode deposition.

• Low Pmax is due to high ohmic loss.

[62]

Electrolyte-

supported

Ni-YSZ/YSZ/

LSM

1.90 Ar-H2 800 0.95 0.018 • YSZ micro-tube as electrolyte and scaffold for anode.

• Ni-YSZ particles were infused into the pores from alcoholic

dispersion whereas Ni is deposited by electroless plating.

[63]

Anode-

supported

Ni-YSZ/YSZ/

LSM-YSZ

1.15 H2 800 ~1.1 0.828 • Higher sintering temperature reduces anode porosity.

• Anode was first pre-treated to obtain denser electrolyte.

[64]

Anode-

supported

Ni-YSZ/YSZ/

LSM-YSZ

6.0 Wet

H2

800 ~1.0 0.848 • Proposed porosity for the anode should be <55% to

minimise resistance.

• Study on the effect of anode current collecting point

distance with the cathode; shorter distant gave better

performance.

[65]

Anode-

supported

Ni-YSZ/YSZ/

LSCM-SDC-

YSZ

1.30 Wet

H2

850 ~1.03 0.513 • Pre-sintered anode is needed for successful electrolyte

sintering.

• Redox stable cathode gave low interfacial polarisation

resistance.

[66]

Anode-

supported

Ni-YSZ/YSZ/ 1.50 Wet

H2

900 ~1.0 1.25 • LSM-SDC was added to the YSZ matrix by impregnation. [67]

Chapter 2

48

LSM-YSZ-

SDC

• Well-connected LSM and the use of SDC is aimed at

extending the TPB.

• Better asymmetric-porous structured H2 electrode: large

micro-channel benefits the fuel transport.

Anode-

supported

Ni-YSZ/YSZ/

LSM-YSZ

1.18 Wet

H2

800 0.84 0.2 • YSZ electrolyte prepared by electrophoretic deposition.

• YSZ sintering under H2 atmosphere to minimise the

problem due to the differences in the thermal expansion

rates of NiO and YSZ; NiO shrinkage rate upon reduction

was expected to be more than the covering YSZ electrolyte.

[68]

Anode-

supported

Ni-YSZ/ScSZ/

LSM-ScSZ

1.65 Wet

H2

800 1.1 1.01 • ScSZ electrolyte reduces ohmic polarisation.

• Ni-YSZ anode with Ni-ScSZ anode functional layer.

[69]

Anode-

supported

Ni-YSZ/ScSZ/

LSM-SDC-

ScSZ

1.5 Wet

H2

650 1.1 0.52 • Small finger-like pores close to electrolyte serves as an

anode functioning layer while the layer with large finger-

like pores serves as a fuel delivery layer.

• Thermal cycles appear to have no adverse effect on the

long-term performance.

[70]

Anode-

supported

Ni-YSZ/ScSZ/

GDC-BCFN

~2.0 Wet

H2

650 1.1 0.72 • Low cell resistance in this study is attributed to improved

electrode/electrolyte contact and anode micro-structure.

• Good stability over 213 h short-term test.

[71]

Anode-

supported

Ni-BZCYYb/

BZCYYb/

LSCF-

BZCYYb

1.60 Wet

H2

600 1.01 0.26 • Cell with BZCYYb electrolyte proton conductor.

• Poor performance at low temperatures as cathode interfacial

polarisation resistance significantly increases with the

reduction in the operating temperature.

[72]

Chapter 2

49

Anode-

supported

Ni-BZCYYb/

BZCYYb/

SSC-BZCYYb

n/a Wet

H2

650 1.1 0.254 • Graded anode obtained due to the asymmetric structure

formed following phase-inversion process.

• Cell with BZCYYb electrolyte proton conductor.

• BZCYYb electrolyte only accounts for ~9% of the ohmic

resistance.

[73]

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

1.40 Wet

H2

550 0.9 0.8 • Dual-layer anode/electrolyte structure is fabricated from co-

extrusion/co-sintering process.

• Low cell performance is associated with contact loss due to

poor contact between silver wire spring and inside micro-

tube lumen.

[74]

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

n/a Wet

H2

550 0.78 1.1 • Micro-tubes with different electrolyte thickness are

fabricated using co-extrusion/co-sintering process.

• Electrolyte thickness is controlled by controlling the

electrolyte dope extrusion rate.

• The thinnest electrolyte (10 µm) gave the best cell

performance.

[75]

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

n/a Wet

H2

600 0.77 2.32 • Dual-layer anode/electrolyte structure is made from co-

extrusion/co-sintering process.

• Long micro-channels (70% of the anode thickness) with

larger pore entrance resulted in superior performance.

[76]

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

n/a Wet

H2

600 0.76 0.69 • Surface modification of Ni nano-particles via solution

impregnation technique.

[77]

Chapter 2

50

• Electronic conductivity and catalytic activity of the anode

are improved after Ni impregnation.

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

1.52 H2 600 0.8 1.21 • Triple layer micro-tube consisting anode/anode functional

layer (AFL)/electrolyte is fabricated in a single step.

• A cell with the thinnest AFL resulted in the best

electrochemical performance.

[59]

Anode-

supported

Ni-GDC/GDC/

LSCF-GDC

2.27 H2 600 0.85 1.07 • Triple layer micro-tube consisting current collector, anode

and electrolyte is fabricated in a single step.

• The Ni-based inner layer enables more uniform current

collection and considerably reduce the ratio of contact loss

in total ohmic loss from 70% down to 6-10 %.

[78]

Anode-

supported

Ni-GDC/GDC

LiO/LSCF-

GDC

1.3 Wet

H2

500 0.33 0.02 • Li-GDC electrolyte is prepared via metal nitrate doping

aimed at improving GDC densification.

• Poor cell performance could be due to moderately dense

electrolyte and appearance of electronic conductivity in Li-

GDC electrolyte.

[79]

Cathode-

supported

Ni-YSZ/YSZ/

LSM-YSZ

n/a H2 850 0.98 0.29 • YSZ/LSM-YSZ dual layer micro-tube is fabricated.

• Higher sintering temperature resulted in thinner YSZ

electrolyte and reduced porosity.

• Co-sintering of cathode/electrolyte is proposed at 1350 °C

to minimise interlayer diffusion.

[80]

*BCFN = Ba0.9Co0.7Fe0.2Nb0.1O3−δ; BZCYYb = BaZr0.1Ce0.7Y0.1Yb0.1O3−δ

Chapter 2

51

2.3.3 Multi-channel anode design

Extensive efforts have been exerted to achieve high-performance SOFC whereby various

strategies have been attempted including material development using doped ceria and doped

lanthanum gallate to replace YSZ as the electrolyte or using nano-structured cathode material

etc [15, 16, 28]. Another way is to study the geometry architecture whereby considerable

research works have been focused on anode-supported cell. In an anode-supported SOFC, the

main role of the anode is to offer proper sites for an electrochemical oxidation of the fuel and

to deliver the produced electrons to the interconnector. Furthermore, the anode itself should

provide sufficient mechanical robustness for the electrolyte and cathode, and act as a gas

diffusion path for the supply of the fuel to reaction sites. An anode-supported cell has been

preferable as such design allows the use of thin electrolyte which minimise ohmic resistance

and eventually results in high performance. Additionally, the flexibility of phase-inversion

process which allows morphology tailoring such as adjustment of pore size and/or porosity

should undergo further investigation.

It is noteworthy that mechanical strength is often been overlooked or even sacrificed during

the attempts to improve electrochemical performance. The major challenge is to find a way to

achieve good cell performance together with sufficient robustness. Flexibility in micro-

structure tailoring and morphology control through phase-inversion process could be exploited

[55] to develop a better structural design for anodes with superior mechanical property [38].

One option to improve the mechanical resistance of the micro-tube is the use of a multi-channel

design, analogous to a honeycomb structure [81]. Such structure is known to exhibit much

greater robustness than the single-channel design. A proof-of-concept study using alumina to

produce various multi-channel capillary tubes via the phase-inversion process has been

demonstrated in our previous work [81][80]. Realising the huge potential of such new design,

with the use of such anode materials (NiO-YSZ), a similar multi-channel substrate could be

Chapter 2

52

made for the development of anode-supported MT-SOFC. Obtaining a robust MT-SOFC with

high-performance would not only help to accelerate its commercialisation but also to be

competitive with the planar design.

2.4 Fabrication of precursor of micro-tubes and micro-monoliths

Prior to developing a complete MT-SOFC, the precursor for micro-tubes/micro-monoliths is

first prepared. The fabrication of ceramic hollow fibres/micro-tubes via a phase-inversion-

based extrusion and sintering technique consist of three consecutive steps: (1) preparation of a

spinning (dope) suspension; (2) extrusion of the precursors; and finally (3) the sintering process

[55, 82].

2.4.1 Preparation of spinning suspension

The spinning suspension is prepared by mixing ceramic particles, solvent, binder and additives.

Ceramic as the main component must be cautiously selected, as particle size, shape and

distribution have substantial effects on the dispersal of the suspension and the characteristics

of the final micro-tubes (i.e. pore size and porosity). Interaction between the binder and the

solvent influences the rheology of the suspension and the solidification process of a micro-

tube. In general, the solvents must have a high exchange rate with non-solvents as it will govern

the structure of the produced micro-tube. Dispersants are added to the suspension for a better

particle dispersion by disrupting the surface interaction between particles which ensure that

they remain separated. To provide high volume fraction of ceramic particles in spinning dope,

a minimum quantity of additive must be added.

2.4.2 Extrusion of precursors of micro-tubes and micro-monoliths

After dope preparation, the next step is the extrusion process, otherwise termed as spinning

process. Before extrusion, the spinning suspension must be degassed under vacuum and

constant stirring to remove gas bubbles, which may affect the micro-tubes’ integrity and

Chapter 2

53

uniformity[55]. After sufficient degassing time, the spinning suspension is extruded through

the custom-designed spinneret, and concurrently, the internal coagulant (bore fluid) is pumped

through the centre bore of the spinneret. Figure 2.11 shows a diagram of extrusion of dope

suspension with bore fluid.

Figure 2.11: Schematic diagram showing the dope suspension and bore fluid are extruded

simultaneously through the concentric orifices.

The extruded micro-tube goes into the non-solvent coagulation bath where it forms a spiral,

thus the term “spinning”. During the phase-inversion process, the exchange between the

solvent and the non-solvent occurs causing the polymer in the suspension to precipitate and

consolidate the ceramic material. Typically, two morphologies have been observed: micro-

channels (finger-like voids) and a sponge-like structure [83, 84], as presented in Figure 2.12.

Chapter 2

54

Figure 2.12: Example of the micro-structure of ceramic micro-tube.

The formation of micro-channels in ceramic membrane precursors can be described by a

hydrodynamically unstable viscous fingering. This phenomenon occurs at the interface

between fluids with different viscosities in the first moments of mixing [84, 85]. There is a

steep concentration gradient once the suspension is in contact with non-solvent, leading to a

solvent and non-solvent exchange. Subsequently, both a rapid increase in local viscosity and

the precipitation of the polymer phase occur. However, due to instabilities at the interface

between the suspension and the precipitant, there is an inclination for viscous fingering to

occur, initiating the formation of micro-channels. Under normal circumstances, a stable

interface would be established between the two phases of differing viscosities, but because of

the presence of an invertible polymer binder, a rapid viscosity increase followed by polymer

precipitation retains the viscous fingering structure [85]. Other work has proposed that the

formation of micro-channels could also be interpreted by the Rayleigh-Taylor instability theory

[86]. Such structure could be formed when two fluids with different density are in contact, the

low-density fluid (i.e. external coagulant) inclines to push into the high-density fluid (i.e.

ceramic suspension) because of the interfacial instability/perturbation, during which solvent

and non-solvent exchange occurs.

Chapter 2

55

Indeed, the thickness of sponge-like structure and the plurality of micro-channels have certain

impacts on the membrane properties such as the mechanical strength and permeation flux. Due

to the flexibility of phase-inversion process, controlling and tailoring the micro-tubes’

morphology and structure is essential for developing a membrane to suit particular uses [85-

87].

2.4.3 Sintering process

Extruded micro-tube precursors are required to go through heat treatment known as sintering.

This process transforms a powder compact into a robust and dense ceramic body. Throughout

sintering, the binders which are typically long-chain polymers soluble in the solvent must be

fully removed. As the sintering process is carried out after the extrusion of the micro-tube, this

method is designated as the “combined phase-inversion-based extrusion and sintering

technique”. The sintering process usually comprises of three successive stages [55]: (Stage 1)

pre-sintering; (Stage 2) thermolysis; and (Stage 3) final sintering.

(1) Pre-sintering is a process to remove any remaining liquid after the fabrication of the micro-

tube precursor and any adsorbed moisture from the surrounding. The adsorbed moisture

may persist in the precursor at temperatures up to 200 °C. Gradual temperature increment

(i.e. 2 °C min−1) is applied, as the expanding vapour within the lattice may otherwise cause

cracks and fractures.

(2) Thermolysis is a procedure which confiscates the organic components, comprised of binder

and dispersant. It occurs when the temperature is raised to about 600-800 °C and dwelled

for 2 hours. This stage is critical to carefully remove the polymer binder, as uncontrolled

thermolysis could cause defects.

(3) Final sintering involves a process of transformation of ceramic particles into a dense and

strong ceramic body. The densification of particles begins at a temperature approximately

2/3 (c. 70 %) of the melting temperature of the material. It is during this period that the

Chapter 2

56

main changes to the pore size and porosity occur. There are three stages during final

sintering: initial, intermediate and final stage, as presented in Figure 2.13.

Figure 2.13: Progress of ceramic microstructure during the sintering process.

During the initial stage, the re-organisations of ceramic particle and neck growth at the contact

points between particles continues until the radius of the grown neck achieves 40-50 % of the

particle radius [55]. Throughout the intermediate stage, the grain boundaries grow as ceramic

particles start to hold together, and pore channels are created alongside the grain edges. By the

end, the pores begin to pinch off and become isolated from each other. This phase takes up

most of the sintering process and causes substantial shrinkage of the ceramic membrane. The

growth of grains primarily occurs during the final stage and the pores are steadily shrunk and

eliminated. Eventually, the ceramic membrane transforms into a dense and compact structure

with appropriate strength [28, 82]. It is common that the asymmetric structure obtained from

the phase-inversion technique is well-preserved after high temperature sintering process. The

sponge-like structures will become gas tight and well densified for certain ceramic materials

whilst micro-channels above a certain size would retain their structure [85].

Chapter 2

57

2.4.4 Co-sintering process

Considering the use of typical materials, the fabrication of a complete single cell made of NiO-

YSZ electrode-supported SOFC with YSZ electrolyte requires at least two high-temperature

sintering processes [28]; (1) co-sintering of the anode substrate and electrolyte and (2) sintering

of the cathode layer. This corresponds to substantial differences of the required sintering

temperatures. A YSZ electrolyte needs a temperature of about 1350-1450 °C to be densified,

while cathode materials, often made from La0.8Sr0.2MnO3−δ (LSM), are usually sintered at much

lower temperature, i.e. ≤ 1200 °C. This two-step process also reduces possible reactions

between cathode and electrolyte during the sintering process. The feasibility of co-sintering

also allows the fabrication of cells with a cathode-side mechanical support instead of anode-

supported, mainly when the anode component requires much lower temperature than both

cathode and electrolyte for sintering. However, the findings in the literature state that cathode-

supported cell performance is much lower than its anode-supported counterpart, making the

latter more desirable [11, 38]. This is due to high polarisation resistance of the cathode tube

leading to a poor electrochemical performance [80].

Less sintering step to fabricate a complete SOFC is preferable. This is due to benefits such as

process simplification by minimising the processing time and input energy, along with reducing

fabrication cost for a better commercial viability [88, 89]. In practice, co-sintering is often

conducted to sinter the anode/electrolyte layer, since temperatures for this purpose are too high

and damaging for cathode sintering, thus, the cathode is typically added in another step.

Recent research progress has shown that protonic (proton-conducting) SOFC appears to be

attractive for lowering the operation temperatures to less than 600 °C [90-92]. Findings have

shown that protonic SOFC could be made through a single-step sintering process utilising a

BZY-based system. The latest work [32] has demonstrated that raw oxides precursors could be

prepared at specific ratio for each cell components; subsequently, all layers are compressed

Chapter 2

58

into a pellet and sintered in a single heat treatment. Success in making a cell via single-sintering

process appears to be attractive in terms of economics and convenience. Nevertheless, such

advancement is still based on lab-scale study and more work still need to be done to improve

the scalability of the fabrication process. Furthermore, co-sintering will also be much practical

where the entire stack comprising of the cells, gas channels and interconnects could be sintered

and prepared concurrently. However, with current material selection for making MT-SOFC

from phase-inversion process, such an approach is very challenging and has yet to be realised.

Continuous effort in searching for suitable materials which allows single sintering to prepare

all cell components for MT-SOFC is required to be competitive with its planar design

counterpart.

2.5 Hydrocarbon-fuelled SOFCs

The versatility in fuel intake for SOFC affords the use of hydrocarbon fuels in place of

hydrogen. There are several modes of operations for hydrocarbon-fuelled SOFCs including an

external fuel reforming prior entering the cell, internal reforming or directly feeding the pure

hydrocarbon into the cell [15, 22]. This is typically selected based on hydrocarbon type and

requirement of power output. Theoretically, the reforming process transforms the hydrocarbon

to H2 and CO, which can then be used directly in a fuel cell. In comparison, SOFCs allow using

the CO produced following hydrocarbon reforming, while most low-temperature fuel cells are

poisoned by CO, helping simplify the fuel process by excluding the CO clean-up step. The

reforming of fuel may be performed externally using a separate fuel processor (reformer) or

internally. A variant of internal reforming is either indirect internal reforming or direct internal

reforming [93]. In the former case, a separate catalyst is integrated within the SOFC stack

upstream of the anode. However, a primary issue is the possible discrepancy between the rates

of endothermic and exothermic reactions. This will cause a sharp decrease in local temperature

close to the entrance of the reformer, resulting in mechanical failure due to thermally induced

Chapter 2

59

stresses [94]. Otherwise, direct internal reforming operates by directly feeding the system with

a mixture of fuels and reforming agents. This mode offers lower overall system costs, reduces

complexity, and requires less maintenance due to the elimination of the external reformer [15].

Finally, considerable interest has been focused on a system in which hydrocarbon-rich fuel is

directly fed into the anode compartment [14]. Since external reforming increases cost and

complication to the whole process, direct utilisation of the hydrocarbon as fuel would offer

substantial benefits. This approach aimed at achieving higher efficiency in addition to the least

fuel processing balance of plant. Direct hydrocarbon SOFCs offer attractive ways for power

generation, particularly with the prospect of gradual internal reforming of hydrocarbons. In this

mode, the anode material must fulfil the required criteria including excellent ionic and

electronic conductivity and high catalytic activity for fuel oxidation [15].

2.5.1 Direct hydrocarbon utilisation

In the literature, several similar terms such as direct conversion, direct oxidation and direct

utilisation have been used. This might lead to misinterpretation of their precise definitions. One

commonly used definition is described by McIntosh and Gorte in which direct hydrocarbon

utilisation in an SOFC involves the use of either dry fuel (for direct oxidation process) or

humidified fuels (approximately 3 vol.% H2O), regardless of the exact reaction steps [14]. It is

reported that the oxidation of hydrocarbon is implausible to take place in one single stage. In

fact, the actual reaction pathways could consist of multi-steps and hence, the electrochemical

reaction of hydrocarbon fuel may progress as follows:

1. Oxidation of cracked carbon and hydrogen and/or

2. Oxidation of oxygenated compounds and/or

3. Oxidation of the intermediates produced from free radical reactions.

Chapter 2

60

Although SOFC has the benefit of fuel flexibility, choice is still restricted by the affinity of

some fuels to foul the typically used Ni-YSZ anode surface [15, 21]. Fouling is due to the

formation of carbon, which is a significant problem which must be addressed for stable

operation. Unless there is a prevalent catalyst that could realise direct oxidation of hydrocarbon,

to minimise this issue, hydrocarbon must be reformed either with steam or CO2 prior to

electrochemical reaction. Another concern is catalyst poisoning by contaminants. Most

hydrocarbons inevitably contain certain levels of impurities such as hydrogen sulphide (H2S),

which may poison a catalyst contained in the anode. Therefore, a pre-treatment process to

remove these contaminants is essential. There have been several comprehensive reviews about

this issue [95, 96].

2.5.2 Anode for hydrocarbon-fuelled SOFCs

Anode materials for SOFCs vary with the types of fuel used. Until now, Ni-YSZ cermet has

been widely used as it exhibits high electrical conductivity, high electrocatalytic activity toward

the oxidation of hydrogen (H2) and good compatibility with YSZ electrolyte [21]. When using

hydrocarbons as fuels, one critical issue to be deliberated is that the anode must exhibit

excellent resistance to carburization and sulfidation [97, 98]. Nickel-based anodes are

susceptible to coking, because nickel is good at catalysing the breaking of hydrocarbon. The

deposition of carbon can block the pores and encapsulate catalyst particles, increasing transport

resistance and reduce the cell performance. To address such issues, there are three approaches

for hydrocarbon-fuelled systems with the typical Ni-based anodes [15]. First is to properly

control the operating conditions which restrain the formation of carbon, i.e. mixing fuel and

reforming agents. Second is to add other carbon-resistant materials (i.e. ceria, Cu, etc.) to the

Ni-cermet formulation and third is to integrate a buffer layer (sometimes denoted as anode

functional layer) or catalyst functional layer onto the anode layer to stimulate internal

reforming process.

Chapter 2

61

The feeding of hydrocarbon with enough reforming agent such as steam or CO2 promotes

internal reforming. This helps minimise carbon deposition by allowing hydrocarbon to be first

reformed into much simpler fuel, hydrogen (H2) and carbon monoxide (CO). It has been

reported that the Ni-YSZ anode-supported SOFCs to be well operated directly with humidified

CH4 and natural gas [99]. The electrochemical oxidation of CH4 is postulated to involve the

initial CH4 cracking followed by the electrochemical oxidation of the resulting solid carbon. It

is, however, worth mentioning that the excessive steam amount will result in fuel dilution and

thus, compromise the overall cell efficiency. The second strategy to modify Ni-based anodes

through doping or incorporating other materials have been attempted [23]. It is shown that the

direct electrochemical oxidation of dry CH4 using Ni-ceria cermet anodes at temperatures from

550 to 650 °C is achievable [100]. The addition of barrier layer of catalyst layer onto the anode

is intended to control the gas composition that will reach the active anode TPB zone. Such

catalytic layer would function to reform hydrocarbon into H2 or CO that is relatively easier to

undergo oxidation reaction.

The other way to prohibit coking problem is by using a Ni-free anode. The substitute must be

catalytically active for the oxidation of hydrocarbons, and inactive for cracking reactions that

form carbon. Other transition metals, for instance, iron (Fe) and cobalt (Co) have unfortunately

similar carbon deposition issue [101]. In contrast, copper (Cu) is a suitable candidate as it is a

poor catalyst for carbon formation and stable at regular oxygen partial pressure. With Cu-based

anode, carbon deposition could be potentially minimised, and this has been reported in several

works [101-104]. Unlike other metals such as Fe, Co and Ni, the mechanism for carbon

formation with Cu is different, as the formed graphite layer on the Cu cermet can be eliminated

by scraping [103]. Gorte and co-workers [14, 101-103] have successfully synthesised and

showed that Cu-based anodes could be applied in direct utilisation of different hydrocarbon

fuels with insignificant carbon deposits. Carbon formation is inhibited since copper is a poor

Chapter 2

62

catalyst for C–C and C–H bond activation. In these composite anodes, the copper mainly

provides electronic conductivity. It is, however, have a low catalytic activity for hydrocarbon

oxidation and thus, an additional oxidation catalyst such as ceria (CeO2) is added into the anode

formulation, improving the catalytic activity for fuel oxidation [14]. The mixed ionic and

electronic conductivity (MIEC) property of ceria helps extend the TPB region for a better anode

performance [20]. Moreover, better performance with carbonaceous fuels (i.e. CO and syngas

fuel) has been described using cells with Cu/CeO2/YSZ anode than Ni-YSZ anode [104].

Unfortunately, copper and its oxides have relatively low melting points, making the fabrication

of Cu-based anodes through standard high-temperature ceramic processing techniques (i.e. tape

casting and sintering) inappropriate. Instead, a cell with Cu-based anode can be prepared first

by creating the porous YSZ matrix via dual-tape-casting methods followed by the impregnation

of Cu and Ceria using their nitrate solutions, followed by decomposition and reduction

processes to form Cu/CeO2/YSZ composites [14]. Due to thermal stability issues, operation for

cells with Cu-based at 800 °C appears to be relatively high for the operation of SOFCs with

Cu-based anode, thus, it is suggested that the operating temperature to be ≤750 °C.

Various cermet formulations such as Cu–CeO2, Cu-GDC, Ni-GDC, and Cu-Ni-GDC, each

with the ability to minimise carbon formation and sulfur poisoning, have been advocated [105].

Cobalt (Co) also displays a high electrical conductivity and electrochemical activity. With

humidified CH4, cells with anode based on Cu-Co mixtures showed better performance and

stability than one derived from Cu or Co alone [106]. An anode consisting 50:50 weight ratio

of Cu and Co survived a long-term stable operation at 800 °C. The incorporation of Co to Cu-

ceria-YSZ anode resulted in better cell performance for a system fed with CO than that of H2

[104]. Furthermore, cells with Cu-Co(Ru)/ZDC anodes displayed comparable performance in

H2 and methanol, whereas with ethanol as fuel the performance, although good in the

beginning, then decreased exponentially [107]. Compared to pure palladium (Pd) anode, Cu-

Chapter 2

63

Pd alloys impregnated into SOFC anodes presented comparable electrochemical performance

or even better carbon tolerance [108, 109]. The addition of small amount of platinum (Pt) [110],

palladium (Pd) [108, 110, 111], ruthenium (Ru) [112], and rhodium (Rh) [110] increased the

catalytic activity of Cu-ceria-YSZ anodes. The incorporation of Rh to the anode cermet showed

substantial improvement in the cell performance with methane as fuel [113]. All of these

precious-metal dopants contributed to greater cell performance for direct utilisation of different

hydrocarbons during operation at 700 °C [110, 111]. Pd/ceria, Pt/ceria and Rh functions as an

excellent catalyst for steam reforming [114]. Many review articles are discussing about anode

materials selection in the literature where more information could be obtained [16, 18, 19, 115,

116].

2.6 Solid oxide electrochemical reactor (SOER)

Figure 2.14: Operating principle of reversible SOFC-SOEC process. Reproduced with


Solid oxide electrochemical reactor (SOER) is an energy conversion device that can be

operated reversibly [117-119]. This technology called reversible (R)-SOER can integrate

generation of electricity during fuel cell mode (SOFC) and fuel production in electrolysis mode

(SOEC). Fundamentally, a solid oxide electrolysis cell (SOEC) is the corresponding solid oxide

Chapter 2

64

fuel cell (SOFC) that work in “reverse” mode. Figure 2.14 shows the operating principle for

the R-SOER (SOFC-SOEC).

During SOEC operation, such a cell works at relatively high temperatures (700-1000 °C), and

uses energy/electricity to split, e.g., steam (H2O) into hydrogen (H2) and oxygen (O2), or carbon

dioxide (CO2) to carbon monoxide (CO) and O2. If steam and CO2 is electrolysed

simultaneously (co-electrolysis operation), a syngas (gas mixture of H2 and CO) is produced

[120]. This syngas can be the starting product for various syntheses of hydrocarbons in the

chemical industry [121].

SOECs can be used to produce fuels (e.g. H2, CO or syngas) from surplus electricity generated

by renewable energies such as solar or wind turbines. The fuel can be stored and reconverted

into electricity under fuel cell mode when the demand increases. This allows the storage of

electricity when production exceeds demand. During electrolysis, the fuel electrode (cathode

in SOEC mode) reactions depend on the feed stream. The reactions for steam (H2O) electrolysis

and carbon dioxide (CO2) electrolysis [122] can be represented as follows:

Fuel electrode

(e.g. Ni-YSZ)

: H2O + 2e− → H2 + O2− (2.16)

CO2 + 2e− → CO + O2− (2.17)

Electrolyte

(e.g. YSZ)

: O2−(fuel electrode) → O2−(oxygen electrode) (2.18)

Oxygen electrode

(e.g. LSM-YZ)

: 12⁄ (2O2− → O2 + 4e−) (2.19)

Overall : H2O → H2 + 12⁄ O2 (2.20)

CO2 → CO + 12⁄ O2 (2.21)

Chapter 2

65

It is worth mentioning that fuel electrode is termed as “cathode” in SOEC mode and “anode”

in SOFC mode.

Figure 2.15: Total energy demand (∆H), electric energy demand (∆G) and heat demand (T∆S)

as a function of temperature for (a) steam electrolysis and (b) CO2 electrolysis. Reproduced

with permission from [122].

The energy demand for electrolysis reaction is the enthalpy change (ΔH) [119] is represented

by:

∆𝐻 = ∆𝐺 + 𝑇∆𝑆 (2.22)

Where ∆G is Gibb’s free energy or the electrical energy demand, ∆S is the entropy change, and

T is temperature. Figure 2.15 shows the energy demand for both steam and CO2 electrolysis.

The desired operational cell voltage during electrolysis is the point upon which no net heat is

produced by the reactor, maximising energy conversion efficiency. This is called thermo-

neutral voltage (Vtn), [122] and can be represented as below:

𝑉𝑡𝑛 = −∆𝐻

𝑛𝑒𝐹

(2.23)

Chapter 2

66

Where ne represents the electron stoichiometry of the overall cell reaction, and F is the Faraday

constant. At 750 °C, the thermo-neutral voltage (Vtn) is 1.285 V for steam electrolysis and 1.464

V for CO2 electrolysis [119, 122].

2.6.1 Steam (H2O) electrolysis

Most works on SOEC are directed on steam (H2O) electrolysis which generates hydrogen (H2)

[123]. As the cell used for electrolysis could be the same cell applied in fuel cell (SOFC) mode,

the material selections for SOEC is essentially similar [123]. Cell with conventional fuel

electrode materials, Ni-cermets such as Ni-YSZ, Ni-GDC etc, have been often applied for

steam electrolysis studies [120, 123-125]. For instance, button cell (Ni-YSZ/YSZ/LSM-YSZ)

was tested in steam electrolysis with 70-30% H2O-H2 [120]. Substantial performance

degradation was detected where the cell degraded over 20% after 200 hours operation. In their

follow-up study [125], further improvement in terms of cell stability has been achieved (e.g.

degradation rate of 1 mV h−1) by using Ni-GDC-supported cell and bi-layer electrolyte (Ni-

GDC/GDC/YSZ/LSM-YSZ).

Jensen et al. [126] tested Ni-YSZ/(10µm) YSZ/YSZ-LSM cell which reached current densities

of up to −3.6 A cm−2 at 1.48 V and 950 °C. This remains as one of the best performances for

steam electrolysis using conventional SOE material.

Another long-term test has been demonstrated by Schefold et al [127] using Ni-

YSZ/YSZ/GDC/LSCF cell for steam electrolysis at 780 °C and −1 A cm−2 for 9000 hours. The

input gas had an absolute humidity of 80 vol. % and a steam utilisation of 36%. More recent

work by the same group [128] using an electrolyte-supported cell consisting Ni-

GDC/6Sc1CeSZ/GDC/LSCF for steam electrolysis has been reported for 23,000 hours. Post-

test analysis showed that the cell had no mechanical damage at electrolyte and fuel electrode

and only a small part of the oxygen electrode was delaminated. Such excellent performance

Chapter 2

67

has shown a prospect for SOEC made from common materials to be applied for steam

electrolysis.

Cells comprising Ni-YSZ/ScSZ/GDC-LSCF have been studied for steam electrolysis with and

without methane (CH4) supplied to the oxygen-electrode [129]. It was found that at 1.3 V and

850 °C, the H2 production rates were 4.53 and 1.74 cm3 min−1 (SCCM) for the system supplied

with CH4/steam and air/steam, respectively. Such finding showed a performance enhancement

could be realised by feeding CH4 to the oxygen electrode side.

A novel approach using a composite of (La,Sr)(Cr,Mn)O3/(Gd,Ce)O2 (LSCM/GDC) as a

cathode in a high temperature SOEC has been investigated for steam electrolysis and steam-

CO2 co-electrolysis [130]. The authors concluded that LSCM could be a promising substitute

to conventional Ni-YSZ and efforts in refining electrode micro-structure and current collection

is still on-going. In recent years, the use of proton conducting oxides has attracted a

considerable attention among researchers to develop electrolysis cells operating at intermediate

temperatures (500 to 700 °C) [90, 131]. These materials are recommended to resolve some

drawbacks of conventional oxide-ion conductors such as high operating temperature and the

requirement for H2 purification from the effluent. Barium zirconate cerates Ba(Zr,Ce)O3 have

been studied to combine the benefit of zirconate excellent stability and the higher grain

boundary conduction of the cerate. However, poor sinterability and high grain-boundary

resistance of BaZrO3 as well as poor stability of BaCeO3 in CO2 and H2O have restricted their

use for SOEC/SOFC applications [6]. Efforts in improving such materials’ properties are still

on-going. Reader are referred to several excellent reviews for additional information regarding

progress in this field [90, 132].

Chapter 2

68

2.6.2 CO2 electrolysis

In comparison to steam electrolysis, high temperature CO2 electrolysis using SOEC has been

less often investigated [133]. CO2 electrolysis energetic demand is typically higher than steam

electrolysis [122, 133]. In the late 1960s, CO2 electrolysis was demonstrated by NASA aimed

to produce O2 for life supports and propulsion in submarines and spacecraft [134]. To date,

there have been several works reported on electrochemical reduction of CO2 to CO using

SOECs [135-138].

Ebbesen and Mogensen [138] tested planar Ni-YSZ-supported cells under various CO2-CO gas

compositions at 850 °C. Key finding from this work has shown that long-term CO2 electrolysis

is feasible using SOEC with Ni-electrode provided suitable operating conditions were applied.

They found that area specific resistances (ASRs) of 0.36 Ω cm2 and 0.37 Ω cm2 were measured

for 50:50% CO2:CO and 70:30% CO2:CO, respectively when operating in electrolysis mode.

In contrast, ASRs reduced to 0.30 Ω cm2 and of 0.31 Ω cm2 for 50:50% CO2:CO and 70:30%

CO2:CO respectively, when operating in fuel cell mode, indicating a higher activity for

oxidation of CO compared to CO2 electrochemical reduction using the same cells.

Kim-Lohsoontorn and co-workers [125] investigated several Ni-GDC-supported cells with

different electrolytes (e.g. GDC, YSZ and bi-layer GDC/YSZ) at 800 °C using different CO2-

CO compositions. The bi-layered (YSZ/GDC) electrolyte cell showed considerably greater

performance than the cell with either YSZ or GDC electrolyte. The same authors [120] has

described that the polarisation resistance was greater with more CO2 content in the gas feed

(i.e. 70:30% CO2:CO > 50:50% CO2:CO) and became more apparent when feeding pure CO2

to the fuel electrode.

Zhan et al. [139] describes that the lower diffusivity of the higher molecular weight CO2

through the porous cathode caused larger concentration polarisation in a cell with a higher CO2

Chapter 2

69

content in a feed mixture. Therefore, improvement in the cathode micro-structure would be

useful in reducing polarisation due to mass transport restrictions. Recent study [140] using Ni-

YSZ/YSZ/GDC/LSCF cells also reported that CO2 electrolysis performance was largely

influenced by gas diffusion, which could be associated with the cathode micro-structure. Ni-

YSZ cathode with higher porosity prepared by adding more pore former during cathode support

fabrication resulted in lower gas resistance and eventually gave better cell performance.

It has been highlighted that during CO2 electrolysis, one major concern is that the gas feed

composition need to be outside of carbon formation regime, i.e. low CO content [119, 138].

Thus, high CO2-CO ratio in the gas feed composition is necessary to ensure such problem will

be minimised.

2.6.3 Steam and CO2 co-electrolysis.

Co-electrolysis CO2 and steam offers the prospect to generate syngas (a mixture of CO and

H2), which would be advantageous for producing synthetic liquid fuels through Fischer-

Tropsch process or in-situ methanation within the electrolyser [141]. Similar consideration for

both electrode and electrolyte materials for a cell apply as for separate CO2 and H2O

electrolysis. It is often reported that pure H2O electrolysis outperformed pure CO2 electrolysis

due to higher cathodic overpotentials for the latter process [125]. Co-electrolysis performance

has been described largely to be identical to steam electrolysis alone or in between H2O and

CO2 electrolysis. In addition, some ambiguity exists about the extent of direct CO2 electrolysis

and chemical reaction of CO2 with electrochemically produced H2 through the (reverse)-water

gas shift reaction (RWGSR) in co-electrolysis mode [119].

Stoots et al. [142] suggested that steam electrolysis and the RWGSR were the only reactions

which took place within this process; the hydrogen produced from steam electrolysis is thought

to be involved for the reduction of CO2 to CO via the RWGSR. The authors [142] proposed

Chapter 2

70

this reaction mechanism having observed a similar ASR for H2O electrolysis and H2O-CO2 co-

electrolysis. Furthermore, the ASR for CO2 electrolysis was observed to be considerably higher

than H2O electrolysis and H2O-CO2 co-electrolysis. Ebbesen et al. [143] also suggested that

“part of or probably most of the CO produced results from the RWGSR” because RWGSR

equilibrium favours the chemical reaction at 850 °C. On the other hand, other works [144, 145]

have claimed otherwise, that CO is generated both from the RWGSR and the electrochemical

reaction during co-electrolysis of CO2 and steam.

As mentioned above, there is on-going debate about the role of the RWGS during co-

electrolysis, with some researchers tend to believe that; during co-electrolysis, only steam

electrolysis is taking place, whereas CO2 is only being converted to CO by the RWGS as CO2

electrolysis is much slower than steam electrolysis. With regards to CO2 conversion, the

H2O/CO2 co-electrolysis using SOECs for syngas production is advantageous for producing

fuels or chemicals. However, it is noteworthy that steam- and co-electrolysis is beyond the

scope of this thesis and further information regarding these issues could be found in the

literature [132, 133].

In general, compared to the low temperature alkaline electrolysers and PEM based

electrolysers, SOEC has the advantages of efficiency and compactness, along with the ability

to produce synthesis gas directly. At high temperature, the electric energy demand decreases

while the heat energy demand increases [122]. This heat could be provided by solar heat or

from exothermal chemical reaction heat in the SOEC stack. Besides, high operating

temperature could also lead to a substantial decrease in the ohmic resistance and acceleration

of the electrode reaction processes. Although the use of R-SOER (SOEC-SOFC) presents a

wide range of potential uses, they are still under continuing development. Several critical issues

must be first addressed which includes electrodes performances and stability, delamination

Chapter 2

71

(common problem for oxygen electrode during long-term electrolysis operation) and matters

related to large-scale production and cell reproducibility.

2.7 Conclusion

In recent years, many studies on SOFCs have been intended to obtain not only high-

performance cell but also improved structural design. Newer micro-tubular design has

appeared to be a strong contender to its conventional tubular pioneer with additional benefits

of having greater thermal resistance, larger volumetric power density, rapid start-up and shut-

down as well as good portable characteristics. Micro-tubes fabricated using phase-inversion

process present unique asymmetric structure which is useful for the development of micro-

tubular (MT)-SOFC. Such process has greater flexibility in tailoring micro-structure of the

micro-tube by adjusting the fabrication parameters and suspension properties. Earlier works

demonstrated in our lab for the preparation of ceramic micro-tubes through phase-inversion

process have shown the success in constructing high-performance MT-SOFC [57-60]. This

could be further explored by fine-tuning the micro-tube fabrication process to obtain more

refine structural design for better SOFC performance. The fabrication of multi-channel micro-

tube or known as micro-monolithic structure has been proposed to increase mechanical

robustness. Earlier proof-of-concept study showed that such micro-monolithic structure could

withstand fracture load several times better than the single-channel design [80]. In addition,

the capability of solid oxide electrochemical reactor to be applied reversibly has attracted

significant interest for further investigation. Such reversible operation allows the same cell to

be applied under both fuel cell (SOFC mode) and electrolysis (SOEC mode) could be of great

advantage for SOER to be commercially feasible.

Chapter 2

72

2.8 References

1. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups

I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate

Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva,

Switzerland, 151 pp.

2. Mekhilef, S., R. Saidur, and A. Safari, Comparative study of different fuel cell

technologies. Renewable and Sustainable Energy Reviews, 2012. 16(1): p. 981-989.

3. Andújar, J.M. and F. Segura, Fuel cells: History and updating. A walk along two

centuries. Renewable and Sustainable Energy Reviews, 2009. 13(9): p. 2309-2322.

4. Kirubakaran, et. al, A review on fuel cell technologies and power electronic interface.

Renewable and Sustainable Energy Reviews, 2009. 13(9): p. 2430-2440.

5. Wilberforce, T., et al., Advances in stationary and portable fuel cell applications.

International Journal of Hydrogen Energy, 2016. 41(37): p. 16509-16522.

6. Fabbri, E., D. Pergolesi, and E. Traversa, Materials challenges toward proton-

conducting oxide fuel cells: a critical review. Chemical Society Reviews, 2010. 39(11):

p. 4355-4369.

7. Abdalla, A.M., et al., Nanomaterials for solid oxide fuel cells: A review. Renewable

and Sustainable Energy Reviews, 2018. 82: p. 353-368.

8. Elmer, T., et al., Fuel cell technology for domestic built environment applications: State

of-the-art review. Renewable and Sustainable Energy Reviews, 2015. 42: p. 913-931.

9. Stambouli, A.B. and E. Traversa, Solid oxide fuel cells (SOFCs): a review of an

environmentally clean and efficient source of energy. Renewable and Sustainable

Energy Reviews, 2002. 6(5): p. 433-455.

10. Ellamla, H.R., et al., Current status of fuel cell based combined heat and power systems

for residential sector. Journal of Power Sources, 2015. 293: p. 312-328.

Chapter 2

73

11. Howe, K.S., G.J. Thompson, and K. Kendall, Micro-tubular solid oxide fuel cells and

stacks. Journal of Power Sources, 2011. 196(4): p. 1677-1686.

12. Lawlor, V., Review of the micro-tubular solid oxide fuel cell (Part II: Cell design issues

and research activities). Journal of Power Sources, 2013. 240: p. 421-441.

13. Armor, J.N., The multiple roles for catalysis in the production of H2. Applied Catalysis

A: General, 1999. 176(2): p. 159-176.

14. McIntosh, S. and R.J. Gorte, Direct Hydrocarbon Solid Oxide Fuel Cells. Chemical

Reviews, 2004. 104(10): p. 4845-4866.

15. Ge, X.-M., et al., Solid Oxide Fuel Cell Anode Materials for Direct Hydrocarbon

Utilization. Advanced Energy Materials, 2012. 2(10): p. 1156-1181.

16. Mahato, N., et al., Progress in material selection for solid oxide fuel cell technology: A

review. Progress in Materials Science, 2015. 72: p. 141-337.

17. Winkler, W., Chapter 3 - Thermodynamics, in High Temperature and Solid Oxide Fuel

Cells, S.C. Singhal and K. Kendall, Editors. 2003, Elsevier Science: Amsterdam. p. 53-

82.

18. Cowin, P.I., et al., Recent Progress in the Development of Anode Materials for Solid

Oxide Fuel Cells. Advanced Energy Materials, 2011. 1(3): p. 314-332.

19. Fergus, J.W., Oxide anode materials for solid oxide fuel cells. Solid State Ionics, 2006.

177(17–18): p. 1529-1541.

20. Sun, C. and U. Stimming, Recent anode advances in solid oxide fuel cells. Journal of

Power Sources, 2007. 171(2): p. 247-260.

21. Shri Prakash, B., S. Senthil Kumar, and S.T. Aruna, Properties and development of

Ni/YSZ as an anode material in solid oxide fuel cell: A review. Renewable and

Sustainable Energy Reviews, 2014. 36: p. 149-179.

Chapter 2

74

22. Hanna, J., et al., Fundamentals of electro- and thermochemistry in the anode of solid-

oxide fuel cells with hydrocarbon and syngas fuels. Progress in Energy and Combustion

Science, 2014. 40: p. 74-111.

23. Kee, R.J., H. Zhu, and D.G. Goodwin, Solid-oxide fuel cells with hydrocarbon fuels.

Proceedings of the Combustion Institute, 2005. 30(2): p. 2379-2404.

24. Tavares, A.C., et al., Novel copper-based anodes for solid oxide fuel cells with samaria-

doped ceria electrolyte. Journal of Power Sources, 2008. 183(1): p. 20-25.

25. Tao, S. and J.T.S. Irvine, Discovery and characterization of novel oxide anodes for

solid oxide fuel cells. The Chemical Record, 2004. 4(2): p. 83-95.

26. Fergus, J.W., Electrolytes for solid oxide fuel cells. Journal of Power Sources, 2006.

162(1): p. 30-40.

27. Zha, S., W. Rauch, and M. Liu, Ni-Ce0.9Gd0.1O1.95 anode for GDC electrolyte-based

low-temperature SOFCs. Solid State Ionics, 2004. 166(3): p. 241-250.

28. Li, T., Micro-structured hollow fibers for micro-tubular solid oxide fuel cells, in

Department of Chemical Engineering. 2015, Imperial College London. p. 230.

29. Ahn, J.S., et al., High-performance bilayered electrolyte intermediate temperature

solid oxide fuel cells. Electrochemistry Communications, 2009. 11(7): p. 1504-1507.

30. Wan, J.-H., J.-Q. Yan, and J.B. Goodenough, LSGM-Based Solid Oxide Fuel Cell with

1.4 W/cm2 Power Density and 30 Day Long-Term Stability. Journal of The

Electrochemical Society, 2005. 152(8): p. A1511-A1515.

31. Fabbri, E., et al., Tailoring the chemical stability of Ba(Ce0.8−xZrx)Y0.2O3−δ protonic

conductors for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Solid

State Ionics, 2008. 179(15–16): p. 558-564.

32. Duan, C., et al., Readily processed protonic ceramic fuel cells with high performance

at low temperatures. Science, 2015. 349(6254): p. 1321-1326.

Chapter 2

75

33. Duan, C., et al., Highly durable, coking and sulfur tolerant, fuel-flexible protonic

ceramic fuel cells. Nature, 2018. 557(7704): p. 217-222.

34. Sun, C., R. Hui, and J. Roller, Cathode materials for solid oxide fuel cells: a review.

Journal of Solid State Electrochemistry, 2010. 14(7): p. 1125-1144.

35. Ding, D., et al., Enhancing SOFC cathode performance by surface modification

through infiltration. Energy & Environmental Science, 2014. 7(2): p. 552-575.

36. Wu, J. and X. Liu, Recent Development of SOFC Metallic Interconnect. Journal of

Materials Science & Technology, 2010. 26(4): p. 293-305.

37. Yang, Z., Recent advances in metallic interconnects for solid oxide fuel cells.

International Materials Reviews, 2013. 53(1): p. 39-54.

38. Jamil, S.M., et al., Recent fabrication techniques for micro-tubular solid oxide fuel cell

support: A review. Journal of the European Ceramic Society, 2015. 35(1): p. 1-22.

39. Hui, S., et al., Metal-supported solid oxide fuel cell operated at 400–600°C. Journal of

Power Sources, 2007. 167(2): p. 336-339.

40. Nielsen, J., et al., Towards High Power Density Metal Supported Solid Oxide Fuel Cell

for Mobile Applications. Journal of The Electrochemical Society, 2018. 165(2): p. F90-

F96.

41. Lee, K., et al., Evaluation of metal-supported solid oxide fuel cells (MS-SOFCs)

fabricated at low temperature (∼1,000 °C) using wet chemical coating processes and

a catalyst wet impregnation method. International Journal of Hydrogen Energy, 2018.

43(7): p. 3786-3796.

42. Sammes, N.M., Y. Du, and R. Bove, Design and fabrication of a 100 W anode

supported micro-tubular SOFC stack. Journal of Power Sources, 2005. 145(2): p. 428-

434.

Chapter 2

76

43. Lawlor, V., et al., Review of the micro-tubular solid oxide fuel cell: Part I. Stack design

issues and research activities. Journal of Power Sources, 2009. 193(2): p. 387-399.

44. Kendall, K. and M. Palin, A small solid oxide fuel cell demonstrator for microelectronic

applications. Journal of Power Sources, 1998. 71(1–2): p. 268-270.

45. Kendall, K., Progress in Microtubular Solid Oxide Fuel Cells. International Journal of

Applied Ceramic Technology, 2010. 7(1): p. 1-9.

46. Yang, C., C. Jin, and F. Chen, Micro-tubular solid oxide fuel cells fabricated by phase-

inversion method. Electrochemistry Communications, 2010. 12(5): p. 657-660.

47. Funahashi, Y., et al., New Fabrication Technique for Series-Connected Stack With

Micro Tubular SOFCs. Fuel Cells, 2009. 9(5): p. 711-716.

48. Majewski, A.J. and A. Dhir, Direct Utilization of Methane in Microtubular-SOFC. ECS

Transactions, 2015. 68(1): p. 2189-2198.

49. Tompsett, G.A., et al., Novel applications for micro-SOFCs. Journal of Power Sources,

2000. 86(1–2): p. 376-382.

50. Du, Y. and N.M. Sammes, Fabrication of tubular electrolytes for solid oxide fuel cells

using strontium- and magnesium-doped LaGaO3 materials. Journal of the European

Ceramic Society, 2001. 21(6): p. 727-735.

51. Du, Y. and N.M. Sammes, Fabrication and properties of anode-supported tubular solid

oxide fuel cells. Journal of Power Sources, 2004. 136(1): p. 66-71.

52. Suzuki, T., et al., Fabrication and characterization of micro tubular SOFCs for

operation in the intermediate temperature. Journal of Power Sources, 2006. 160(1): p.

73-77.

53. Liang, Z. and S. Blackburn, Design and characterisation of a co-extruder to produce

trilayer ceramic tubes semi-continuously. Journal of the European Ceramic Society,

2001. 21(7): p. 883-892.

Chapter 2

77

54. Loeb, S. and S. Sourirajan, Sea Water Demineralization by Means of an Osmotic

Membrane, in Saline Water Conversion?II. 1963, AMERICAN CHEMICAL

SOCIETY. p. 117-132.

55. Li, K., Preparation of Ceramic Membranes, in Ceramic Membranes for Separation and

Reaction. 2007, John Wiley & Sons, Ltd. p. 21-57.

56. Othman, M.H.D., et al., Single-step fabrication and characterisations of

electrolyte/anode dual-layer hollow fibres for micro-tubular solid oxide fuel cells.

Journal of Membrane Science, 2010. 351(1–2): p. 196-204.

57. Othman, M.H.D., High performance micro-tubular solid oxide fuel cell, in Department

of Chemical Engineering. 2011, Imperial College London. p. 227.

58. Othman, M.H.D., et al., Dual-layer hollow fibres with different anode structures for

micro-tubular solid oxide fuel cells. Journal of Power Sources, 2012. 205: p. 272-280.

59. Li, T., Z. Wu, and K. Li, Co-extrusion of electrolyte/anode functional layer/anode

triple-layer ceramic hollow fibres for micro-tubular solid oxide fuel cells–

electrochemical performance study. Journal of Power Sources, 2015. 273: p. 999-1005.

60. Li, T., Z. Wu, and K. Li, Single-step fabrication and characterisations of triple-layer

ceramic hollow fibres for micro-tubular solid oxide fuel cells (SOFCs). Journal of

Membrane Science, 2014. 449: p. 1-8.

61. Cooper, S.J., Quantifying the transport properties of solid oxide fuel cell electrodes, in

Department of Materials. 2016, Imperial College London: London. p. 204.

62. Wei, C.C. and K. Li, Yttria-Stabilized Zirconia (YSZ)-Based Hollow Fiber Solid Oxide

Fuel Cells. Industrial & Engineering Chemistry Research, 2008. 47(5): p. 1506-1512.

63. Kanawka, K., et al., Microstructure and Performance Investigation of a Solid Oxide

Fuel Cells Based on Highly Asymmetric YSZ Microtubular Electrolytes. Industrial &

Engineering Chemistry Research, 2010. 49(13): p. 6062-6068.

Chapter 2

78

64. Yang, N.-T., et al., Fabrication of Anode Supported Micro Tubular SOFCS by Dip-

Coating Process on NiO/YSZ Hollow Fibers. ECS Transactions, 2009. 25(2): p. 881-

888.

65. Jin, C., et al., Electrochemical properties analysis of tubular NiO–YSZ anode-

supported SOFCs fabricated by the phase-inversion method. Journal of Membrane

Science, 2009. 341(1): p. 233-237.

66. Zhang, X., et al., An anode-supported micro-tubular solid oxide fuel cell with redox

stable composite cathode. International Journal of Hydrogen Energy, 2010. 35(16): p.

8654-8662.

67. Yang, C., C. Jin, and F. Chen, Performances of micro-tubular solid oxide cell with

novel asymmetric porous hydrogen electrode. Electrochimica Acta, 2010. 56(1): p. 80-

84.

68. Droushiotis, N., et al., Phase Inversion and Electrophoretic Deposition Processes for

Fabrication of Micro-Tubular Hollow Fiber SOFCs. Journal of The Electrochemical

Society, 2014. 161(3): p. F271-F279.

69. Sun, W., et al., Fabrication of anode-supported Sc2O3-stabilized-ZrO2 electrolyte

micro-tubular Solid Oxide Fuel Cell by phase-inversion and dip-coating.

Electrochemistry Communications, 2012. 20: p. 117-120.

70. Yang, C., et al., Intermediate temperature micro-tubular SOFCs with enhanced

performance and thermal stability. Electrochemistry Communications, 2013. 34: p.

231-234.

71. Yang, C., et al., High performance intermediate temperature micro-tubular SOFCs

with Ba0.9Co0.7Fe0.2Nb0.1O3−δ as cathode. International Journal of Hydrogen Energy,

2013. 38(35): p. 15348-15353.

Chapter 2

79

72. Zhao, F., et al., Fabrication and characterization of anode-supported micro-tubular

solid oxide fuel cell based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte. Journal of Power

Sources, 2011. 196(2): p. 688-691.

73. Zhao, L., et al., Micro-tubular solid oxide fuel cells with graded anodes fabricated with

a phase inversion method. Journal of Power Sources, 2011. 196(3): p. 962-967.

74. Droushiotis, N., et al., Novel co-extruded electrolyte–anode hollow fibres for solid

oxide fuel cells. Electrochemistry Communications, 2009. 11(9): p. 1799-1802.

75. Othman, M.H.D., et al., Electrolyte thickness control and its effect on electrolyte/anode

dual-layer hollow fibres for micro-tubular solid oxide fuel cells. Journal of Membrane

Science, 2010. 365(1–2): p. 382-388.

76. Othman, M.H.D., et al., High-Performance, Anode-Supported, Microtubular SOFC

Prepared from Single-Step-Fabricated, Dual-Layer Hollow Fibers. Advanced

Materials, 2011. 23(21): p. 2480-2483.

77. He, B., et al., Effect of nickel impregnated hollow fiber anode for micro tubular solid

oxide fuel cells. Journal of Power Sources, 2014. 258: p. 391-394.

78. Li, T., Z. Wu, and K. Li, High-efficiency, nickel-ceramic composite anode current

collector for micro-tubular solid oxide fuel cells. Journal of Power Sources, 2015. 280:

p. 446-452.

79. Jamil, S.M., et al., Dual-layer hollow fiber MT-SOFC using lithium doped CGO

electrolyte fabricated via phase-inversion technique. Solid State Ionics, 2017. 304: p.

113-125.

80. Meng, X., et al., Fabrication of Y2O3-stabilized-ZrO2(YSZ)/La0.8Sr0.2MnO3−α–YSZ

dual-layer hollow fibers for the cathode-supported micro-tubular solid oxide fuel cells

by a co-spinning/co-sintering technique. Journal of Power Sources, 2013. 237: p. 277-

284.

Chapter 2

80

81. Lee, M., et al., Micro-structured alumina multi-channel capillary tubes and monoliths.

Journal of Membrane Science, 2015. 489: p. 64-72.

82. Wu, Z., et al., A controlled sintering process for more permeable ceramic hollow fibre

membranes. Journal of Membrane Science, 2013. 446: p. 286-293.

83. Wang, B. and Z. Lai, Finger-like voids induced by viscous fingering during phase

inversion of alumina/PES/NMP suspensions. Journal of Membrane Science, 2012. 405–

406: p. 275-283.

84. Paiman, S.H., et al., Morphological study of yttria-stabilized zirconia hollow fibre

membrane prepared using phase inversion/sintering technique. Ceramics International,

2015. 41(10): p. 12543-12553.

85. Kingsbury, B.F.K. and K. Li, A morphological study of ceramic hollow fibre

membranes. Journal of Membrane Science, 2009. 328(1–2): p. 134-140.

86. Lee, M., et al., Formation of micro-channels in ceramic membranes – Spatial structure,

simulation, and potential use in water treatment. Journal of Membrane Science, 2015.

483: p. 1-14.

87. Lee, M., B. Wang, and K. Li, New designs of ceramic hollow fibres toward broadened

applications. Journal of Membrane Science, 2016. 503: p. 48-58.

88. Vohs, J.M. and R.J. Gorte, High-Performance SOFC Cathodes Prepared by

Infiltration. Advanced Materials, 2009. 21(9): p. 943-956.

89. Tucker, M.C., Progress in metal-supported solid oxide fuel cells: A review. Journal of

Power Sources, 2010. 195(15): p. 4570-4582.

90. Hossain, S., et al., A review on proton conducting electrolytes for clean energy and

intermediate temperature-solid oxide fuel cells. Renewable and Sustainable Energy

Reviews, 2017. 79: p. 750-764.

Chapter 2

81

91. Medvedev, D.A., et al., Advanced materials for SOFC application: Strategies for the

development of highly conductive and stable solid oxide proton electrolytes. Progress

in Materials Science, 2016. 75: p. 38-79.

92. Fabbri, E., et al., Towards the Next Generation of Solid Oxide Fuel Cells Operating

Below 600 °C with Chemically Stable Proton-Conducting Electrolytes. Advanced

Materials, 2012. 24(2): p. 195-208.

93. Dokmaingam, P., et al., Modeling of IT-SOFC with indirect internal reforming

operation fueled by methane: Effect of oxygen adding as autothermal reforming.


94. Aguiar, P., et al., Modelling of an indirect internal reforming solid oxide fuel cell.

Chemical Engineering Science, 2002. 57(10): p. 1665-1677.

95. Zha, S., et al., Sulfur Poisoning and Regeneration of Ni-Based Anodes in Solid Oxide

Fuel Cells. Journal of The Electrochemical Society, 2007. 154(2): p. B201-B206.

96. Niakolas, D.K., Sulfur poisoning of Ni-based anodes for Solid Oxide Fuel Cells in H/C-

based fuels. Applied Catalysis A: General, 2014. 486: p. 123-142.

97. Aguiar, P., C.S. Adjiman, and N.P. Brandon, Anode-supported intermediate

temperature direct internal reforming solid oxide fuel cell. I: model-based steady-state

performance. Journal of Power Sources, 2004. 138(1–2): p. 120-136.

98. Pillai, M., et al., Stability and coking of direct-methane solid oxide fuel cells: Effect of

CO2 and air additions. Journal of Power Sources, 2010. 195(1): p. 271-279.

99. Liu, J. and S.A. Barnett, Operation of anode-supported solid oxide fuel cells on

methane and natural gas. Solid State Ionics, 2003. 158(1–2): p. 11-16.

100. Murray, E.P., T. Tsai, and S.A. Barnett, A direct-methane fuel cell with a ceria-based

anode. Nature, 1999. 400(6745): p. 649-651.

Chapter 2

82

101. Gorte, R., J. Vohs, and S. Park, Direct oxidation of hydrocarbons in a solid-oxide fuel

cell. Nature, 2000. 404: p. 265-267.

102. Gorte, R.J., et al., Anodes for Direct Oxidation of Dry Hydrocarbons in a Solid-Oxide

Fuel Cell. Advanced Materials, 2000. 12(19): p. 1465-1469.

103. Gorte, R., J. Vohs, and H. Kim, Novel SOFC anodes for the direct electrochemical

oxidation of hydrocarbon. Journal of Power Sources, 2002. 106(1-2): p. 10-15.

104. Costa-Nunes, O., R.J. Gorte, and J.M. Vohs, Comparison of the performance of Cu–

CeO2–YSZ and Ni–YSZ composite SOFC anodes with H2, CO, and syngas. Journal of

Power Sources, 2005. 141(2): p. 241-249.

105. Shaikh, S.P.S., et al., A review on the selection of anode materials for solid-oxide fuel

cells. Renewable and Sustainable Energy Reviews, 2015. 51: p. 1-8.

106. Lee, S.-I., J.M. Vohs, and R.J. Gorte, A Study of SOFC Anodes Based on Cu-Ni and

Cu-Co Bimetallics in CeO2-YSZ. Journal of The Electrochemical Society, 2004. 151(9):

p. A1319.

107. Cimenti, M. and J.M. Hill, Direct utilization of methanol and ethanol in solid oxide fuel

cells using Cu–Co(Ru)/Zr0.35Ce0.65O2−δ anodes. Journal of Power Sources, 2010.

195(13): p. 3996-4001.

108. Bi, Z.H. and J.H. Zhu, Cu1−xPdx/CeO2-impregnated cermet anodes for direct oxidation

of methane in LaGaO3-electrolyte solid oxide fuel cells. Journal of Power Sources,

2010. 195(10): p. 3097-3104.

109. Zhang, J.-T., et al., Enhanced electrochemical performance of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ

electrodes for hydrogen and methane oxidation in solid oxide fuel cells by Pd or

Cu0.5Pd0.5 impregnation. Journal of Power Sources, 2012. 200: p. 29-33.

Chapter 2

83

110. McIntosh, S., J.M. Vohs, and R.J. Gorte, Effect of Precious-Metal Dopants on SOFC

Anodes for Direct Utilization of Hydrocarbons. Electrochemical and Solid-State

Letters, 2003. 6(11): p. A240-A243.

111. Costa-Nunes, O., J.M. Vohs, and R.J. Gorte, A Study of Direct-Conversion SOFC with

n-Butane at Higher Fuel Utilization. Journal of The Electrochemical Society, 2003.

150(7): p. A858-A863.

112. Cimenti, M. and J.M. Hill, Direct utilization of ethanol on ceria-based anodes for solid

oxide fuel cells. Asia-Pacific Journal of Chemical Engineering, 2009. 4(1): p. 45-54.

113. Putna, E.S., et al., Ceria-Based Anodes for the Direct Oxidation of Methane in Solid

Oxide Fuel Cells. Langmuir, 1995. 11(12): p. 4832-4837.

114. Wang, X. and R.J. Gorte, A study of steam reforming of hydrocarbon fuels on Pd/ceria.

Applied Catalysis A: General, 2002. 224(1–2): p. 209-218.

115. Zhu, W.Z. and S.C. Deevi, A review on the status of anode materials for solid oxide

fuel cells. Materials Science and Engineering a-Structural Materials Properties

Microstructure and Processing, 2003. 362(1-2): p. 228-239.

116. Jiang, S. and S. Chan, A review of anode materials development in solid oxide fuel cells.

Journal of Materials Science, 2004. 39(14): p. 4405-4439.

117. Gómez, S.Y. and D. Hotza, Current developments in reversible solid oxide fuel cells.

Renewable and Sustainable Energy Reviews, 2016. 61: p. 155-174.

118. Minh, N.Q. and M.B. Mogensen, Reversible Solid Oxide Fuel Cell Technology for

Green Fuel and Power Production. The Electrochemical Society Interface, 2013.

22(4): p. 55-62.

119. Kleiminger, L., Solid oxide electrochemical reactors and processes for carbon dioxide

and water splitting, in Department of Chemical Engineering. 2015, Imperial College

London. p. 381.

Chapter 2

84

120. Kim-Lohsoontorn, P. and J. Bae, Electrochemical performance of solid oxide

electrolysis cell electrodes under high-temperature coelectrolysis of steam and carbon

dioxide. Journal of Power Sources, 2011. 196(17): p. 7161-7168.

121. Kleiminger, L., et al., Syngas (CO-H2) production using high temperature micro-

tubular solid oxide electrolysers. Electrochimica Acta, 2015. 179: p. 565-577.

122. Hansen, J.B., Solid oxide electrolysis – a key enabling technology for sustainable

energy scenarios. Faraday Discussions, 2015. 182(0): p. 9-48.

123. Laguna-Bercero, M.A., Recent advances in high temperature electrolysis using solid

oxide fuel cells: A review. Journal of Power Sources, 2012. 203: p. 4-16.

124. Ferrero, D., et al., Reversible operation of solid oxide cells under electrolysis and fuel

cell modes: Experimental study and model validation. Chemical Engineering Journal,

2015. 274: p. 143-155.

125. Kim-Lohsoontorn, P., N. Laosiripojana, and J. Bae, Performance of solid oxide

electrolysis cell having bi-layered electrolyte during steam electrolysis and carbon

dioxide electrolysis. Current Applied Physics, 2011. 11(1, Supplement): p. S223-S228.

126. Jensen, S.H., P.H. Larsen, and M. Mogensen, Hydrogen and synthetic fuel production

from renewable energy sources. International Journal of Hydrogen Energy, 2007.

32(15): p. 3253-3257.

127. Schefold, J., et al., Nine Thousand Hours of Operation of a Solid Oxide Cell in Steam

Electrolysis Mode. Journal of The Electrochemical Society, 2011. 159(2): p. A137-

A144.

128. Schefold, J., et al., 23,000 h steam electrolysis with an electrolyte supported solid oxide

cell. International Journal of Hydrogen Energy, 2017. 42(19): p. 13415-13426.

Chapter 2

85

129. Lee, K.-J., M.-J. Lee, and H. Hwang, High-temperature steam electrolysis combined

with methane partial oxidation by solid oxide electrolyzer cells. Applied Surface

Science, 2019. 473: p. 746-749.

130. Yue, X. and J.T.S. Irvine, (La,Sr)(Cr,Mn)O3/GDC cathode for high temperature steam

electrolysis and steam-carbon dioxide co-electrolysis. Solid State Ionics, 2012. 225: p.

131-135.

131. Emiliana, F., P. Daniele, and T. Enrico, Electrode materials: a challenge for the

exploitation of protonic solid oxide fuel cells. Science and Technology of Advanced

Materials, 2010. 11(4): p. 044301.

132. Bi, L., S. Boulfrad, and E. Traversa, Steam electrolysis by solid oxide electrolysis cells

(SOECs) with proton-conducting oxides. Chemical Society Reviews, 2014. 43(24): p.

8255-8270.

133. Zhang, L., et al., Electrochemical reduction of CO2 in solid oxide electrolysis cells.

Journal of Energy Chemistry, 2017. 26(4): p. 593-601.

134. Elikan, L., Morris, J. P., Wu, C. K., Development of a solid electrolyte carbon dioxide

and water reduction system for oxygen recovery. 1972.

135. Zhan, Z. and L. Zhao, Electrochemical reduction of CO2 in solid oxide electrolysis

cells. Journal of Power Sources, 2010. 195(21): p. 7250-7254.

136. Kleiminger, L., et al., CO2 splitting into CO and O2 in micro-tubular solid oxide

electrolysers. RSC Advances, 2014. 4(91): p. 50003-50016.

137. Lu, J., et al., Efficient CO2 electrolysis with scandium doped titanate cathode.


138. Ebbesen, S.D. and M. Mogensen, Electrolysis of carbon dioxide in Solid Oxide

Electrolysis Cells. Journal of Power Sources, 2009. 193(1): p. 349-358.

Chapter 2

86

139. Zhan, Z., et al., Syngas Production By Coelectrolysis of CO2/H2O: The Basis for a

Renewable Energy Cycle. Energy & Fuels, 2009. 23(6): p. 3089-3096.

140. Wang, T., et al., Effect of NiO/YSZ cathode support pore structure on CO2 electrolysis

via solid oxide electrolysis cells. Journal of the European Ceramic Society, 2018.

38(15): p. 5051-5057.

141. Zheng, Y., et al., A review of high temperature co-electrolysis of H2O and CO2 to

produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced

materials and technology. Chemical Society Reviews, 2017. 46(5): p. 1427-1463.

142. Stoots, C.M., et al., Syngas Production via High-Temperature Coelectrolysis of Steam

and Carbon Dioxide. Journal of Fuel Cell Science and Technology, 2008. 6(1): p.

011014-011014-12.

143. Ebbesen, S.D., C. Graves, and M. Mogensen, Production of Synthetic Fuels by Co-

Electrolysis of Steam and Carbon Dioxide. International Journal of Green Energy,

2009. 6(6): p. 646-660.

144. Ni, M., An electrochemical model for syngas production by co-electrolysis of H2O and

CO2. Journal of Power Sources, 2012. 202: p. 209-216.

145. Ebbesen, S.D., R. Knibbe, and M. Mogensen, Co-Electrolysis of Steam and Carbon

Dioxide in Solid Oxide Cells. Journal of The Electrochemical Society, 2012. 159(8): p.

F482-F489.

Chapter 3

87

3 CHAPTER 3

Fabrication of micro-structured NiO-YSZ anode for micro-tubular solid oxide fuel cell

(MT-SOFC)

Chapter 3 describes the development of MT-SOFC using a common anode material, NiO-YSZ

cermet. The aim for this work is to introduce the benefit of hierarchical porous materials applied

to electrode design. A single-channel NiO-YSZ anode design with long micro-channels and

larger entrance has not been reported before. Such structure is obtained based on modified-

phase-inversion process using solvent-based bore fluid and the cell developed from this new

anode design has shown improved performance in hydrogen (H2)-fuelled system.

This chapter is partially based on author’s publication; Lu, X., et al., The application of

hierarchical structures in energy devices: new insights into the design of solid oxide fuel cells

with enhanced mass transport. Energy & Environmental Science, 2018. 11(9): p. 2390-2403.

DOI: 10.1039/C8EE01064A (Open access)

Abstract

The anode-supported solid oxide fuel cell (SOFC) has emerged as the standard cell

configuration owing to the use of thin electrolyte which lowers ohmic resistance (RΩ) and

eventually led to high electrochemical performance. Flexibility in structural design and

morphology tailoring during the phase-inversion process shows great promise for further

optimising an anode substrate utilising common materials for the development of micro-tubular

(MT)-SOFC. Herein, an anode micro-tube with long micro-channels and an open channel

entrance has been fabricated using solvent-based bore fluid whereby its performance was

compared with the conventional micro-tube design. This new anode structure gave superior

electrochemical performance, reaching 16-32 % higher power density than that achieved by its

Chapter 3

88

conventional design counterpart. This study has demonstrated that high-performance MT-

SOFC made of standard materials could be realised through micro-structural optimisation.

3.1 Introduction

The fuel cell technology has appeared as an encouraging way to generate energy by directly

converting the chemical energy in fuel into electric power through an electrochemical reaction

[1]. Among different types of fuel cells, solid oxide fuel cell (SOFC) have attracted

considerable attention because of its use of a solid-state electrolyte, which resolves problems

such as corrosion and electrode wetting which are often encountered in polymer exchange

membrane fuel cell (PEMFC). SOFC also have the benefits of fuel flexibility, low emission of

pollutants and quiet operation, since there is no vibration or moving parts in the system. For

the past few decades, there have been significant development in materials for making SOFC

[2-4], however, commercial SOFC products are limited exclusively to traditional Ni-cermet

anode. Its high temperature requirement restricting the number of suitable materials which are

economical and stable for long-term operation. With this constraint, one direct approach to

realise high-performance SOFC is by optimising cell geometry, associated with micro-

structural improvement [5, 6]. The micro-structure of a cell plays an important role in the

transport of gases to and from the triple phase boundary (TPB) region and influences the

concentration polarisation [7]. To date, most frequently used cell geometries are flat and

tubular. Alternatively, a newer cell design, the micro-tubular (MT)-SOFC, was introduced in

the 1990s [8]. Such design offers additional advantages which include smaller sealing area,

better thermal resistance, suitable for rapid start-up or shut-down and higher power density as

the cell diameter decreases [9, 10].

Most recent SOFC research has involved the use of anode-supported cells incorporating

common materials such as nickel (Ni) and yttria stabilised zirconia (YSZ) [11-13]. This design

appears to be a strong contender for the pioneer electrolyte-supported cells as a thinner

Chapter 3

89

electrolyte ranging between 3 and 30 µm [5, 14-16] could be applied, and may considerably

reduce ohmic loss [17]. Furthermore, there are ongoing efforts to reduce the operating

temperature of SOFCs from 1000 °C to ≤ 800 °C, aimed at better economical aspect and

enhanced reliability. For this purpose, an anode-supported cell with a thin electrolyte appears

to be a favourable option.

One attractive way to produce anode micro-tubes is through phase-inversion process. Such a

process, as opposed to the conventional paste extrusion method, has the benefit of tailoring the

anode micro-structure, in which a highly asymmetric structure can be made [18, 19]. Two

commonly observed structures, sponge-like and micro-channels, have their own functions. The

former contributes to triple phase boundary (TPB) and provides mechanical robustness, while

the latter plays an important role in reducing mass transport resistance. An anode micro-

structure, which is largely determined during the fabrication procedure, has a strong influence

on the performance of an SOFC. For instance, it has been reported that substrate consists of

longer conical-shaped micro-channel gave better performance than the shorter one attributable

to lower concentration polarisation [20].

Although alternatives such as doped-ceria, gallate-based materials and so on have been

proposed as anodes for SOFC, they appear to be rather expensive and not as stable as doped

zirconia [2, 3]. While significant consideration has been made to seek alternate anode materials,

the use of standard material remains attractive from economic point of view with micro-

structural optimisation expected to be beneficial in improving cell performance. Therefore, the

main emphasis in current work is the enhancement of the micro-structural design using Ni-

YSZ cermet, to obtain high performance MT-SOFC. One approach is to minimise the

concentration polarisation of the cell by tailoring the micro-structure of the anode substrate.

For this purpose, such unique phase-inversion technique was applied using a solvent-based

bore fluid. Through this modification, a better micro-structure of the substrate could be

Chapter 3

90

designed for enhanced mass transport and for further advancement of the MT-SOFC system.

Anode design with long and open-channels structure has not been reported before. This novel

anode structure will be developed into a complete single cell and compared with the

conventional micro-tube structure for its electrochemical performances with pure hydrogen

(H2) as a fuel.

3.2 Experimental

3.2.1 Materials

Yttria-stabilised zirconia (8 mol% YSZ, mean particle size 0.1-0.4 μm) and nickel oxide (NiO,

mean particle size 0.5-1.5 μm) were obtained from Inframat Advanced Materials, USA and

used as received. Polyethersulfone (PESf) (Radal A300, Ameco Performance, USA), 30-

dipolyhydroxystearate (Arlacel P135, Uniqema), and N-methyl-2-pyrrolidone (NMP, HPLC

grade, VWR UK) were used as the polymer binder, dispersant, and solvent, respectively.

Polyvinyl alcohol (PVA, M.W. approx. 145000, Merck Schuchardt OHG, Germany) was used

to prepare an aqueous PVA bore fluid. De-ionised (DI) water was used as external coagulant

throughout the micro-tube fabrication process.

3.2.2 Fabrication of micro-tube

NiO-YSZ micro-tubes with varying micro-structures were fabricated through a phase-

inversion-assisted extrusion technique. The dope suspension was firstly prepared using

planetary ball-milling to acquire homogenous dispersion where details on the process has been

described elsewhere [10,12]. Milling of the suspension was carried out over four days using a

planetary miller (SFM-1 Desk-top Miller, MTI Corporation, USA) to assure good

homogeneity. Prior to spinning process (extrusion of the micro-tube precursor), the dope

suspension was degassed for two hours under vacuum by placing the suspension-containing

beaker in a vacuum chamber with constant stirring to remove air bubbles. The set-up for the

degassing process is shown in Figure 3.1.

Chapter 3

91

Figure 3.1: Set-up for the degassing process.

Both dope suspension and bore fluid were transffered into two stainless steel syringes and

extruded through a spinneret produced in-house with a precisely controlled syringe pumps

(PHD 2000 Programmable, HARVARD APPARATUS). Detailed compositions of the

spinning suspensions and fabrication parameters for different anode substrates are listed in

Table 3.1.

Table 3.1: Parameters for NiO-YSZ anode fabrication.

Suspension composition Composition (wt.%)

YSZ 25.2

NiO 37.8

Solvent (NMP) 28.0

Polymer binder (PESf) 8.4

Dispersant (Arlacel P135) 0.6

Electrolyte ink

YSZ powder (~700 nm) 28.2

Chapter 3

92

Ethanol 67.8

Arlacel P135 1.2

Butvar 1.8

Polyethylene glycol 1.0

Fabrication conditions NiO-YSZ (BF-S) NiO-YSZ (BF-P)

Extrusion rate (ml min−1) 8 8

Bore fluid rate (ml min−1) 10 10

Air gap (cm) 0 0

Sintering conditions

Temperature (°C) 1450

Dwelling period (h) 6

The fabrication process is illustrated in Figure 3.2(a). Two different anode structures could be

prepared by applying two different types of bore fluids (BFs): (1) BF-S which stands for

solvent-based (NMP-Ethanol, 60:40 wt.%) and (2) BF-P stands for aqueous solution of

polyvinyl alcohol (10 wt.%), for making the anode substrates designated as NiO-YSZ (BF-S)

and NiO-YSZ (BF-P), respectively. Both dope suspension and bore fluid were extruded

simultaneously through the spinneret (Figure 3.2b) into an external coagulant bath containing

DI water. During the fabrication of both substrate design, a zero air-gap was applied, in which

the spinneret was slightly immersed in the external coagulant bath. The micro-tube precursors

were left immersed in the bath for a day to ensure complete solidification process.

Chapter 3

93

Figure 3.2: (a) Schematic diagram of the fabrication process of anode micro-tubes; (b)

photographic image of spinneret from the bottom and detailed dimensions; (c) as-prepared

micro-tubular SOFCs. Reproduced from Ref. [21] with permission from the Royal Society of

Chemistry.

3.2.3 Fabrication of a complete cell and reactor assembly

Upon complete phase-inversion process, the micro-tube precursor was then straightened and

dried under ambient conditions. Subsequently, YSZ electrolyte layer was added onto the anode

substrate precursor by dip-coating using a home-made electrolyte ink which composition is

listed in Table 3.1. Once dried, the YSZ coated anode substrate underwent a co-sintering

process using a tubular furnace (TSH17/75/450, ELITE) where the furnace was firstly heated

to 600 °C with ramping rate of 2 °C min−1 and dwelled for 2 h to remove the organic polymer

binder, followed by the sintering at target temperature (1450 °C) for 6 h with a ramping rate of

10 °C to ensure proper electrolyte densification. Finally, a dual-layer cathode was dip-coated

Chapter 3

94

onto the co-sintered micro-tube, with the first layer composed of YSZ-LSM mixture (50 wt. %

each) and second layer composed of pure LSM. After both layers were dried, a sintering

process at 1000 °C was undertaken for 2 hours to deliver a complete single cell. Schematic

representation of a single cell is shown in Figure 3.3(a).

Figure 3.3: Schematic representation of (a) a single cell (NiO-YSZ|YSZ|YSZ-LSM/LSM) and

(b) reactor assembly and sealing for a single MT-SOFC. *Not to scale

Both ends of a complete MT-SOFC were fixed into two gas-tight alumina tubes (Almath

Crucibles, UK) with an outer diameter (Do) of 6 mm using a ceramic sealant (Aremco, USA).

Three smaller alumina supporting tubes (3 mm Do) were applied to reinforce the reactor design

for practical operation. The complete assembly for a single MT-SOFC is shown in Figure

3.3(b).

Chapter 3

95

3.2.4 Characterisations

The morphology of the micro-tube was examined using scanning electron microscopy (SEM)

(JEOL JSM-5610). Samples were gold-coated under vacuum at 20 mA for 60 seconds

(EMITECH Model K550), and SEM images with various magnifications were acquired.

The gas-tightness of the electrolyte layer was examined using a nitrogen (N2) gas-leakage test

(N2 permeation test), which has been described elsewhere [10, 16]. A pressure change in the

system over a specified period (8 hours) was monitored with a pressure gauge and logged

automatically. Gas permeability was measured based on the change of pressure with time using

the following equation:

𝑃 =𝑉

𝑅𝑇 × 𝐴𝑚𝑡𝑙𝑛 (

𝑝0 − 𝑝𝑎

𝑝𝑡 − 𝑝𝑎)

(3.1)

𝐴𝑚 = [2𝜋(𝑅0 − 𝑅𝑖𝑛)𝐿]/𝑙𝑛 (𝑅0/𝑅𝑖𝑛) (3.2)

The mechanical strength was studied by a three-point bending test using a tensile tester (Instron

Model 5544) with a load cell of 1 kN. Figure 3.4 shows the setup for the testing. The sample

was positioned onto two sample holders with a gap of 30 mm. 5 samples were tested for each

anode design. The bending strength (σF) was calculated from the obtained fracture force using

the following equation:

𝜎𝐹 =8F𝐿𝐷𝑂

𝜋(𝐷𝑂4 − 𝐷𝑖

4)

(3.3)

Where F is fracture load (N) and L, Do and Di represent the length (m), the outer and inner

diameters of the micro-tube (m), respectively.

Chapter 3

96

Figure 3.4: Schematic diagram for three-point bending test.

The average porosity of the reduced (Ni-YSZ) micro-tube samples was measured using a

pycnometer (Micromeritics Accupyc II 1340). Porosity (εV) was calculated using the equations

below:

𝜀𝑉 =𝜌𝑝𝑦𝑐 − 𝜌𝑚𝑖𝑐𝑟𝑜−𝑡𝑢𝑏𝑒

𝜌𝑝𝑦𝑐

(3.4)

𝜌𝑚𝑖𝑐𝑟𝑜−𝑡𝑢𝑏𝑒 =4𝑚𝑠𝑎𝑚𝑝𝑙𝑒

𝜋(𝐷𝑂4 − 𝐷𝑖

4)𝐿𝑠𝑎𝑚𝑝𝑙𝑒

(3.5)

where ρpyc signifies the skeleton density (g cm−3) measured through pycnometer, m, L, Do and

Di represent the mass, length, outer and inner diameters of the sample (cm), respectively. The

assumption has been made that each sample has a uniform structure with identical dimension

all through the micro-tube.

3.2.5 Electrochemical performance test

For current collection from the electrodes, silver wires of 0.2 mm diameter (99.99% purity,

Advent Research Materials Ltd, UK) were wrapped along the cathode and on the exposed

anode with additional silver paste for contact enhancement. Wires from both electrodes were

connected to a potentiostat/galvanostat (Model: IviumStat, Ivium Technologies B.V.,

Netherlands). Prior to electrochemical testing, the cells were reduced in-situ at 700 °C for at

least 3 hours.

Chapter 3

97

Current–Voltage (I-V) curves were measured with 30 ml min−1 of pure hydrogen (H2) supplied

to anode and 50 ml min−1 of air fed to cathode. The electrochemical performance of the cells

were determined in a range of temperatures from 700 to 800 °C. The AC impedance spectra

were measured using Iviumstat (IviumSoft™ Electrochemistry Software) with frequency

ranging from 0.1 Hz to 1 MHz using a signal amplitude of 10 mV under open-circuit voltage

(OCV) conditions. The active area used for all the calculations relating to fuel cell performance

was selected to be the area of the cathode, as measured by:

𝐴𝑟𝑒𝑎𝑐𝑒𝑙𝑙 = 2𝜋𝑟𝑜𝐿 (3.6)

where, ro and L represent outer radius and length of the cathode, respectively.

3.3 Results and discussion

3.3.1 Morphology

Figure 3.5 shows scanning electron microscopy (SEM) images of the different anode substrate.

These two different structure were fabricated using two different types of bore fluid (internal

coagulant). An aqeuous solution of PVA (10 wt.%) was applied as a bore fluid to make NiO-

YSZ (BF-P). During the fabrication process, bore fluid exchanges with the solvent within the

suspension from the centre bore and form short finger-like voids, also known as micro-

channels. At the same time, the external coagulant penetrates the radial thickness of the micro-

tube, forming a percolated micro-channel. From Figure 3.5(a-ii), it can be observed that the

formation of micro-channels is initiated from both micro-tube surfaces, leaving two sponge-

like skin layers at each surface and a third sponge-like structure sandwiched between the two

micro-channel layers. In addition to these three sponge-like layers (two sponge-like skin layers

in interior and exterior surfaces and a central sponge-like layer), the walls in between the micro-

channels are also sponge-like structures, formed during the growth of the micro-channels.

Chapter 3

98

Figure 3.5: SEM images of two different anode substrate; (a) NiO-YSZ (BF-P) and (b) NiO-

YSZ (BF-S) where (i), (ii) and (iii) represent the overall cross-section, magnified cross-section

and the inner surface, respectively.

A distinct micro-structural design has been found for an anode substrate prepared with solvent-

based bore fluid, denote as NiO-YSZ (BF-S). From Figure 3.5(b-ii), it is observed that micro-

channels initiated from the exterior surface and penetrated through the membrane wall, leaving

a plurality of openings on the interior surface. Instead of aqueous solution, a solvent-based bore

fluid was used to delay polymer precipitation at the interior surface. This allowed the

Chapter 3

99

sustainable growth of the micro-channels from the exterior surface until they penetrate through

the inner surface. NMP, which is a solvent used in the dope suspension preparation, was used

as part of the bore fluid, with a ratio of NMP-Ethanol of 60-40 wt. %. Such composition was

chosen to minimise the strength of solvent-based bore fluid which tends to partially eliminate

readily formed micro-channel when it diffuses outward into the external coagulant bath and

partially dissolves the interior surface. This process resulted in a distinct pore entrance diameter

for the two designs, as seen at the inner surfaces, showing that NiO-YSZ (BF-S) has

considerably larger channel entrances. For both designs, the commonly observed sponge-like

region and micro-channels were preserved following the sintering process, indicating that the

sintering only removed the organic materials, but did not significantly alter the overall structure

of the micro-tube.

In Figure 3.6, the cross-sectional images depict a cell consisting of an anode and a thin YSZ

electrolyte layer which are well bounded, signifying the success of co-sintering process. The

SEM images of electrolyte outer surface (Figure 3.6b and d) presented proper densification has

been achieved. Anode-supported design allows for a thinner electrolyte to be applied and such

design would lead to better electrochemical performance as lower ohmic loss is expected for a

cell with thin electrolyte layer. However, extra caution must be taken to prevent the formation

of microcracks, and also to ensure the electrolyte layer is fully dense and gas-tight, impervious

to both fuel and oxidant streams.

Chapter 3

100

Figure 3.6: SEM images for (a) NiO-YSZ (BF-P) and (c) NiO-YSZ (BF-S) anode substrate

with YSZ electrolyte, respectively where the inserts show electrolyte layer thickness and (b)

and (d) show electrolyte outer surface for NiO-YSZ (BF-P) and NiO-YSZ (BF-S), respectively.

3.3.2 Bending strength, electrolyte gas-tightness property and porosity

To support other cell components, anode micro-tubes should have sufficient mechanical

strength to be developed into MT-SOFC. Herein, mechanical robustness was measured using

a three-point bending test. The NiO-YSZ (BF-P) substrate had a fracture load of 14.1 ± 1.1 N

with bending strength 253.2 MPa whereas the NiO-YSZ (BF-S) substrate had much lower

mechanical properties with 9.3 ± 1.5 N and 179.2 MPa for fracture load and bending strength,

respectively. The superior mechanical strength of NiO-YSZ (BF-P) is predictable, as such

substrate has a thicker sponge-like structure compared to the NiO-YSZ (BF-S) counterpart.

The long micro-channels observed in the latter design also contribute to lower mechanical

strength of the substrate, since micro-tube robustness is largely dependent on the proportions

Chapter 3

101

of the sponge-like structure itself. Nevertheless, as described in earlier work [16], anode-

supported micro-tube with a bending strength of ~150 MPa could be made into a complete

MT-SOFC. Therefore, both anode designs could be well developed into a full cell.

Appropriate gas-tightness for the electrolyte layer is crucial for the success of SOFC operation.

A micro-tube can be considered gas-tight if the nitrogen (N2) permeance is of the magnitude

10−10 mol m−2 s−1 Pa−1 [22]. From the N2 gas leakage test, the measured N2 gas permeances

were 7.30 × 10−10 and 6.11 × 10−10 mol m−2 s−1 Pa−1 for NiO-YSZ (BF-S) and NiO-YSZ (BF-

P), respectively. Such values are well within the range to be considered gas-tight. This gas-

tight property shown by the sintered half-cell (NiO-YSZ/YSZ) from both anode design agrees

well with the observation under SEM wherein dense YSZ layer could be seen (Figure 3.5).

Sufficient electrolyte densification and gas-tightness property are crucial to prevent the mixing

of fuel and oxidant.

Minimum gas transport resistance is crucial for better movement of the fuel towards the TPB.

TPB is a region in which electronic, ionic and gas phase meet for the reaction to occur, making

mixed conduction and adequate porosity vital to performance. It has been described that the

ideal anode porosity should be in between 30 and 40% [23, 24]. Under pycnometry analysis,

the porosity of the reduced samples were 32.5% and 30.1 % for Ni-YSZ (BF-S) and Ni-YSZ

(BF-P), respectively. Such values are well within the requirements for optimum anode porosity.

As described above, all characteristics required for a good anode support should be achieved

in order to be aptly developed into a complete MT-SOFC. Therefore, careful control during the

fabrication process is crucial to acquire suitable design to fulfil all of these requirements.

Chapter 3

102

3.3.3 Electrochemical performances

From Figure 3.7, all cells show an open circuit voltage (OCV) of about 1.15 V when fuelled

by pure hydrogen (H2) with a flow rate of 30 ml min−1. This value is close to theoretical value

predicted by Nernst equation, indicating a proper gas-tightness property of the 5 µm YSZ

electrolyte and good cell sealing. The OCV values obtained here are in agreement with other

work [13].

Cells with conventional anode design, denoted as NiO-YSZ (BF-P), reached maximum power

densities (Pmax) of 0.22, 0.44 and 0.84 W cm−2 at 700, 750 and 800 °C, respectively. Such

values indicated considerable improvements than other tubular systems with similar cell

materials reported earlier [13, 16, 25, 26]. Enhanced electrochemical performances were

obtained with an evolved anode design, NiO-YSZ (BF-S), achieving 15.5 to 31.8% higher

power density than those obtained with NiO-YSZ (BF-P) counterpart. Maximum power

densities were measured at 0.29, 0.57 and 0.97 W cm−2 at 700, 750 and 800 °C, respectively.

This could be attributed to the presence of long micro-channels observed in NiO-YSZ (BF-S)

substrate which is useful in decreasing gas transport resistance within the anode and eventually

lead to higher electrochemical performance.

In addition, it is apparent that NiO-YSZ (BF-P) shows fuel starvation towards the current

density (2.5 A cm−2) at 750 °C and this is more pronounced at 800 °C, as indicated by the

dashed red boxes in Figure 3.7. This could be a consequence of faster exchange current density

and oxygen ions transport with the increase in temperature. However, no sign of fuel depletion

is observed for NiO-YSZ (BF-S) at similar current density, which demonstrates the

effectiveness of micro-channels in mass transport optimisation.

Chapter 3

103

Figure 3.7: j-V curves and j-P curves for cell developed from (a) NiO-YSZ (BF-P) substrate

and (b) NiO-YSZ (BF-S) substrate. Reproduced from Ref. [21] with permission from the Royal

Society of Chemistry.

Chapter 3

104

The overall electrochemical performances for both anode designs are listed in Table 3.2.

Table 3.2: Power densities of cells with different anode substrate at various temperature.

Temp. Maximum power density, Pmax (W cm−2) Increment

(°C) NiO-YSZ (BF-P) NiO-YSZ (BF-S) (%)

700 0.22 0.29 31.8

750 0.44 0.57 29.5

800 0.84 0.97 15.5

Cells developed from two different substrates were investigated for their stability under similar

operating conditions. The short-term tests were conducted by leaving the cells under constant

0.7 V for 30 hours at 750 °C. From Figure 3.8, both cells show relatively stable performance

with negligible performance degradation, indicating an appropriate reactor design. It is

noteworthy that the duration for short-term test was limited by the safety issue, where the

system used for such test is not suitable for unattended testing for long-term.

Figure 3.8: Short-term stability test under 0.7 V at 750 °C for cells derived from different anode

substrates.

Chapter 3

105

The performance enhancement shown by Ni-YSZ (BF-S) against Ni-YSZ (BF-P) could be

explained by assessing the electrochemical impedance spectra (EIS), as shown in Figure 3.8.

Two arcs which are commonly observed under EIS measurement represent activation

polarisation (ηa) (arc on the left-hand side) and concentration polarisation (ηc) (arc on the right-

hand side, between 1 Hz and 100 Hz). From literatures, the first intercept on x-axis, under the

high frequency region, denotes ohmic resistance (RΩ), whereas the intercept at low frequency

region represents the total cell polarisation (η). It is noteworthy that the spectra obtained in this

study showed no intercepts on the x-axis at high frequency region. This could be due to the

limitation of the potentiostat/galvanostat (Model: Iviumstat, Ivium Technologies B.V.,

Netherlands) where it was difficult to obtain the value for the imaginary parts of the impedance

under high frequency measurement, thus, some points at this region were omitted.

Nevertheless, as can be estimated from the spectra, the contribution of ohmic resistance (RΩ)

is trivial compared with activation polarisation (ηa) and concentration polarisation (ηc) owing

to thin (5 µm) electrolyte. For both anode design, the RΩ is relatively low signifying the

appropriate electronic conductivity and ionic conductivity of the cells.

Under all testing conditions, the total cell polarisation for Ni-YSZ (BF-S) substrate was lesser

than the Ni-YSZ (BF-P). Similar trend was observed with temperature increment whereby for

both designs, the higher operating temperature caused considerable reduction in activation

polarisation due to improved reaction kinetics and greater electrolyte conductivities.

Major difference was observed for concentration polarisation as Ni-YSZ (BF-S) had much

smaller value, whereby open-channel design effectively reduced concentration polarisation

(approximately 0.9 Ω cm vs 1.3 Ω cm, as estimated from the EIS spectra). Such a reduction

may be attributed to the distinctive micro-structure of the anode substrate. It is recognised that

the fuel oxidation reaction occurs at the active triple phase boundary (TPB) region, which has

been described to be located near the anode/electrolyte interface, with several studies reporting

Chapter 3

106

that it is located within 10-20 µm from the electrolyte towards the anode [7, 27]. For Ni-YSZ

(BF-S), the plurality of micro-channels with open pore are effective in lowering resistance to

fuel diffusion, resulting in improved cell performance due to minimised concentration

polarisation resistance.

Figure 3.9: Electrochemical impedance spectra (EIS) for cells from (a) Ni-YSZ (BF-P) and (b)

Ni-YSZ (BF-S).

Chapter 3

107

This is in a good agreement with the observation of 2D distributions of reactant fluxes, H2

pressure and concentration loss as modelled by our research collaborator1, shown in Figure

3.10.

Figure 3.10: Comparisons of the 2D distributions of reactant fluxes, pressure and concentration

overpotential for different designs (Sp: spongy layer; Ch: channels).1 Reproduced from Ref.

[21] with permission from the Royal Society of Chemistry.

1 Simulation and modelling work were conducted by research collaborator from Imperial College London and

University College London (UCL).

Chapter 3

108

The micro-structural and mass transport parameters determined using multi-length scale X-ray

CT and CFD simulations, performed by our research collaborator from Imperial College

London and University College London (UCL) are used to define the 2D electrochemical

simulation models. Figure 3.10 shows the distributions of simulated field values for two anode

designs, Ni-YSZ (BF-S) and Ni-YSZ (BF-P), obtained at 1.3 A cm−2 and 800 °C. In both

design, similar electrochemical parameters were applied to study the influence of having micro-

channels. The reactant flux maps represented by Figure 3.10a and b show that the reactants are

drawn from the centre-bore into the micro-channels and permeate through the spongy layer. It

is observed that the mass flux in the spongy layer beyond the micro-channels in Ni-YSZ (BF-

S) is lower compared to Ni-YSZ (BF-P). This could be associated with the different diffusion

path lengths from the micro-channels tip to the anode and electrolyte interface where the active

TPB region is located and the H2 gas transported outside of such region accumulates. Figure

3.10c and d show the effect of micro-channels length in which the longer they are, the greater

the local H2 molar concentration.

In comparison, a relatively small concentration loss is detected in Ni-YSZ (BF-S) than the Ni-

YSZ (BF-P) as shown in Figure 3.10e and f. This has proved that the micro-channels with

larger pore entrance are useful in reducing the concentration loss by assisting the permeation

of the reactants through the spongy layer. Furthermore, the concentration loss ahead of the

micro-channels tip is much lower in Ni-YSZ (BF-S) where this can also be due to the longer

micro-channels and thinner sponge-like region. Further discussion on the simulation and

modelling work for this distinct anode design could be found in reference [21].

Overall, the distinct design of Ni-YSZ (BF-S) with long micro-channels and larger pore

entrance has been demonstrated to be an attractive way to reduce mass transport resistance.

Such improvement in gas transport property ultimately results in higher maximum power

density, as demonstrated by the j-P curves (Figure 3.7).

Chapter 3

109

3.4 Conclusion

This study showed that cell developed from micro-tubes made from standard anode material

could deliver decent performances through micro-structure optimisation. Two different anode

structures were fabricated and compared in terms of electrochemical performance. A

substantial increase in electrochemical performance has been demonstrated by the anode

substrate with superior structural design fabricated via the use solvent-based bore fluid. The

use of modified-phase-inversion process with solvent-based bore fluid to produce NiO-YSZ

(BF-S) micro-tube resulted in a unique structure consisting of longer micro-channels with

larger pore opening, greatly reducing concentration polarisation. This improved cell design

gave up to 31.8% higher peak power densities than the conventional substrate design. Much

lower total cell polarisation was observed for cell developed from NiO-YSZ (BF-S) substrate,

mainly corresponding to the minimisation of concentration polarisation, as corroborated from

the EIS analysis and 2D modelling on fuel distribution. This work has indicated that common

cell materials would provide high performance by optimising cell’s micro-structure.

3.5 References









Chapter 3

110

5. Suzuki, T., et al., Impact of Anode Microstructure on Solid Oxide Fuel Cells. Science,

2009. 325(5942): p. 852-855.

6. Hanifi, A.R., et al., Tailoring the Microstructure of a Solid Oxide Fuel Cell Anode

Support by Calcination and Milling of YSZ. Scientific Reports, 2016. 6: p. 27359.

7. Brown, M., S. Primdahl, and M. Mogensen, Structure/Performance Relations for

Ni/Yttria‐Stabilized Zirconia Anodes for Solid Oxide Fuel Cells. Journal of The

Electrochemical Society, 2000. 147(2): p. 475-485.

8. Kendall, K. and M. Palin, A small solid oxide fuel cell demonstrator for microelectronic

applications. Journal of Power Sources, 1998. 71(1–2): p. 268-270.



10. Lawlor, V., Review of the micro-tubular solid oxide fuel cell (Part II: Cell design issues

and research activities). Journal of Power Sources, 2013. 240: p. 421-441.

11. Wang, H., Z. Gao, and S.A. Barnett, Anode-Supported Solid Oxide Fuel Cells

Fabricated by Single Step Reduced-Temperature Co-Firing. Journal of The

Electrochemical Society, 2016. 163(3): p. F196-F201.

12. Arias-Serrano, B.I., et al., High-performance Ni–YSZ thin-walled microtubes for

anode-supported solid oxide fuel cells obtained by powder extrusion moulding. RSC

Advances, 2016. 6(23): p. 19007-19015.

13. Zhang, X., et al., An anode-supported micro-tubular solid oxide fuel cell with redox

stable composite cathode. International Journal of Hydrogen Energy, 2010. 35(16): p.

8654-8662.

14. Li, T., et al., A highly-robust solid oxide fuel cell (SOFC): simultaneous greenhouse

gas treatment and clean energy generation. Energy Environ. Sci., 2016. 9(12): p. 3682-

3686.

Chapter 3

111




16. Yang, C., et al., Fabrication and characterization of an anode-supported hollow fiber

SOFC. Journal of Power Sources, 2009. 187(1): p. 90-92.

17. Droushiotis, N., et al., Comparison Between Anode-Supported and Electrolyte-

Supported Ni-CGO-LSCF Micro-tubular Solid Oxide Fuel Cells. Fuel Cells, 2014.

14(2): p. 200-211.




19. Yang, N., X. Tan, and Z. Ma, A phase-inversion/sintering process to fabricate

nickel/yttria-stabilized zirconia hollow fibers as the anode support for micro-tubular

solid oxide fuel cells. Journal of Power Sources, 2008. 183(1): p. 14-19.

20. Othman, M.H.D., et al., Morphological studies of macrostructure of Ni–CGO anode

hollow fibres for intermediate temperature solid oxide fuel cells. Journal of Membrane

Science, 2010. 360(1–2): p. 410-417.

21. Lu, X., et al., The application of hierarchical structures in energy devices: new insights

into the design of solid oxide fuel cells with enhanced mass transport. Energy &

Environmental Science, 2018. 11(9): p. 2390-2403.

22. Bohn, H.G. and T. Schober, Electrical Conductivity of the High-Temperature Proton

Conductor BaZr0.9Y0.1O2.95. Journal of the American Ceramic Society, 2000. 83(4): p.

768-772.

Chapter 3

112

23. Zhao, F. and A.V. Virkar, Dependence of polarization in anode-supported solid oxide

fuel cells on various cell parameters. Journal of Power Sources, 2005. 141(1): p. 79-

95.

24. Dong, D., et al., Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell

by gelcasting. Journal of Power Sources, 2007. 165(1): p. 217-223.

25. Kanawka, K., et al., NI/NI-YSZ Current Collector/Anode Dual Layer Hollow Fibers for

Micro-Tubular Solid Oxide Fuel Cells. Fuel Cells, 2011. 11(5): p. 690-696.



84.


Reviews, 2004. 104(10): p. 4845-4866.

Chapter 4

113

4 CHAPTER 4

Electrode design for direct-methane micro-tubular solid oxide fuel cell (MT-SOFC)

Chapter 4 describes the development of a MT-SOFC combining the electrode design reported

in Chapter 3 with a Ni-free anode. This study utilised yttria-stabilised zirconia (YSZ) as an

electrolyte material to form such structure. This work first focused on the development of a

suitable electrolyte substrate, which was then developed into a complete single cell by

employing a copper-ceria (Cu-CeO2) anode. The main purpose of the cell was aimed at direct

operation with dry methane (CH4).

This chapter is based on author’s publication: Rabuni, M.F., et al., Electrode design for direct-

methane micro-tubular solid oxide fuel cell (MT-SOFC). Journal of Power Sources, 2018. 384:

p. 287-294. https://doi.org/10.1016/j.jpowsour.2018.03.002 (Open access)

Abstract

In this chapter, a micro-structured electrode design was fabricated through phase-inversion

with solvent-based bore fluid. A thin yttria-stabilised zirconia (YSZ) electrolyte was integrated

with a highly porous structure functioning as a suitable scaffold for anode incorporation in a

single-step process. A continuous and well-distributed layer of copper-ceria (Cu-CeO2) was

impregnated inside the micro-channels of the anode scaffold. The combination of well-

organised micro-channels and nano-structured Cu-CeO2 anode contributed to an increase in

electrochemical reaction sites which promoted charge-transfer and the unique structure

facilitated easier mass transport within the electrode and led to increased performance. Fuel

cell operation using both hydrogen (H2) and CH4 as fuel demonstrated outstanding

electrochemical performance. A power density of 0.16 Wcm−2 achieved with CH4 as a fuel at

750 °C which is one of the highest ever reported values for a cell with similar anode materials.

Chapter 4

114

4.1 Introduction

Solid oxide fuel cells (SOFCs) are a variety of fuel cells which are characterised by their ability

to operate at high temperatures, typically between 600 and 1000 °C [1-3]. In addition to having

the common advantages of fuel cell technology, such as, high efficiency and environmentally

friendly operation, SOFCs have additional benefits of full solid-state operation and flexibility

for fuel intakes [4, 5]. Due to their high working temperature, SOFC can be operated using a

variety of fuels including hydrogen (H2), hydrocarbons, biogas, ethanol and others. H2 remains

the most common fuel, with studies often reporting excellent electrochemical performances.

The vast majority of H2 is generated from the hydrocarbons through steam reforming, which

sacrifices about 20 to 30% of the potential fuel value [6]. For that reason, direct hydrocarbon

utilisation for SOFC operation has been proposed to overcome this and attracted research

interest globally [7-10]. The main problem with direct hydrocarbon utilisation arises from

carbon formation on the conventional nickel (Ni)-cermet anodes [11, 12]. Carbon formation on

the anode negatively impacts the fuel cell and results in lower performance and cell

degradation, eventually leading to failure of the fuel cell system. Therefore, alternative anode

materials to Ni are desirable to avoid such problems. An excellent review on various anode

materials by Ge et al. described that while Ni-based anodes have superior electrocatalytic

activity than the copper (Cu)-based anodes, but the latter has the advantage of greater fuel

flexibility and a greater resistance towards carbon formation [2]. A number of studies by Gorte

and co-workers reported the development of planar SOFCs with Cu-based anodes operated

using various types of hydrocarbons where good cell performances was observed with no or

minor carbon formation [6, 12-14].

A cell which utilises a copper-based anode seems to be attractive option to reduce the risk of

carbon formation when using a hydrocarbon fuel source. However, the fabrication process

provides a major challenge in preparing such a cell. This is mainly due to the low melting

Chapter 4

115

points of copper and its oxides, i.e. 1085 °C, 1326 °C and 1232 °C for Cu, CuO, and Cu2O,

respectively [15, 16]. The preparation of a cell using traditional ceramic processing techniques,

e.g. tape casting and sintering require high temperatures which, due to the low melting point

of Cu-based materials, are unfeasible. As an alternative, impregnation has been almost

exclusively applied to incorporate Cu based materials as an anode [12, 14, 17]. In terms of cell

geometry, a micro-tubular design offers extra benefits compared to the typical planar/flat and

tubular designs such as greater thermal shock resistance, rapid start-up and shut-down, and

higher volumetric power density [18, 19].

Anode design is a crucial aspect for the success of direct use of hydrocarbons in SOFC

operation. Inspired by the new micro-tube design as reported in the previous chapter, herein, a

micro-structured yttria-stabilised zirconia (YSZ) micro-tube has been prepared using a

modified phase-inversion-assisted process utilising a solvent-based bore fluid. The YSZ micro-

tube was subsequently developed into a complete MT-SOFC with a Cu-based anode which is

fuelled by dry CH4. Such unique micro-tube design was acquired by exploiting the advantage

of phase-inversion assisted process that allows the flexibility in control and tailoring of the

micro-structure. The hierarchical structured YSZ scaffold consisted of self-organised micro-

channels and a thin sponge-layer at the outer section. The thin and dense sponge-layer functions

as an electrolyte, whereas the micro-channels subsequently perform as a substrate/scaffold for

the anode materials (Cu-CeO2). These anodic materials are securely placed into the porous

structure of the scaffold and thus, prevents their thermal movement. Furthermore, this distinct

micro-tube feature helps avoid delamination and defect formation, contributing towards better

structural integrity. Upon completion of a full cell, a performance test was conducted using dry

CH4.

Chapter 4

116

4.2 Experimental

4.2.1 Materials

YSZ and lanthanum strontium manganite (LSM) powders were purchased from Inframat

Advance Materials (USA). Polyethersulfone (PESf) (Radel A300, Mw = 42500 g mol−1, Ameco

Performance, USA), N-Methyl-2-pyrrolidone (NMP) (Mw =99.13 g mol−1 from VWR, UK),

and Arlacel P135 (polyethyleneglycol 30-dipolyhydroxystearate, Uniquema) were used as

binder, solvent, and dispersant, respectively. NMP was also applied as part of bore fluid after

mixing with ethanol (VWR, UK, NMP/Ethanol=60/40 in weight). De-ionised (DI) water was

applied as external coagulant. Anode components: 99 % copper (III) nitrate trihydrate

(Cu(NO3)2.3H2O) and 99.5 % cerium nitrate hexahydrate (Ce(NO3)3.6H2O) were obtained

from Fisher Scientific (UK). Ethylene glycol (99+ %) from Acros Organic (UK) was applied

as cathode ink vehicle.

4.2.2 Fabrication of micro-tube

The ceramic suspension was prepared by mixing the YSZ powder with the solvent (NMP) and

dispersant. The resultant mixture was milled for 60 hours (MTI Corporation model SFM-1

Desktop Planetary Ball Miller). PESf was then added to the suspensions and milled for another

60 hours to attain a homogeneous mixture. Before spinning, the suspension was degassed by

stirring under vacuum for 2 hours to eliminate air bubbles. The suspension was transferred to

a stainless-steel syringe and extruded through a custom-designed spinneret into an external

coagulation bath of DI water. The spinneret was slightly immersed in the coagulant bath to

remove any air gap. The precise control on the extrusion rates of the suspension and the bore

fluid were done by two Harvard PHD 22/2000 Hpsi syringe pumps. The micro-tube precursors

were left in water bath overnight to ensure complete phase-inversion before sintering. Table

4.1 presents the fabrication parameters for the preparation of YSZ micro-tube.

Chapter 4

117

Table 4.1: Parameter for YSZ micro-tubes fabrication.

Suspension composition

Ceramic – YSZ (wt.%)

Solvent - NMP (wt.%)

PESf (wt.%)

Dispersant (wt.%)

56.5

34.2

8.6

0.7

Fabrication conditions

Extrusion rate (ml min−1)

Bore fluid rate (ml min−1)

Air gap (cm)

8

10

0

Sintering conditions

Temperature (°C)

Dwelling period (h)

1450

6

4.2.3 Fabrication of a complete fuel cell

Figure 4.1: Schematic diagram of the steps involved in the overall fabrication of cell

components of MT-SOFC.

Chapter 4

118

Figure 4.1a shows the custom-made spinneret for the fabrication of a single channel YSZ

micro-tube. The procedure for the preparation of a complete single cell is represented in Figure.

4.1b. Following YSZ micro-tube sintering at 1450 °C for 6 hours, a dual-layer cathode

consisting of an inner LSM-YSZ layer (LSM/YSZ=50/50 by weight) and an outer LSM layer

was brush-painted onto the YSZ micro-tube. The dual-layer cathode was sintered at 1000 °C

for two hours. Then, the anode materials were incorporated into the electrolyte scaffold via

vacuum-assisted co-impregnation process. An aqueous solution of copper nitrate and cerium

nitrate was prepared prior to co-impregnation, with a concentration of 4 and 1 molar,

respectively. Following each impregnation cycle, the co-impregnated micro-tube underwent a

heat treatment at 450 °C for an hour to decompose the nitrates. This impregnation was repeated

until a sufficient anode material loading was achieved.

Figure 4.2: Set-up for co-impregnation process.

Chapter 4

119

Figure 4.2 shows the complete set-up of the vacuum-assisted co-impregnation process. The

YSZ micro-tube outer surface was first wrapped by PTFE tape to prevent the deposition of

anode materials onto the electrolyte and cathode surfaces. The quartz tube was pre-vacuumed

by opening Valve 1 and closing Valve 2. After five minutes, Valve 1 was closed, and Valve 2

was opened to allow the anode solution to enter the quartz tube and completely immerse the

micro-tube. Afterwards, Valve 2 was closed, and Valve 1 was carefully opened to start the co-

impregnation process.


The morphology of the micro-tube was examined using scanning electron microscopy (SEM)

(JEOL JSM-5610 and LEO Gemini 1525 FEGSEM). Samples were gold-coated under vacuum

at 20 mA for 60 seconds (EMITECH Model K550), and the SEM images with various

magnifications were acquired. Energy dispersive spectrometry (EDS, JEOL JSM-6400

electron microscope) analysis was used to evaluate the elemental distribution of anodic

materials. The gas-tightness of the sintered micro-tubes was assessed using nitrogen (N2)

permeation and the mechanical strength was examined using a three-point bending method

with a tensile tester. The same procedures have been described in Section 3.2.4 in Chapter 3.

4.2.5 Cell sealing and reactor assembly

The single cell was sealed into two alumina supporting tubes (Almath Crucibles Ltd, UK) with

a ceramic sealant (Aremco, Ceramabond 552-VFG) as shown in Figure 4.3. The current

collection from the cathode was done by wrapping silver wires (99.99 % purity, temper

annealed, Advent Research Materials Ltd, UK) on cathode surface with additional silver paste

to enhance the contact. For current collection at the anode, silver wires were passed through

the lumen, with additional silver wool (Sigma Aldrich, UK) and silver paste (Alfa Aesar, UK)

to improve contact between the anode surface and silver wires. Both wires from anode and

cathode were connected to a potentiostat/galvanostat (Model: IviumStat, Ivium Technologies

Chapter 4

120

B. V., Netherlands) for current-voltage (I-V) measurement and impedance analysis. The cell

current-voltage measurement was performed at a temperature between 650 and 750 °C by

supplying 30 ml min−1 of fuel (dry H2 or dry CH4) to the anode and 50 ml min−1 of air as the

oxidant to the cathode. Electrochemical impedance spectroscopy (EIS) was taken at open

circuit voltage (OCV) in the frequency range of 105 - 0.01 Hz with a signal amplitude of 10

mV.

Figure 4.3: Reactor assembly and sealing.

Chapter 4

121


4.3.1 Micro-structure of micro-tube

From Figure 4.4, it can be seen that the YSZ micro-tube displays a highly asymmetric structure

which consist of self-organised micro-channels growing through the entire cross-section and a

thin and dense sponge layer (also known as skin layer) at the outer section. The formation of

such micro-channels could be explained by Rayleigh-Taylor instability theory [20]. It has been

described that when two fluids of varying density are in contact, the external coagulant with

lower density tends to ‘push’ into the high-density ceramic suspension as a result of interfacial

instability/perturbation, during which solvent/non-solvent exchange takes place. A sponge-like

region is formed when the phase-inversion is close to completion. However, in this work, a

mixture of 60/40 wt. % of NMP/ethanol was used as the bore fluid (internal coagulant) in order

to delay the precipitation of polymer phase, which allows the formation of micro-channels from

the external surface to grow and penetrate through the whole cross section, creating open

entrances to the inner surface [21]. Such a structure is observed in Figure 4.4a and b. It should

be noted that one undesirable property of micro-tube with large and long micro-channels is a

limited sponge-like region, which is the main contributor to the triple phase boundary (TPB),

a region at which the electrochemical reactions occurs.

The average outer and inner diameter of the sintered micro-tubes are 1.44 ± 0.01 and 0.96 ±

0.01 mm, respectively. From Figure 4.4c and d, a thin (ca. 10 µm) and dense YSZ skin layer

could be observed which is suited as the electrolyte material, whereas the numerous micro-

channels and porous structures are suitable for the incorporation of anode materials, which in

this work was a composite of Cu-CeO2. In addition, long micro-channels with open entrances

have been described to be useful in reducing the resistance to gas transport. The concentration

polarisation resistance through the pores may be considered trivial if the pore radius is larger

than 1 μm, which is adequate for facilitating the diffusion of gaseous fuels and the reaction

Chapter 4

122

products [22, 23]. From Figure 4.4e, it can be estimated that the pore entrances are about 30-

40 µm. Such large pore entrance would be advantageous for SOFC operation as it facilitates

the fuel transport to the TPB area which would eventually results in better electrochemical

performance due to a reduced concentration polarisation.

Figure 4.4: SEM images of the sintered YSZ micro-tube: (a) overall cross-section, (b)

magnified cross-section, (c) and (d) cross-section and magnified cross-section showing the

thickness of the skin-layer region, respectively, (e) and (f) inner and outer surface, respectively.

Chapter 4

123

4.3.2 Gas-tightness and mechanical property

Other than offering oxygen (O2−) ions conductivity from the cathode to the anode, adequate

gas-tightness of electrolyte is essential to avoid mixing between the fuel and the oxidant. Gas

tightness was ensured using a nitrogen (N2) gas leakage test. It is reported that membranes

could be considered as gas-tight if the N2 permeance is of the magnitude 10−10 mol m−2 s−1 Pa−1

[24]. The average permeance of the prepared YSZ micro-tubes was 5.3 × 10−10 mol m−2 s−1

Pa−1, showing that the micro-tube is suitable for the fabrication of a complete cell.

Micro-tubes should also have sufficient mechanical strength to support other cell components

and this property is also important to cell lifetime. The mechanical strength of the micro-tubes

was measured using a three-point bending test. The average bending strength of the green YSZ

micro-tubes was measured to be 264 MPa, which is considerably higher than micro-tubes with

similar structure reported before [25]. It has been suggested by other researchers that anode-

supported micro-tubes with a bending strength of ~178 MPa are appropriate for the

development into a complete single cell [26, 27], suggesting that our YSZ micro-tube is a

suitable support for preparation of the MT-SOFC.

4.3.3 Vacuum-assisted co-impregnation process

The anode material (Cu-CeO2) was incorporated using a vacuum-assisted co-impregnation

process. Firstly, the effect of impregnation time on the single-cycle loading was studied. The

outer skin layer of the YSZ micro-tube was properly densified after sintering to serve as an

electrolyte, however, the bulk of the scaffold is still porous with pore size down to less than

100 nm (Figure 4.5). Therefore, sufficient time is required to remove gas bubbles trapped

inside.

.

Chapter 4

124

Figure 4.5: SEM image showing the porous YSZ bulk.

Subsequent to each heat treatment to decompose the nitrate, the mass of impregnated micro-

tube was measured. Figure 4.6 shows that after 30 minutes of impregnation the mass increase

was about 1.5%. The mass increase is influenced by impregnation time up to 1 hour, beyond

which the loading reaches a plateau, with only slight increases between 1 and 2 hours. For time

efficiency, 1 hour was selected for each cycle.

Figure 4.6: Percentage mass increase after a single cycle at different impregnation time.

Chapter 4

125

Several impregnation cycles are needed to ensure sufficient anode materials to form a

continuous phase. The relationship between the anode loading and number of impregnation

cycles is shown in Figure 4.7. After 15 cycles of imprenation, the anode loading is 28 wt.% of

the full cell (65:35 wt.% for CuO:CeO2). It can be observed that the anode loading increased

almost linearly with the number of cycles, signifying that the inpregnation process could be

easily controlled.

Figure 4.7: Anode loading after 15 cycles of impregnation.

Under pycnometry analysis, the porosity of the YSZ scaffold was measured to be 43%, and

reduced to 36% for the micro-tube that has undergone 15 cycles of the co-impregnation

process. The porosity reduces as the anode materials occupied the void of the YSZ scaffold.

Nevertheless, such values are well within the optimal anode porosity range which is reported

to be between 30% and 40% [28].

Under high temperature operation, previous work has reported the tendency of CuO to

agglomerate and form isolated and spherical clusters [29]. Similar agglomeration was observed

in this work, with the micro-tube impregnated with copper nitrate showing agglomeration of

CuO particles after the heat treatment, as shown in Figure 4.8.

Chapter 4

126

Figure 4.8: SEM images of copper (Cu)-impregnated micro-tube; (a) cross-section and (b)

inner surface.

This problem can be associated with the low thermal stability of Cu and its oxides due to their

relatively low melting temperatures. When using the co-impregnation approach, no metallic

phase agglomeration or blockage of the channel entrance was observed, as shown in Figure

4.9b. CeO2 has greater thermal stability than Cu and its oxides, due to its higher melting

temperature of 2602 °C [2]. Therefore, the presence of CeO2 may assist in distributing the Cu

throughout the entire area, besides increasing the thermal stability of the anode composite.

It is worth mentioning that one of the main advantages of applying a composite Cu-CeO2 anode

is that both materials do not form a solid solution, with each species, retaining their

functionality. In such a system, Cu provides the electronic conductivity whereas the main role

of CeO2 is largely as a catalyst for fuel oxidation. One important characteristic of an anode is

to offer continuous electronic conducting phase to allow smooth electron transport, reducing

ohmic losses. From Figure 4.9a and b, both materials are homogeneously dispersed over the

entire region of the inner surface and into the micro-channels. The vacuum-assisted co-

impregnation process applied has shown good Cu continuity, which is important for a

successful fuel cell operation.

Chapter 4

127

Figure 4.9: SEM images of Cu-CeO2-YSZ anode; low-resolution: (a) and (b) cross-section and

the inner surface, respectively and high-resolution: (c) and (d); inner surface and the magnified

inner surface, respectively, (e) magnified cross-section and (f) Cu-CeO2 coating layer.

Chapter 4

128

The homogeneous distribution of both particles onto the inner surface of the micro-tube could

also be demonstrated using elemental mapping with EDS. The chemical mapping of the co-

impregnated micro-tube is presented in Figure 4.10 which shows both materials (Cu and CeO2)

are homogeneously distributed over the entire region of the inner surface and inside the micro-

channels. Additionally, the higher resolution of the SEM images (Figure 4.9c and d) of the co-

impregnated micro-tubes show the flake structure of the Cu-CeO2 and the thickness of the

anode catalyst layer is about 5 µm (Figure 4.9f). This structure could result in larger surface

area, contributing to increased TPB, where the electrochemical reaction occurs.

Figure 4.10: Chemical mapping of the co-impregnated micro-tube: (a) SEM images, (b)

distribution of Cu element and (c) distribution of Ce element; (i) cross-section and (ii) inner

surface.

Chapter 4

129


Electrical performance tests were performed at 650, 700, and 750 °C with a ramping of 5 °C

min−1 and 30 ml min−1 of dry H2 or CH4 as fuel, and 50 ml min−1 of air as the oxidant. For this

test, the co-impregnated cell (15 cycles) were used and three identical cells were studied with

a standard deviation of less than 10 %. Using the outer diameter of the micro-tube, the

estimated active area was calculated to be approximately 0.45 cm2. The open circuit voltage

(OCV) was ~1.19 V for a system fuelled with H2 at 650 and 700 °C and ~0.95 V for CH4

system, proving a suitable gas-tightness of the electrolyte. Note that the OCV for a cell

operating with CH4 is slightly lower than the theoretical value measured from Nernst potential

equation (~1.04 V) [30]. This discrepancy for the CH4 system could be due to the presence

non-electrochemical reactions such as CH4 cracking [31]. Nevertheless, both OCV values for

systems fuelled by either dry H2 or CH4 as described here were in good agreement with

literature [6, 16, 32-34].

Figure 4.11 shows the electrochemical performance for a cell fuelled by dry H2. It reached 0.25

and 0.41 W cm−2 at 650 and 700 °C, respectively while at 750 °C, a reasonably high power

density of up to 0.55 W cm−2 was obtained. This cell achieved a considerable improvement

over other SOFCs with similar Cu-CeO2-YSZ composite anodes [16, 35], which could be

related to the distinct hierarchical structure of the micro-tubes. The large number of long micro-

channels with larger entrances may assist in facilitating gas transport to the reaction zone. The

obtained maximum power density is also comparable to previously reported cells with Ni-

based anodes [25], showing that the cell with Cu-CeO2 composite anode has a potential to

substitute the common Ni-cermet [3, 36, 37].

On the other hand, much lower power densities were achieved with CH4 as a fuel than that of

H2 (Figure 4.11b). This could be associated with the catalytic limitation, where it is believed

that electrochemical oxidation of CH4 is sluggish compared to a much simpler reaction

Chapter 4

130

involving H2 as a fuel. Previous work has described that CH4 is less reactive than H2 in

heterogeneous oxidation [38] and has lower reactivity at the anode [14]. Another reason for

lower performance in the CH4 system could be linked to the rate-limiting electrochemical steps

at the anode. Performing direct and complete oxidation of CH4 to produce carbon dioxide (CO2)

and steam (H2O) in a single step appears to be challenging [39], and instead, it has been

anticipated that several parallel reactions take place when using hydrocarbons fuel [40]. This

means that CH4 may either be partially oxidised by the oxide (O2−) ions to form carbon

monoxide (CO) and hydrogen (H2) (Equation 4.6) or fully oxidised to CO2 and steam (Equation

4.7). Moreover, there is a likelihood of gradual internal steam and dry reforming (Equation 4.8

and 4.9, respectively) occurring within the system.

CH4 + O2− ⟶ CO + 2H2 + 2e− (4.6)

CH4 + 4O2− ⟶ CO2 + 2H2O + 8e− (4.7)

CH4 + H2O → 3H2 + CO (4.8)

CH4 + CO2 → 2H2 + 2CO (4.9)

2CO CO2 + C (4.10)

CH4 → C + 2H2 (4.11)

Unfortunately, a system fuelled by dry CH4 is also susceptible to carbon formation, according

to Equation (4.10) and (4.11). Equations (4.6)-(4.9) show that there are several reacting species

at the anode (CH4, H2 and CO), contributing to the more complex anodic reactions as compared

to the H2-fuelled system. Furthermore, issues surrounding the transport limitations could also

be related to the larger molecular mass of CH4, which causes slower diffusion and may lead to

greater concentration polarisation. This could be minimised by operating the cell with internal

reforming process where the added reforming agents would reform the CH4 into H2 and CO

Chapter 4

131

which are rather easier to undergo electrochemical reaction. However, the main aim of this

work was to study the direct utilisation of dry CH4, therefore, no additional reforming agents

were considered.

Figure 4.11: Current-voltage curves for system fuelled by (a) dry H2 and (b) dry CH4.

Chapter 4

132

The difference in the electrochemical performance between these fuels is reduced when the

cell is operated at higher temperatures. The MT-SOFCs fuelled with CH4 achieved maximum

power densities of about 0.05, 0.09 and 0.16 W cm−2 at 650, 700, and 750 °C, respectively.

Operation using dry CH4 at 750 °C demonstrated a power density of 0.16 W cm−2 which, to

the best of our knowledge, is the highest compared to other reported values for similar anode

materials (Cu-CeO2), as listed in Table 4.2.

Table 4.2: SOFCs with Cu–cermet anodes fed with methane (CH4) as fuel.

Cell configuration

Anode / Electrolyte / Cathode

Fuel Temp.

(°C)

Pmax

(Wcm−2)

Ref.

Cu-CeO2-LDC / LSGM / SCF Dry CH4 800 0.07 [17]

Cu-CeO2-LDC / LSGM / SCF Dry CH4 800 0.11 [33]

Cu-CeO2-YSZ / YSZ/ LSM-YSZ

Cu-Fe-CeO2-YSZ / YSZ/ LSM-

YSZ

1-wt% Pd-Cu–Fe–ceria–YSZ

/YSZ/ LSM-YSZ

Dry CH4

Dry CH4

Dry CH4

800

800

800

0.07

0.09

0.12

[34]

[34]

[34]

Cu-Co-CeO2 / YSZ (0.23µm) /

LSM

CH4-H2

(80:20)

750 0.09 [41]

Cu-CeO2-YSZ/YSZ (180µm)

/LSM-YSZ

Dry CH4 800 0.11 [42]

Cu-CeO2-YSZ/YSZ (10µm) /LSM-

YSZ

Dry CH4 750 0.16 This work

Chapter 4

133

Figure 4.12 shows the impedance analysis for the cell with a signal amplitude of 10 mV at the

frequency range from 105 - 0.01 Hz. The high-frequency intercept on the x-axis indicates the

ohmic resistance of the cell (RΩ), consist of the ionic resistance in the electrolyte, both ionic

and electronic resistance in the electrodes, and the contact resistance at the interface and current

collectors. The first semi-circle (at high-frequency region) corresponds to the activation

polarisation (ηa), which is associated with the TPB area and the number of reactive sites, while

the second semi-circle (at low-frequency region) is related to the concentration polarisation

(ηc), encompassing the mass transport resistance of gaseous reactants and products through

electrodes and interfaces. When combined, these three components give the total cell resistance

(η), which in principle, can be represented by the intercept at low-frequency.

It can be estimated from the EIS spectra that the total cell resistance (η) reduces with the

increases in operating temperature for both fuels, with this reduction being more noticeable for

the CH4-fuelled operation. This is in agreement with the observation that greater power density

achieved during operation at higher temperature. There is only a small difference in ohmic

resistance (RΩ) for cells operated at different temperatures, measured at 0.3-0.35 and 0.3-0.5 Ω

cm2 under H2 and CH4 operation, respectively. In comparison, it can be anticipated from the

EIS spectra pattern that for CH4, concentration polarisation dominates total cell resistance, with

this value being much greater than H2. This is consistent with the trend of the measured power

density, in which the poorer performance of CH4-fuelled systems could be associated with

more complex reactions occurring. A higher concentration polarisation for the CH4-fuelled

system might be related to the rate-determining step of the anode reaction [41]. Direct oxidation

of CH4 in a single step, involving an eight-electron transfer, appears to be difficult. Therefore,

it is expected that the CH4 oxidation reaction consist of several paths including the complete

oxidation to form steam and CO2 and partial oxidation involving the conversion to H2 and CO.

These reaction products must be transported away from the electrolyte/anode interface and

Chapter 4

134

through the porous anode to the fuel stream. Such diffusion of several reactants and products

(i.e. CH4, H2, CO, CO2, and H2O) from the reaction site could results in the greater

concentration polarisation. Meanwhile, the larger activation polarisation of the CH4-fuelled

system may be attributed to the catalytic reaction constraint.

Figure 4.12: The AC impedance spectra of MT-SOFCs under open-circuit conditions at various

operating temperature fuelled by (a) dry H2 and (b) dry CH4.

Chapter 4

135

The resistance against carbon formation of this cell was also studied for 30 hours operation

time with dry CH4. A stability test has been conducted under constant operation at 750 °C at

0.7 V. It can be observed from Figure 4.13 that the current density of the single cell was

relatively stable at 0.14 A cm−2 and had less than 1% of degradation. It is worth mentioning

that a short-term period was used due to safety issue that constrains the long-term operational

time.

Figure 4.13: Current density for operation at 750 °C with dry CH4 under constant 0.7 V.

Following the 30 hours exposure to dry CH4, the presence of carbon (C) at the anode was

inspected via post-test analysis with SEM-EDS by examining two different regions of anode

surface. From Figure 4.14, the chemical mapping indicates the distribution of C element on the

anode surface where negligible amount could be observed. Moreover, only a small peak

representing the C element is shown by the spectrum. This is consistent with the steady

electrochemical performance monitored during the short-term stability test given that rapid

performance degradation will be perceived if there is severe carbon deposition on the anode.

Chapter 4

136

Figure 4.14: SEM-EDS carbon mapping post operation at 750 °C with dry CH4 under 0.7 V

for 30 hours; (a) and (d) the SEM images of the mapped area, (b) and (e) distribution of C

element and (c) and (f) sum spectrum.

Chapter 4

137

4.4 Conclusion

In this work, a unique YSZ scaffold was fabricated using a modified phase-inversion and

sintering process. The scaffold is composed of thin and dense outer skin layer acting as the

electrolyte and a series of micro-channels that are suitable for the support of anode materials.

The distinctive structure allows the utilisation of copper (Cu), which has much lower melting

point than the required temperature for the YSZ micro-tube sintering process, to be

incorporated as the anode. In addition, the micro-structural design offers well distributed gas

flow passages and assists in lowering fuel transport resistance. Fuel cell operation with both

dry H2 and CH4 demonstrated excellent electrochemical performance. Electrochemical testing

for a system fuelled with dry CH4 achieves a power density of 0.16 W cm−2, which is one of

the highest ever reported for Cu-CeO2 anode. It has been found that only minor carbon

formation could be detected, and negligible performance degradation has been observed for 30

hours short-term test. This demonstrates that the cell prepared from the micro-structured YSZ

micro-tube contributes to a better cell structure and also suitable for direct CH4 utilisation.

4.5 References

1. Sun, C. and U. Stimming, Recent anode advances in solid oxide fuel cells. Journal of

Power Sources, 2007. 171(2): p. 247-260.








Chapter 4

138



6. Gorte, R., J. Vohs, and H. Kim, Novel SOFC anodes for the direct electrochemical

oxidation of hydrocarbon. Journal of Power Sources, 2002. 106(1-2): p. 10-15.

7. Lin, Y., et al., Direct operation of solid oxide fuel cells with methane fuel. Solid State

Ionics, 2005. 176(23–24): p. 1827-1835.

8. Murray, E.P., T. Tsai, and S.A. Barnett, A direct-methane fuel cell with a ceria-based

anode. Nature, 1999. 400(6745): p. 649-651.

9. Finnerty, C.M., et al., Carbon formation on and deactivation of nickel-based/zirconia

anodes in solid oxide fuel cells running on methane. Catalysis Today, 1998. 46(2–3):

p. 137-145.

10. Zhan, Z. and S.A. Barnett, Operation of ceria-electrolyte solid oxide fuel cells on iso-

octane–air fuel mixtures. Journal of Power Sources, 2006. 157(1): p. 422-429.



12. Gorte, R. and S. Mclntosh, Direct Hydrocarbon Solid Oxide Fuel Cells. Chemical

Reviews, 2004. 104(10): p. 4845-4866.

13. Gorte, R.J., J.M. Vohs, and S. McIntosh, Recent developments on anodes for direct fuel

utilization in SOFC. Solid State Ionics, 2004. 175(1–4): p. 1-6.

14. Gorte, R., J. Vohs, and S. Park, Direct oxidation of hydrocarbons in a solid-oxide fuel

cell. Nature, 2000. 404: p. 265-267.

15. Lashtabeg, A. and S.J. Skinner, Solid oxide fuel cells-a challenge for materials

chemists? Journal of Materials Chemistry, 2006. 16(31): p. 3161-3170.

Chapter 4

139

16. He, H., J.M. Vohs, and R.J. Gorte, Effect of Synthesis Conditions on the Performance

of CuCeO2YSZ Anodes in SOFCs. Journal of The Electrochemical Society, 2003.

150(11): p. A1470-A1475.

17. Bi, Z.H. and J.H. Zhu, Cu1−xPdx/CeO2-impregnated cermet anodes for direct oxidation

of methane in LaGaO3-electrolyte solid oxide fuel cells. Journal of Power Sources,

2010. 195(10): p. 3097-3104.




gas treatment and clean energy generation. Energy Environ. Sci., 2016.

20. Sharp, D.H., An overview of Rayleigh-Taylor instability. Physica D: Nonlinear

Phenomena, 1984. 12(1): p. 3-18.

21. Zhentao Wu, Benjamin F.K. Kingsbury, and Kang Li, Microstructured Ceramic

Hollow-Fiber Membranes: Development and Application, in Membrane Fabrication,

Nidal Hilal, Ahmad Fauzi Ismail, and C.J. Wright, Editors. 2015, CRC Press. p. 317-

346.

22. Li, K., J. Kong, and X. Tan, Design of hollow fibre membrane modules for soluble gas

removal. Chemical Engineering Science, 2000. 55(23): p. 5579-5588.

23. Virkar, A.V., et al., The role of electrode microstructure on activation and

concentration polarizations in solid oxide fuel cells. Solid State Ionics, 2000. 131(1–

2): p. 189-198.



768-772.

Chapter 4

140

25. Meng, X., et al., Highly stable microtubular solid oxide fuel cells based on integrated

electrolyte/anode hollow fibers. Journal of Power Sources, 2015. 275: p. 362-369.

26. Yang, N.-T., et al., Fabrication of Anode Supported Micro Tubular SOFCS by Dip-

Coating Process on NiO/YSZ Hollow Fibers. ECS Transactions, 2009. 25(2): p. 881-

888.

27. Yang, N., X. Tan, and Z. Ma, A phase-inversion/sintering process to fabricate

nickel/yttria-stabilized zirconia hollow fibers as the anode support for micro-tubular

solid oxide fuel cells. Journal of Power Sources, 2008. 183(1): p. 14-19.

28. Zhao, F. and A.V. Virkar, Dependence of polarization in anode-supported solid oxide

fuel cells on various cell parameters. Journal of Power Sources, 2005. 141(1): p. 79-

95.

29. Kim, H., et al., Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide

Fuel Cells. Journal of The Electrochemical Society, 2002. 149(3): p. A247.

30. Winkler, W., Chapter 3 - Thermodynamics, in High Temperature and Solid Oxide Fuel

Cells. 2003, Elsevier Science: Amsterdam. p. 53-82.


Reviews, 2004. 104(10): p. 4845-4866.

32. Park, S., R.J. Gorte, and J.M. Vohs, Tape Cast Solid-Oxide Fuel Cells for the Direct

Oxidation of Hydrocarbons. Journal of The Electrochemical Society, 2001. 148(5): p.

A443-A447.

33. Bi, Z.H. and J.H. Zhu, A Cu-CeO2-LDC Composite Anode for LSGM Electrolyte-

Supported Solid Oxide Fuel Cells. Electrochemical and Solid-State Letters, 2009.

12(7): p. B107-B111.

Chapter 4

141

34. Kaur, G. and S. Basu, Physical characterization and electrochemical performance of

copper–iron–ceria–YSZ anode-based SOFCs in H2 and methane fuels. International

Journal of Energy Research, 2015. 39(10): p. 1345-1354.

35. Jung, S., et al., Influence of composition and Cu impregnation method on the

performance of Cu/CeO2/YSZ SOFC anodes. Journal of Power Sources, 2006. 154(1):

p. 42-50.

36. Shaikh, S.P.S., A. Muchtar, and M.R. Somalu, A review on the selection of anode

materials for solid-oxide fuel cells. Renewable and Sustainable Energy Reviews, 2015.

51: p. 1-8.

37. Park, S., et al., Direct Oxidation of Hydrocarbons in a Solid Oxide Fuel Cell: I.

Methane Oxidation. Journal of The Electrochemical Society, 1999. 146(10): p. 3603-

3605.

38. Farrauto, R.J., et al., Catalytic chemistry of supported palladium for combustion of

methane. Applied Catalysis A: General, 1992. 81(2): p. 227-237.

39. Nabae, Y. and I. Yamanaka, Alloying effects of Pd and Ni on the catalysis of the

oxidation of dry CH4 in solid oxide fuel cells. Applied Catalysis A: General, 2009.

369(1–2): p. 119-124.

40. Atkinson, A., et al., Advanced anodes for high-temperature fuel cells. Nat Mater, 2004.

3(1): p. 17-27.

41. Fuerte, A., et al., Effect of cobalt incorporation in copper-ceria based anodes for

hydrocarbon utilisation in Intermediate Temperature Solid Oxide Fuel Cells. Journal

of Power Sources, 2011. 196(9): p. 4324-4331.

42. Sarıboğa, V. and M.A. Faruk Öksüzömer, Cu-CeO2 anodes for solid oxide fuel cells:

Determination of infiltration characteristics. Journal of Alloys and Compounds, 2016.

688(Part B): p. 323-331.

Chapter 5

142

5 CHAPTER 5

Fabrication of novel micro-monolithic anodes for solid oxide fuel cell

Chapter 5 describes the development of novel anode substrates showing improvement in both

mechanical reliability and electrochemical performances. The strategy to obtain highly robust

anode was by employing multi-channel design. Anode substrate with different number of

channels were eventually developed into a complete fuel cell, so-called “micro-monolithic

solid oxide fuel cell” and tested for their electrochemical performance with pure hydrogen (H2).

This work is partially based on author’s manuscript: Li, T., et al., Design of next-generation

ceramic fuel cells and real-time characterization with synchrotron X-ray diffraction computed

tomography. Nature Communications, 2019. 10(1): p. 1497. (Open access under

http://creativecommons.org/licenses/by/4.0/)

https://www.nature.com/articles/s41467-019-09427-z

Abstract

Solid oxide fuel cells (SOFCs) have been accepted as a promising technology for sustainable

energy generation. Geometrically, the micro-tubular design (MT-SOFC) has additional

advantages such as large volumetric surface area, easier sealing, better thermal shock resistance

and good portable characteristics. However, this technology is yet to be commercially applied

owing to some major bottlenecks, such as the lack of a cost-effective manufacturing process

and problematic robustness. In this chapter, a novel micro-structured micro-monolithic anode

for the fabrication of SOFC has been prepared via a phase-inversion-assisted extrusion process.

This method integrates the benefits of micro-structure tailoring and enhanced mechanical

property. An asymmetric structure has been attained for the anode substrate, comprising micro-

channels and a thin sponge-like structure, allowing a more uniform distribution of fuel gases

and reduced concentration polarisation. The fracture load measured via three-point bending

indicated that the resistance towards the external impact is up to 8 times better for multi-channel

http://creativecommons.org/licenses/by/4.0/

https://www.nature.com/articles/s41467-019-09427-z

Chapter 5

143

design than the single-channel counterpart, without compromising gas transport property.

Maximum power density (Pmax) of 1.27 W cm−2 at 800 °C demonstrated by the 7-channel cell

is one of the highest reported values for Ni-YSZ/YSZ/LSM-based tubular SOFCs to date.

5.1 Introduction

Micro-tubular geometry is one of the design types of solid oxide fuel cells (SOFC) that has

been introduced in the early 1990’s by Prof. Kendall from the University of Birmingham [1].

Along with the typical benefits of SOFC including high efficiency, fuel flexibility and being

environmentally friendly, such design could offer additional distinctive features. For instance,

better thermal shock resistance, rapid start-up and shut down, and improved volumetric power

density. All these advantages have resulted in an increasing level of interest to utilise micro-

tubular design. However, there still exist several concerns that impede the scale-up of micro-

tubular systems which include lack of a cost-effective manufacturing route, difficult current

collection and relatively low mechanical robustness compared to the conventional tubular and

planar counterparts.

Compared to ram-extrusion-based process, phase-inversion-assisted extrusion for making

ceramic tubes offers more flexibility and controllability over morphologies and micro-

structures tailoring [2, 3], as has been ascertained by previous studies [4-7]. This alternative

method of producing ceramic micro-tube has been reported to present positive impacts on the

cell performances, mainly due to its asymmetric structure [8]. Such structure consists of self-

organised micro-channels and sponge-like region that provide reactuve site for electrochemical

reactions whereby the radial micro-channels have a major role in lowering the permeation

resistance to gas [2]. However, they would compromise the membrane's mechanical property

since the sponge-like region is responsible for providing robustness of the micro-tube; the

smaller the sponge-like region, the weaker the micro-tube [3]. Regarding mechanical strength,

it is one of the main criteria required for single-cell handling and cell stack assembling. If MT-

Chapter 5

144

SOFCs are to be applied in circumstances where mechanical shocks, vibration or habitual start-

ups or shut-down exist, the optimisation of the tube’s mechanical property will be

indispensable.

Relatively low mechanical strength is the major challenge associated with micro-tubular

SOFC. One strategy for improving the resistance of the micro-tube towards external impacts

is by having a large cross-sectional area [8]. This could be achieved either using a single

channel configuration with a larger cross-sectional area by increasing the diameters or the wall

thickness. By having a larger diameter or thicker wall for single channel micro-tube appears to

be relatively straightforward, but, there are some challenges during the phase-inversion

fabrication process. The precipitation rate of the nascent fibre decreases rapidly once the

thickness, or the diameter exceeds a certain value causing the suspension to not be able to

solidify in time [8]. Consequently, severe elongation due to the gravity will lead to failure in

obtaining the preferred dimension of the micro-tube. Therefore, the diameter and the thickness

of micro-tubes are restricted. Although larger micro-tubes could be prepared, it is difficult to

achieve good reproducibility and it will compromise the micro-tube structure, i.e. surface area

to volume ratio.

Another approach to obtain robust micro-tube is by implementing a multi-channel design.

Multi-channel ceramics, sometimes known as ceramic honeycomb, have been widely applied

in industries, usually as a catalyst substrate and for filtration purposes. A common feature of

honeycomb structure consists of an array of hollow channels, which have various shapes

including rectangular, cylindrical or hexagonal, to be arranged between thin vertical walls.

There have been several attempts to utilise honeycomb ceramic for SOFC application, either

as electrolyte- or electrode-supported cell (as listed in Table 5.3). However, the ceramic body

derived from conventional fabrication routes, such as paste extrusion and injection moulding,

is of a symmetric structure (sponge-like), leading to considerable mass and heat transfer

Chapter 5

145

resistance, thus, poor electrochemical performances [2]. Therefore, a phase-inversion process

appears to be a promising way in producing micro-tubes that could be tailored into having

appropriate micro-structure suitable for the SOFC application.

This chapter reports on the development a new “micro-monolithic” design that integrates the

benefits from phase-inversion process with the multi-channel structural design. Anode

substrates with different number of channels were successfully fabricated, characterised and

compared to the normal single-channel counterpart. Eventually, the cells were tested for their

electrochemical performance using hydrogen (H2) and air as fuel and oxidant, respectively.

5.2 Experimental

5.2.1 Materials

Yttria-stabilised zirconia (8 mol% YSZ), nickel oxide (NiO) and lanthanum strontium

manganite (La0.8Sr0.2MnO3−δ) were obtained from Inframat Advanced Materials (USA). 30-

dipolyhydroxystearate (Arlacel P135, Uniqema), Polyethersulfone (PESf) (Radal A300,

Ameco Performance, USA), dimethyl sulfoxide (DMSO, HPLC grade, VWR, UK) and ethanol

(HPLC grade, VWR, UK) were used as the dispersant, polymer binder, and solvent,

respectively. De-ionised (DI) water was applied as both internal and external coagulant.

5.2.2 Fabrication of anode substrate

The micro-monolithic anode support was prepared by a phase-inversion assisted method,

which is similar to a conventional micro-tube (hollow fibre), as described in previous chapter.

A suspension consists of ceramic particles, solvent and polymer binder was mixed for 4 days

using planetary ball milling (SFM-1 Desk-top Miller, MTI Corporation, USA) to achieve

appropriate homogeneity. The miller was operated at 263 rpm with 40 yttria-stabilised zirconia

(YSZ3) milling balls (10 mm diameter) and the details of the dope suspension composition

were listed in Table 5.1. Similar to the fabrication of a single channel micro-tube described in

the previous chapters, the NiO-YSZ dope suspension was first degassed under vacuum with

Chapter 5

146

stirring for at least 2 hours to ensure air bubbles trapped inside were completely removed before

being transffered into a stainless steel syringe. The spinning process is controlled by the syringe

pumps (Harvard PHD22/200 HPsi) for precision over extrusion rates of both dope suspension

and internal coagulant (bore fluid).

The internal coagulant was de-ionised (DI) water, which is split into required number of

streams through a custom-designed nozzle before in contact with the dope suspension to form

the multi-channel structure. The precursor substrates were left in the external coagulant bath

overnight and taken out after the phase-inversion is completed. Subsequently, they are

straightened and allowed to dry at room environments before sintering step. Figure 5.1 shows

the spinning set-up and spinneret design. It is noteworthy that the different types of a spinneret

having different nozzle numbers could be easily changed.

Table 5.1: Composition of the spinning suspension and fabrication and sintering conditions.

Single

channel

3-channel 4-channel 7-channel

Suspension

composition

(wt.%)

Ceramic-metal 63.0 (NiO:YSZ = 3:2)

Solvent 28.0

8.4

0.6

PESf

Dispersant

Extrusion rate

(ml min−1)

8 8 8 8

Fabrication

conditions

Bore fluid rate

(ml min−1)

6 7 8 10

Air gap (cm) 1

Sintering

conditions

Temp. (°C) 1400, 1450 and 1500

Dwelling time (h) 6

Chapter 5

147

Figure 5.1: Schematic diagram of (a) spinning set-up and (b) changeable multi-nozzle spinneret

designs.


Systematic characterisations have been performed which includes scanning electron

microscopy (SEM) imaging, mechanical strength, gas permeation and electrochemical

performance test. The morphology of the ceramic micro-tubes was observed with SEM (JEOL

JSM-5610). Samples were gold-coated under vacuum at 20 mA for 60 seconds (EMITECH

Model K550) before analysis, and the images were obtained at various magnifications. The

gas-tightness of the sintered electrolyte layer was assessed by nitrogen (N2) leakage test. The

mechanical strength was assessed by a three-point bending test using a tensile tester (Instron

Model 5544) with a load cell of 1 kN. 5 specimens were tested for each type of sample. The

detailed experimental procedures for the gas-tightness measurement and mechanical testing

have been described in Section 3.2.4 in Chapter 3.

Mean flow pore size and gas transport property was investigated via N2 permeation tests at

room temperature by gas-liquid displacement method using a bubble point porometer

(PoroluxTM 1000, POROMETER nv, Belgium) and a specific wetting liquid, Porefil

Chapter 5

148

(POROMETER nv, Belgium). Reduced samples were sealed into the system using epoxy resin

and the permeation flux of N2 was recorded with increased pressure difference. The gas

permeance was subsequently calculated using the following equation:

𝑃 =𝑄

𝜋𝐿𝐷𝑂 . ∆𝑝

(5.1)

Here P denotes the permeability of N2 (mol m-2 s-1 Pa-1), L and Do represent length and the

outer diameters of the micro-tube (m), respectively, and ∆𝑝 is the pressure difference across

the micro-monolithic anode (Pa). The mean flow pore size was calculated using Young-

Laplace’s equation:

𝑟𝑝 =2𝛾

∆𝑝𝑐𝑜𝑠𝜃

(5.2)

Here 𝑟𝑝 denotes the pore radius (m) behaving as gas paths and contributing to the gas flow at

each operating pressure; 𝛾 represents surface tension of the wetting liquid 16 mN m−1); θ is the

contact angle of the wetting liquid on the membrane surface, which is 0°. The mean flow pore

size is obtained from the intersection between half-dry curve corresponding to 50% gas flow

through the dry membrane sample and the wet curve. The half dry curve is computed as half

of the dry curve.

5.2.4 Fabrication of a single cell and reactor assembly

A complete single cell was constructed prior to the electrochemical performance test. YSZ

electrolyte was dip-coated onto green anode precursor and dried at room temperature. The

composition of the coating ink is similar to the one applied in Chapter 3 (Table 3.1). After they

dried, NiO-YSZ/YSZ (anode/electrolyte) co-sintering step has been undertaken at 1450 °C for

6 hours to properly densify the electrolyte layer. A dual-layer cathode with a length of 10 mm

was subsequently dip-coated onto sintered anode/electrolyte half-cell. The inner layer was

composed of YSZ and LSM (YSZ/LSM=50/50 by weight) and the outer layer was pure LSM.

Chapter 5

149

After both layers have been dried, a sintering step was undertaken at 1000 °C for 2 hours with

a heating rate of 5 °C min−1. The total thickness of the cathode is about 30 µm.

The current collection of the cathode was prepared by wrapping silver (Ag) wire (Advent

Research Materials Ltd, UK) on cathode surface with a small amount of Ag paste added onto

the surface to enhance the contact between cathode and wire. For current collection at the

anode, silver wire was coiled at the exposed anode substrate with additional Ag paste to

improve the adhesion. The single cell was fixed into two gas-tight alumina tubes (Almath

Crucibles Ltd, UK) and sealed with a ceramic sealant (Aremco, Ceramabond 552-VFG). Both

wires from the electrodes were connected to a potentiostat/galvanostat (Model: IviumStat,

Ivium Technologies B.V., Netherlands) for current-voltage (I-V) measurement and

electrochemical impedance spectroscopy (EIS) analysis.


The electrochemical performance test was conducted at a temperature ranging from 650 to 800

°C by feeding 30 ml min−1 of pure H2 to the anode whereas the cathode was exposed to ambient

air. Electrochemical impedance spectra (EIS) was measured under open circuit voltage (OCV)

in the frequency range of 10−1 to 105 Hz with a signal amplitude of 10 mV. To ensure the cells

repeatability, at least three identical cells have been tested with standard deviation of less than

10 %.

Chapter 5

150


5.3.1 Morphology

Figure 5.2: SEM images of sintered NiO-YSZ anode substrate; (a)–(d) anodes with different

number of channels, (e)–(g) magnified anode region showing sponge-like structure and micro-

channels, (h) anode with YSZ electrolyte.

Chapter 5

151

Figure 5.2 (a-d) show the SEM images of anode substrate with a different number of channels.

The anode substrate with multi-channels structure showed relatively uniform channels, which

are organised in the same configuration as the design of spinnerets. It can be perceived that as

the number of channel increases, there is an apparent increase in outer diameter, from 1.96 ±

0.02 mm of single-channel to 2.50 ± 0.05 mm of 7-channel, even though the spinnerets used

have the identical outer diameter. This could have been due to the splitting of the internal

coagulant which resulted in a more even distribution, and consequently faster precipitation.

Therefore, the elongation effect from the gravity of the nascent precursor has been inhibited.

Table 5.2 listed the outer diameter (Do) and channel diameter for different micro-tubes with

different number of channels.

Table 5.2: Dimensions of single-channel and multi-channel samples.

Do (mm) Channel diameter (mm)

Single-channel 1.96 ± 0.02 1.65 ± 0.01

3-channel 2.10 ± 0.05 0.67 ± 0.03

4-channel 2.30 ± 0.04 0.70 ± 0.02

7-channel 2.50 ± 0.05 0.69 ± 0.03

The increment in diameter with increasing number of channels signifies the effectiveness of

such multi-channel design in enlarging the cross-sectional area of the tube. However, it is

noteworthy that the more the channels, the harder it was to retain a uniform dimension for every

channel. This issue requires considerable attention as a constant and uniform dimension is

preferred to obtain consistent cell dimensions for stack assembling. Several trials have been

attempted to achieve this, mainly by adjusting the bore fluid extrusion rate in ensuring stable

spinning process as well as maintaining uniform shape of the channels to be formed.

Chapter 5

152

Figure 5.2 (e-g) presents the micro-structure of the cross-section in which micro-channels and

sponge-like structure could be observed in both the multi-channel anodes and single-channel

counterpart. A plurality of micro-channels, which are distributed around each channel, were

formed during the phase-inversion process as a result of solvent/non-solvent exchange and well

preserved throughout sintering stage, as described in earlier works [2, 3]. The presence of such

micro-channels has been reported to help improve fuel gas mixing and thus, leading to better

and more uniform fuel gas distribution. The sponge-like region, on the other hand, is the main

contributor to triple-phase boundary for electrochemical reactions and present mechanical

robustness to support the cell.

Figure 5.3: Microscope images showing different sponge-region thickness for (a) 3-channel

and (b) 7-channel anode substrate, respectively.

A short air gap of 1 cm applied during the spinning process did not provide enough time for

the solvent to exchange with the bore fluid (internal coagulant), leading to simultaneous phase-

inversion processes from multi-directions and thus the formation of the micro-channels near

the exterior surface and throughout the entire cross-section. It can also be observed in Figure

5.3 that the multi-channel anodes displayed an interesting feature, which is the changeable

thickness of the sponge-like region, with the thinnest part having a thickness of approximately

Chapter 5

153

40-46 µm. This thin sponge region could function as the short path for gas diffusion and might

as well reduce the mass transfer resistance.

After NiO was reduced to Ni in hydrogen (H2), the mean flow pore size of various micro-

monolithic anode substrates was tested using a gas-liquid displacement porosimeter. It should

be noted that only the neck size of open pores could be determined via this method. Different

micro-monolithic samples had a similar mean flow pore size of approximately 0.33 ± 0.02 µm

in the sponge region, comparable to their conventional single-channel counterparts.

5.3.2 Gas permeation and mechanical property

Figure 5.4: Nitrogen (N2) gas permeance for anode with a different number of channels.

The N2 gas permeation analysis as presented in Figure 5.4 indicated that all reduced anode (Ni-

YSZ) substrate showed an adequate gas permeation values which increase with increasing

number of channel.

Chapter 5

154

Such observation could be associated with the anode substrate with more channel exhibit higher

percentage of thinner sponge-like region (shown in Figure 5.5), leading to a better gas transport

properties. This could also mean potentially more efficient transportation of fuel gases into the

reaction zone, known as the active triple-phase boundary (TPB), a region where electrode,

reactants and electrolyte meet. Such region has been described to be located near the

electrode/electrolyte interface, extending approximately 10-20 µm from the electrolyte [9]. The

presence of finger-like voids as well as thinner sponge-like structure would lead to better gas

permeability within the anode, allowing easier passage of gaseous fuel towards such region for

the electrochemical reactions to occur.

Figure 5.5: Schematic diagram of the gas diffusion pathway; (left) conventional single-channel

anode and (right) 3-channel anode.

Chapter 5

155

Mechanical strength is another fundamental requirement for SOFC, not only for a single-cell

handling but also for stack assembling. Long and slim micro-tubes are most likely to be

fractured by a shear force applied perpendicularly to the longitudinal direction, and this is most

vulnerable for brittle materials like ceramics. Therefore, the resistance towards a bending force

is important for ceramic micro-tubes. Such resistance can be estimated by three-point bending

tests which indicate the maximum bending force needed to break a ceramic sample.

In general, enhancement in gas transport may result in decreased mechanical strength, which

is one of the essential properties required for the single-cell handling, stack assembling and

long-term stability. Despite various sources of stress, such as axial compression, diametrical

compression or bending, it is thought that bending is more suitable to mimic the type of stress

that stacks may experience from vibration in vehicular applications, as the high-aspect ratio

micro-tubes are extremely fragile against a longitudinally-directed force. Therefore,

mechanical strength was characterised by the three-point bending method; results are presented

in Figure 5.6.

It is apparent that higher sintering temperature increased bending loads prior to fracture for

both micro-monolithic anode supports and the conventional single channel designs. However,

with increasing sintering temperature, porosities or gas permeation rates would decrease

accordingly, requiring optimisation.

Chapter 5

156

Figure 5.6: Fracture force (N) for different types of anode substrates.

Instead, employing micro-monolithic design appears to be more attractive. Figure 5.6 shows

the average fracture force (N) measured for five sample for each anode design with standard

deviation of less than 10%. It showed micro-monolithic anodes to sustain significantly greater

fracture forces than the conventional, single-channel micro-tubes. For instance, at 1450 °C

which is a temperature used for YSZ electrolyte densification in this study, the bending load

was improved from about 3 to 13 N for a 3-channel anode and 3 to 24 N for a 7-channel anode.

This demonstrated the effectiveness of the micro-monolithic geometry in resisting the bending

load, which is the most detrimental type of stress for high-aspect ratio micro-tubes. After

calculating the cross-section area, the bending strength values of ca. 210 ± 20 MPa for micro-

monolithic anodes were found to be comparable with that for the single-channel geometry.2

2 Bending strength for different samples were calculated with the assistance of Dr. Bo Wang, post-doctoral

associate in our research group.

Chapter 5

157


Current density-voltage (j-V) and current density-power density (j-P) curves of cells with

various designs are shown in Figure 5.7, with the anode fuelled by 30 ml min−1 of pure

hydrogen (H2) whereas the cathode is exposed to the ambient air. The measured open circuit

voltages (OCVs) for various designs are between 1.15 and 1.18 V in the temperature range

650-800 °C, which are relatively close to the theoretical values, signifying proper gas-tightness

achieved by the 5 µm electrolyte (Figure 5.2h) and appropriate cell sealing.

Figure 5.7: j-V and j-P curves at various temperatures for different micro-monolithic cells.

From Figure 5.7, excellent power densities have been achieved for micro-monolithic cells. The

maximum power densities (Pmax) of single-channel cell were 0.07, 0.14, 0.31 and 0.58 W cm−2

at 650, 700, 750 and 800 °C, respectively. All three types of micro-monolith design have

Chapter 5

158

showed the capability in achieving much better electrochemical performances than the single-

channel counterpart. In general, at same operating temperatures, an increase in the number of

channels leads to a higher maximum power density. For instance, the maximum power

densities achieved at 800 °C were 0.67, 1.05 and 1.27 W cm−2 for 3-channel, 4-channel and 7-

channel designs, respectively, corresponding to improvements of 16 %, 81 % and 119 %,

respectively. These increases are in good agreement with the nitrogen permeation results (as

shown in Figure 5.4), which validated the effectiveness of micro-monolithic anode in reducing

gas transport resistance. Maximum power density of 1.27 W cm−2 achieved by the 7-channel

cell is by far one of the highest ever reported for Ni-YSZ/LSM-based tubular SOFCs.

In addition, excellent long-term stability of the design was demonstrated by dwelling the best-

performing, 7-channel cell at 750 °C and 0.7 V. From Figure 5.8, there is no observable

degradation (< 1%) in current density after 200 hours of operation. Considering the significant

increase in mechanical properties, an encouraging combination of better mechanical robustness

and high-performance has been obtained using such micro-monolithic anode design.

Figure 5.8: Stability test using 7-channel cell design under constant 0.7 V at 750 °C.

0 50 100 150 200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Curr

ent density (

A c

m−2)

Time (h)

Current density

Power density

0.0

0.2

0.4

0.6

0.8

1.0P

ow

er

density (

W c

m−2)

7-channel cell at 0.7 V and 750 C

Chapter 5

159

The influence of number of anode channels on electrochemical performance can be explained

further by electrochemical impedance spectroscopy (EIS), as shown in Figure 5.9. Each

impedance spectra are composed of two major arcs and the low-frequency arc between 1 Hz

and 100 Hz represents concentration polarisation. While the shape of the low-frequency arcs

was relatively independent of operating temperature, the high-frequency arc, due to activation

polarisation, reduced significantly as the temperature was increased, which is in good

agreement with literatures [10]. It is noted that, in all cases, there is no x-axis intercept at the

high-frequency region which is corresponds to the of ohmic resistance (RΩ). Nevertheless, it

can be estimated from the spectra that the ohmic resistance is relatively small compared to

other losses.

In addition, the impedance, corresponding to activation polarisation remains basically the same

for different multi-channel designs, whereas the concentration polarisation was 1.10, 0.75 and

0.45 Ω cm2 for 3-channel, 4-channel and 7-channel designs, respectively. The concentration

polarisation of 7-channel design is more than halved than that of 3-channel, which might be

due to the difference in sponge-layer thickness, as shown in Figure 5.3. It could be observed

that, despite the channels being not entirely uniform, the thickness of the thinner sponge-like

regions for 7-channel design ranges between 40-80 µm, while the value for 3-channel design

is much greater, up to 130 µm. For the single-channel anode, the fuel gases must diffuse

through a relatively thick sponge to reach the active area, leading to larger concentration

polarisation compared to those of multi-channel design.

Chapter 5

160

Figure 5.9: Effects of number of channels of micro-monolithic SOFCs and temperature on

electrochemical impedance spectra (10−2 to 105 Hz) at open circuit.

As described in the literature, the actual region wherein the triple-phase boundary (TPB) is

regarded as an active zone extends no more than 10-20 µm from the anode/electrolyte interface

[9]. Therefore, the majority region in the anode remains inactive and functions as mechanical

support and current collector. As for the 7-channel design, those thinner sponge-like areas

could be considered as highly-active zone for electrochemical reactions with adequate fuel inlet

flow, whilst the geometry of the total cross-section provides suitable mechanical properties to

the whole cell.

Chapter 5

161

Table 5.3: Comparison of volumetric power density with reported honeycomb-SOFCs.

Temperature

(°C)

Volumetric power density

(W cm−3)

Ref.

This

work

Single

800

9.6

N/A

3-channel 10.4

4-channel 14.9

7-channel 17.0

Electrolyte

Planar

SOFC

N/A N/A 3.0 [11]

N/A 650 10.0 [12]

YSZ 850 9.8 [13]

Honey-

comb-

SOFC

*LSGM 800 0.50 [14]

YSZ 850 0.29 [15]

YSZ 850 0.35 [16]

YSZ 900 0.78 [17]

LSGM 800 0.60 [18]

YSZ 1000 0.16 [19]

YSZ 850 0.39 [20]

*LSGM: lanthanum strontium gallium magnesium oxide

The miniaturisation and introduction of monolithic structure in the anode support kept the cell

dimension within the “micro-tubular” range, which was essential for a high volumetric power

density. The maximum volumetric power density that micro-monolithic SOFCs could achieve

was estimated, based on the closest triangle packing where the results are presented in Table

Chapter 5

162

5.3. Compared to the reported honeycomb-SOFCs with cross-sectional geometries of ca. 1 cm,

the micro-monolithic SOFC with YSZ electrolyte presented considerably greater volumetric

power densities, for instance, the 7-channel geometry had more than one order of magnitude

performance enhancement (17 W cm−3). In general, all micro-monolithic SOFC geometries

showed superior volumetric power densities compared to planar SOFC stacks, which are

currently considered to deliver the greatest performances, with the 7-channel cell’s

performances nearly twice that of planar counterparts. However, it is worth mentioning that

these lab-scale electrochemical performance results will not scale linearly with length. For an

SOFC stack, the performance loss due to the contact resistance (interconnector) is unavoidable

and could lower the performance by up to 50% compared to the single cell’s performance.

Nevertheless, findings from this study indicates that micro-monolithic geometric design offers

great potential to be competitive with conventional planar design in terms of electrochemical

performance.

5.4 Conclusion

In summary, a holistic strategy has been attempted to produce high-performance SOFC by

applying a micro-monolith geometry for the anode support. Anode micro-tubes with 3, 4, and

7 channels have been fabricated via a phase-inversion-assisted extrusion process. Systematic

characterisations have been performed on these various micro-monolithic designs and

compared to the single-channel counterpart. It could be concluded that this multi-channel

design is efficient to improve the mechanical property. The bending load that the micro-tube

could withstand improved by 4-8 times with the increase in number of channel while the

bending strength remains comparable.

The split of internal coagulant during the fabrication process greatly influence the precipitate

rate and thus, the cross-section morphology. Given by the important feature of non-uniform

sponge thickness, the micro-monolith design showed better gas transport properties whereas

Chapter 5

163

the mean flow pore size remains the same. Such micro-structural improvement led to high-

quality electrochemical performances, with all three multi-channels design demonstrated much

higher maximum power density than the single-channel counterpart. The 7-channel achieved

the highest power density of 1.27 W cm−2 at 800 °C. The impedance spectroscopy confirmed

the effectiveness in realising more efficient gas transport by showing much smaller

concentration polarisation from 7-channel design. This study has shown that micro-monolithic

SOFCs exhibit both decent mechanical property and outstanding electrochemical

performances.

5.5 References



2. Jamil, S.M., et al., Recent fabrication techniques for micro-tubular solid oxide fuel cell

support: A review. Journal of the European Ceramic Society, 2015. 35(1): p. 1-22.

3. Lee, M., B. Wang, and K. Li, New designs of ceramic hollow fibres toward broadened

applications. Journal of Membrane Science, 2016. 503: p. 48-58.

4. Othman, M.H.D., High performance micro-tubular solid oxide fuel cell, in Department

of Chemical Engineering. 2011, Imperial College London. p. 227.

5. Othman, M.H.D., et al., Dual-layer hollow fibres with different anode structures for

micro-tubular solid oxide fuel cells. Journal of Power Sources, 2012. 205: p. 272-280.







Chapter 5

164

8. Zhentao Wu, B.F.K.K., and Kang Li, Chapter 10 Microstructured Ceramic Hollow-

Fiber Membranes: Development and Application, in Membrane Fabrication, A.F.I.

Nidal Hilal, and Chris J. Wright, Editor. 2015, CRC Press. p. 317-346.




10. Brown, M., S. Primdahl, and M. Mogensen, Structure/performance relations for

Ni/Yttria-stabilized Zirconia anodes for solid oxide fuel cells. Journal of the


11. FCO Power develops high power density, low-cost SOFC stack. Fuel Cells Bulletin,

2013. 2013(11): p. 10.

12. Wachsman, E.D. and K.T. Lee, Lowering the Temperature of Solid Oxide Fuel Cells.

Science, 2011. 334(6058): p. 935-939.

13. Cable, T.L. and S.W. Sofie, A symmetrical, planar SOFC design for NASA's high

specific power density requirements. Journal of Power Sources, 2007. 174(1): p. 221-

227.

14. Zhong, H., et al., Ag current collector for honeycomb solid oxide fuel cells using

LaGaO3-based oxide electrolyte. Journal of Power Sources, 2009. 186(2): p. 238-243.

15. Kotake, S., H. Nakajima, and T. Kitahar, Flow Channel Configurations of an Anode-

Supported honeycomb SOFC. ECS Transactions, 2013. 57(1): p. 815-822.

16. Fukushima, A., H. Nakajima, and T. Kitahara, Performance evaluation of an anode-

supported honeycomb solid oxide fuel cell. ECS Transactions, 2013. 50(48): p. 71-75.

17. Ruiz-Morales, J.C., et al., Performance of a novel type of electrolyte-supported solid

oxide fuel cell with honeycomb structure. Journal of Power Sources, 2010. 195(2): p.

516-521.

Chapter 5

165

18. Zhong, H., et al., Self-supported LaGaO3-based honeycomb-type solid oxide fuel cell

with high volumetric power density. Solid State Ionics, 2008. 179(27-32): p. 1474-1477.

19. M. Wetzko, et al., Solid oxide fuel cell stacks using extruded honeycomb type elements.

Journal of Power Sources, 1999. 83(1-2): p. 148-155.

20. Nakajima, H., et al., Three-dimensional flow channel arrangements in an anode-

supported honeycomb solid oxide fuel cell. Heat and Mass Transfer, 2017. 54(8): p.

2545-2550.

Chapter 6

166

6 CHAPTER 6

Micro-monolithic SOFC for extended application

This chapter discusses on SOFC application for both hydrogen (H2)-fuel cell and carbon

dioxide (CO2) electrolysis using a novel design of 6-channel micro-monolithic cell.

This chapter is based on author’s manuscript: Rabuni, M.F., et al., High performance micro-

monolithic solid oxide fuel cell (SOFC) for extended application: CO2 electrolysis. To be

submitted to Journal of Power Sources soon.

Abstract

Reversible solid oxide electrochemical reactors should work efficiently in both fuel cell and

electrolysis modes in order to be considered as a practical technology for the energy field. In

addition to improved performance, excellent electrode reversibility and stability for long-term

operation are crucial for such reactors. Herein, high-performance 6-channel solid oxide

electrochemical reactors for reversible operation has been successfully developed using a

phase-inversion and sintering method. A unique morphology has been obtained where micro-

channels were formed from multiple directions and the interchangeable thickness of sponge-

like region between each channel and the exterior surface. Such micro-structured cells exhibit

superior performances for hydrogen (H2) fuel cell achieving 1.62 W cm−2 at 800 °C. Similarly,

excellent performance for carbon dioxide (CO2) electrolysis has been demonstrated, achieving

current densities up to 6.3 (3.1) A cm−2 under 1.8 (1.5) V at 800 °C. To the best of our

knowledge, such high performances during the H2-fuel cell and CO2 electrolysis are the highest

reported values using a common Ni-YSZ-based cell. This outstanding performance, coupled

with superior mechanical robustness, promises a long-awaited alternative to the single-channel

counterpart that would allow micro-tubular system to be commercially applied in the near

future.

Chapter 6

167

6.1 Introduction

A solid oxide electrochemical reactor (SOER) is a type of electrochemical devices that has

been intensively explored in the field of sustainable energy generation and storage [1, 2]. Its

main advantage lies in the flexibility to allow reversible operations using the same cell. A

reversible (R)-SOER is capable of working in both fuel cell and electrolysis modes. In fuel cell

mode (SOFC), electricity is produced by electrochemical combination of a fuel (i.e. hydrogen,

hydrocarbons, etc.) with oxygen. In the electrolysis mode, in which external energy is supplied,

it functions as a solid oxide electrolyser (SOEC), producing hydrogen (H2) from steam (H2O)

electrolysis, carbon monoxide (CO) from carbon dioxide (CO2) electrolysis or syn-gas (CO-H2

mixture) from co-electrolysis of both steam and CO2 [1, 2].

There are great prospects for CO2 electrolysis with renewable energy, as it could facilitate

progress of such energy sources by offering an economical approach of distributing such

energy to end users. SOEC has been demonstrated to effectively split CO2 electrochemically

into CO and O2 in earlier studies [1-4]. Due to its operational flexibility, R-SOER is particularly

attractive for allowing the use and storage of surplus renewable energy under electrolysis mode

and the stored energy (H2 or CO) could then be reconverted into electricity under fuel cell mode

whenever demand increases. To make CO2 electrolysis through high-temperature electrolysis

feasible for practical operations, low power demand and high gas productivity must be

achieved [5, 6]. The former requires minimal internal electrochemical resistance of the cells,

whereas the latter is governed by a CO2 conversion rate. All of these factors are largely

influenced by the cell’s materials and properties as well as its structural design. In essence, the

reactions for carbon dioxide (CO2) electrolysis can be represented as follows [1, 2]:

Fuel electrode

(e. g. Ni-YSZ)

: CO2 + 2e− → CO + O2− (6.1)

Chapter 6

168

Electrolyte

(e. g. YSZ)

: O2−(fuel electrode) → O2−(oxygen electrode) (6.2)

Oxygen electrode

(e. g. LSM-YZ)

: 12⁄ (2O2− → O2 + 4e−) (6.3)

Overall : CO2 → CO + 12⁄ O2 (6.4)

There have been several studies on CO2 electrochemical reduction to CO using planar cell

design developed from different fuel electrode materials, including the conventional Ni-cermet

based electrode or infiltrated scaffold [5-7]. In contrast, relatively few studies of CO2

electrolysis with micro-tubular design are available in the literature [2]. In this study, a novel

6-channel micro-tubular solid oxide cell or named as micro-monolithic cell have been

developed using a standard material via a phase inversion and sintering technique. Such

fabrication method presents advantages of micro-structure tailoring in which the distinctive

structure for the fuel electrode could be made, ultimately leading to outstanding

electrochemical performances. Micro-monolithic cells were first tested in fuel cell mode using

pure H2 and subsequently studied for their CO2 electrolysis performance. In addition, the

reversibility of these cells was examined by short-term cycling between electrolyser and fuel

cell modes.

6.2 Experimental

6.2.1 Materials

Nickel oxide (NiO) with a mean particle size of 0.5−1.5 µm (d50) and 8 mol % yttria-stabilised

zirconia (YSZ8) powders with a mean particle size of 0.5−1.5 µm (d50) were purchased from

Fuelcell materials, USA, and Inframat Advance Materials, USA, respectively. Both materials

were used as fuel electrode materials. Polyethylene glycol 30-dipolyhydroxystearate (Arlacel

P135), N-methyl-2-pyrrolidone (NMP, MW = 99.13 g mol−1, Merck, Germany) and polymethyl

Chapter 6

169

methacrylate (PMMA, Radel A-300, Ameco Performance, Greenville, SC) were used as the

dispersant, solvent, and polymer binder for spinning suspension, respectively. De-ionised (DI)

water was applied as external coagulant and mixed with ethanol (ethanol/DI water=50/50 by

weight) (VWR International LLC, UK) for internal coagulant (bore fluid). YSZ powder

(particle size of ~700 nm), polyvinyl butyral (PVB, Butvar), and polyethylene glycol (MW =

950-1050) were purchased from Sigma Aldrich (UK) and homogenously mixed in ethanol prior

to being applied as the electrolyte ink. The first layer of oxygen electrode materials comprised

of YSZ (Inframat Advance Material, USA) and La0.8Sr0.2MnO3−δ (LSM, d50=1.1 µm, Praxair

Surface Technologies, USA), and the outer layer was of pure LSM.

Silver wire (99.99 % purity, temper annealed with 0.25 mm diameter, Advent Research

Materials, UK) was applied as current collector, together with the silver conductive adhesive

paste (Alfa Aesar, USA) to improve contact. To assemble the single-cell reactor, ceramic

cement (Ceramabond 552-VFG) obtained from Aremco, USA, was used as a sealant.

6.2.2 Fabrication of fuel electrode

The phase-inversion based extrusion technique has been used to fabricate NiO-YSZ substrate

(fuel electrode). The spinning suspension was first prepared from NiO, YSZ, dispersant

(Arlacel P135) and solvent (NMP). The suspension was mixed for 60 hours using a planetary

ball miller (MTI Corporation model SFM-1 Desktop Planetary Ball Miller). Afterwards, the

polymer binder (PMMA), was added in this suspension and milled for another 60 hours to

obtain a homogeneous suspension.

Details of the compositions for suspension, electrolyte ink and oxygen electrode are

summarised in Table 6.1.

Chapter 6

170

Table 6.1: Compositions of spinning suspension, electrolyte ink and oxygen electrode.

Materials Values (wt. %)

Spinning suspension

NiO powder

37.8

YSZ powder 25.2

Solvent – NMP 28.0

Polymer binder – PMMA 8.4

Dispersant – Arlacel P135 0.6

Electrolyte ink

YSZ powder (~700 nm) 24.0

Ethanol 71.9

Arlacel P135 1.2

Butvar 1.9

Polyethylene glycol 1.1

Oxygen electrode

Inner layer

YSZ 25

LSM 25

Ethylene glycol 50

Outer layer

LSM 50

Ethylene glycol 50

Chapter 6

171

To prevent defects in the micro-monolith, air bubbles were completely removed from the

suspension through degassing process, carried out by stirring under vacuum condition for at

least two hours. In extrusion process, the spinning suspension and internal coagulant, which

was composed of de-ionised (DI) water-ethanol (50 wt. % each), were transferred to two

stainless steel syringes. Then, they were pumped synchronously through the custom-designed

spinneret, extruding the micro-monolith vertically downwards into the non-solvent coagulant

bath containing DI water with zero air gap (i.e. spinneret was slightly immersed in the bath).

During phase inversion, the solvent/non-solvent exchange took place, causing the precipitation

of the polymer in the suspension which consolidate the ceramic material together to form the

precursor of micro-monolith. Upon complete precipitation by immersing in DI water overnight,

they were straightened at room temperature and cut to the desired length. Figure. 6.1 shows the

procedure involved during the preparation of a complete single cell.

Figure 6.1: (a) in-house made spinneret and (b) schematic diagram of the steps involved in the

overall fabrication of cell components of micro-monolithic SOFC.

Chapter 6

172


A high-resolution scanning electron microscope (SEM) (LEO Gemini 1525 FEGSEM) was

used to observe the micro-monolith’s morphology. A sample was mounted onto a metal sample

holder, followed by coating with gold under vacuum for 1.5 minutes at 20 mA (EMITECH

Model K550). SEM images at various magnifications were obtained.

The micro-monolith’s gas tightness characteristic was assessed using a nitrogen (N2)

permeation test using a similar setup as reported elsewhere [8, 9]. The micro-monolith, which

is sealed on the one end, was fixed to a sample holder using epoxy resin, and then test vessel

was pressurised with N2, reaching the initial pressure at around 50 psi. The changes in pressure

were monitored for 8 hours. The calculation for the N2 permeance of the tested sample was

carried out using the equation 3.1 and 3.2 [8, 9].

The mechanical strength of micro-monolith was assessed by a three-point bending method,

using a tensile tester (Instron Model 5544) with a loaded cell of 1 kN. The sample was cut into

5 cm in length before being placed on sample holders with a distance of 3 cm. 5 samples were

tested with standard deviation of less than 10%.

6.2.4 Fabrication of a complete single cell

The fuel electrode (NiO-YSZ) substrate was first pre-sintered in a tubular furnace

(TSH17/75/450, ELITE) at 1150 °C for 3 hours to obtain adequate robustness prior to the

incorporation of other cell components. The pre-sintered substrate was dip-coated with the

electrolyte ink and after drying at room temperature, the electrolyte was sintered at 1350 °C

for 6 hours to densify the electrolyte layer. Dual-layer oxygen electrode was then incorporated

to the NiO-YSZ/YSZ half-cell by brush painting. The first layer was composed of YSZ-LSM

(50 wt. % each) acting as a functional electrode layer and the outer layer was pure LSM to

improve electrical conductivity. Upon drying, the oxygen electrode was sintered at 1000 °C for

Chapter 6

173

2 hours with a heating up rate of 5 °C min−1. The final complete single cell had a cathode length

of 1 cm, which was later used to calculate the active area. For current collection, silver wire

was wrapped around both oxygen electrode and the exposed fuel electrode substrate, and

subsequently fixed with additional silver paste.

6.2.5 Reactor assembly and sealing

The single cell was assembled using alumina tubes (6 mm diameter, Almath Crucibles Ltd.,

UK) with supporting tubes and sealed using ceramic sealant. After drying at room temperature,

the sealant was heat-treated at 95 °C and 260 °C, with 2 hours at each temperature, using a

heating rate of 5 °C min−1. The procedure was similar to the one described in Section 3.2.3 in

Chapter 3.


Wires from both fuel and oxygen electrodes were connected to a potentiostat/galvanostat

(Model: IviumStat, Ivium Technologies B.V., Netherlands) for current-voltage (I-V)

measurement and electrochemical impedance analysis. Experiments were conducted at 700,

750 and 800 °C, with 30 ml min−1 of pure H2 being applied as a fuel in fuel cell operation,

while a mixture of CO2-CO (CO2/CO=90/10 by volume) with a flow rate of 40 ml min−1 was

used during electrolysis mode. In all cases, the oxygen electrode was exposed to ambient air.

The fuel flow rates were controlled by automated mass-flow controllers (Omega Engineering

Inc., USA), and temperature was controlled in a tubular furnace (Carbolite, UK). The

electrochemical impedance analysis was measured under open-circuit voltage (OCV) with a

signal amplitude of 10 mV and frequency range of 10−1−105 Hz. A gas chromatography (Varian

3900) was used to analyse the effluent gas from the system during electrolysis study.

Chapter 6

174


6.3.1 Morphology

Figure 6.2: SEM images of cross-sectional NiO-YSZ micro-monolith: (a) overall cross-section,

(b) – (c) magnified cross-section of the wall between channels and outer surface, (e) magnified

cross-section of centre wall, and (d) and (f) cross-section of wall between channels.

SEM images were obtained at various magnification to observe the morphology of the sintered

6-channel NiO-YSZ substrate. Figures 6.2(a) and (b) show the overall and magnified cross-

section of the micro-monolithic anode, respectively, where a typical asymmetric structure from

Chapter 6

175

phase-inversion process composed of micro-channels and a sponge-like structure was obtained.

Such structure was well preserved following the sintering step at 1350 °C for 6 hours. A

plurality of micro-channels at both exterior and inner structure of the micro-monolith was

obtained during polymer precipitation, when the dope suspension contacted with the internal

and external coagulant simultaneously, allowing the exchange between solvent and non-

solvent to occur from various directions.

Figure 6.2(c) shows the structure between channels and the outer surface where micro-channels

initiated from both exterior and interior surface and thin sponge-like region could be seen at

both surfaces as well. The applied bore fluid was a mixture of DI water and ethanol, causing

the polymer precipitation at the interior surface to be relatively slower than the exterior surface,

providing slightly porous sponge-like region, as shown in Figure 6.3. In contrast, for the

exterior surface, the suspension is exposed to abundant fresh DI water coagulant bath (non-

solvent) which resulted in rather denser sponge-like skin layer due to instantaneous polymer

binder precipitation. As the solvent/non-solvent exchange occurs simultaneously at the interior

and exterior surface, the micro-channels from both surfaces could not be connected as shown

in Figure 6.2(c), as the solvent in dope suspension diffuses out in two directions to both the

internal and external coagulant. Figure 6.2(d) and (f) show the thin wall between each channel,

whereas Figure 6.2(e) shows the centre wall of the micro-monolith with micro-channels formed

from various direction.

Chapter 6

176

Figure 6.3: SEM images of (a) inner surface and (b) outer surface of the sintered 6-channel

NiO-YSZ substrate.

Most of the active triple phase boundary (TPB) is in the sponge-type porosity area in the

proximity of the electrode/electrolyte interface in which a decrease in mass transport resistance

is highly likely to improve the cell performance. From literatures, such effective

electrochemical reaction area has been described to be located 10-20 µm from electrolyte/fuel

electrode interface [10]. From Figure 6.2, the variation in thickness of thinner sponge-like

region between each channel and the exterior surface could be estimated, ranging from 50 to

80 µm. The majority of those thinner sponge-like regions could be, therefore, considered as

highly-active zone for electrochemical reactions as well as providing sufficient fuel supply.

Meanwhile, the geometry of the total cross-section of the substrate provides appropriate

mechanical supports to the whole cell and the inactive region of Ni-YSZ electrode functions

as a current collector.

On the other hand, the plurality of micro-channels facilitates gas transport and thus, lowering

mass transfer resistance. The reduction in gas transport resistance could help minimise

concentration polarisation which is expected to result in improved cell performances. In

essence, the asymmetric structure of the electrode would be advantageous for the application

in solid oxide electrochemical cell. Nevertheless, such unique structure has been described to

Chapter 6

177

have certain impacts on the mechanical property and gas permeation [11]. Therefore, the

fabrication process needs to be carefully controlled to achieve the appropriate substrate for the

development of high-performance cell.

6.3.2 Gas-tightness and mechanical property

An excellent electrolyte should not only allow transport of oxygen ions from one side to the

other side, but also must be dense and free of pin holes. Sufficient gas-tightness of the

electrolyte is vital to avoid gas crossover between fuel electrode and oxygen electrode.

Otherwise, gas leakage between electrodes might occur, resulting in lower open-circuit voltage

(OCV). This gas-tight property was studied via an N2 permeation test whereby permeability

should be within the magnitude of 10−10 mol m−2 s−1 Pa−1 to be considered gas-tight [12]. The

average permeability of the prepared NiO-YSZ/YSZ hall-cell was 8.97 × 10−10 mol m−2 s−1

Pa−1, well within the required range. This signifies that the thin electrolyte layer has excellent

densification and gas-tightness for electrochemical testing and in a good agreement with SEM

observation presented in Figure 6.4.

Figure 6.4: SEM images of (a) cross section of NiO-YSZ substrate with YSZ electrolyte and

(b) outer surface of YSZ electrolyte.

Chapter 6

178

Figure 6.4(a) shows the electrolyte layer on top of the NiO-YSZ substrate, which was dense

with a thickness of about 5 µm. Such thin electrolyte would minimise the ohmic resistance.

The thinner sponge-like region observed at the exterior surface of the NiO-YSZ substrate is

expected to give lower resistance for gas transport. This is because of the shorter passage for

the gas to travel towards the TPB, the region in which the gaseous reactant, electronic and ionic

conducting phase all meet, which is located near the boundary between electrode and

electrolyte. Easier passage to the TPB due to the presence of micro-channels, as well as the

short path given by the thin sponge-like region, is expected to allow better cell performances.

Upon reduction process prior to electrochemical test, the porosity of the fuel electrode will be

even higher.

Mechanical strength is another important factor to determine the lifetime of cell which herein

was estimated by a three-point bending test. The average fracture load of the 6-channel anode

was measured to be approximately 21.5 ± 1.3 N, comparable to other multi-channels design

derived from phase-inversion process reported elsewhere [11, 13]. By comparison, this 6-

channel anode has a larger fracture loads than the single channel counterpart [2], suggesting

that multi-channel design with larger cross-sectional area is more robust.


6.3.3.1 Fuel cell operation with pure H2 feed

Prior to the electrochemical performance test, the cell reduction process was performed in-situ

at 700 °C for at least 4 hours. The test was subsequently conducted by feeding 30 ml min−1 of

pure H2 to the fuel electrode and the oxygen electrode was exposed to ambient air. The open-

circuit voltage (OCV) was 1.17 V at 700 °C (Figure 6.5), proving good cell sealing and

excellent electrolyte characteristics. It is noteworthy that OCVs values measured during

operation between 700 and 800 °C were close to the values theoretically predicted using Nernst

Chapter 6

179

equation. Three identical cells prepared through similar procedure were tested in which

comparable performance was achieved, as presented in Figure 6.6, showing the current density-

voltage (j-V) and current density-power density (j-P) curves at 750 °C. The effective area for

the reaction is approximated by referring to the oxygen electrode length, with a length of 1 cm

and the outer diameter (Do) of the sintered 6-channel substrate was about 0.22 cm which gave

an effective area of approximately 0.69 cm2.

Figure 6.5: j-V and j-P curves for pure H2-fuelled system at different operating temperatures.

From Figure 6.5, the cell reached maximum power densities of 0.65, 1.20, and 1.62 W cm−2 at

700, 750 and 800 °C, respectively. The maximum power densities reported here are

significantly higher than those of other works using similar cell materials (Ni-YSZ/YSZ/YSZ-

LSM), as listed in Table 6.2.

Chapter 6

180

Figure 6.6: j-V and j-P curves for three identical cells operated under pure H2-fuel cell mode

at 750 °C.

Table 6.2: The electrochemical performances of H2-fuel cell with similar cell materials.

Cell configuration

(Anode / Electrolyte / Cathode)

Temp.

(°C)

Maximum power

density (W cm−2)

Ref.

Planar Ni-YSZ/ YSZ (8 µm) /

YSZ-LSM

800 1.08 [14]


YSZ-LSM

707 0.25 [15]

752 0.45

788 0.69


YSZ-LSM

650 0.10 [16]

750 0.30

850 0.50

Chapter 6

181


YSZ-LSM

700

750

800

0.60

0.90

1.20

[17]

Micro-

tubular

Ni-YSZ/ YSZ (15 µm) /

YSZ-LSM

850 0.60 [18]

Micro-

tubular


YSZ-LSM-SDC

800

850

900

0.54

0.71

1.25

[19]

Micro-

tubular


YSZ-LSM

700

750

800

0.14

0.18

0.20

[20]

Micro-

tubular


YSZ-LSM

600 0.12 [21]

700 0.29

800 0.38

Micro-

tubular


YSZ-LSM

700

750

0.16

0.20

[22]

Micro-

tubular


YSZ-LSM

700

750

800

0.29

0.57

0.97

[23]

Micro-

tubular


YSZ-LSM

700

750

800

0.42

0.79

1.27

7-channel cell

(in Chapter 5)

Micro-

tubular


YSZ-LSM

700

750

800

0.65

1.20

1.62

This work

Chapter 6

182

The increase in maximum power densities with operating temperature could be further

explained by analysing the electrochemical impedance spectroscopy (EIS), as shown in Figure

6.7. The EIS measurement was conducted under OCV, collecting data from 105 to 0.1 Hz and

10 mV of signal amplitude. Generally, cell polarisation can be divided into three major parts:

ohmic loss, activation and concentration polarisation. In general, the first x-axis intercept at

high frequency region corresponds to ohmic loss, while the first arc (on the left-hand side) was

related to the activation polarisation, and the second arc (on the right-hand side) characterised

the concentration polarisation. The x-axis intercept at low frequency indicates the overall cell

polarisation, which combined all these polarisations. However, at high frequency region, there

is no x-axis intercept could be observed (Figure 6.7) in which this could be due to the limitation

of the potentiostat/galvanostat equipment. It was difficult to obtain the value for the imaginary

parts of the impedance under high frequency measurement.

Figure 6.7: Electrical impedance spectra (EIS) showed the effect of temperature on polarisation

in fuel cell mode using pure H2 as a fuel.

Chapter 6

183

The EIS obtained at various operating temperatures indicate that the reduction in total

polarisation at higher operating temperatures was consistent with an increase in power density,

mainly due to reduced ohmic and activation polarisation. As can be estimated from the spectra,

the ohmic loss is relatively small because the cell has a thin electrolyte (~5µm) which provides

a shorter ionic pathway. Besides, the increase in temperature also improved the ionic

conductivity of YSZ electrolyte and the reaction kinetics, eventually leading to superior cell

performance. It could be seen that the activation polarisation is more apparent at lower

temperature operation (700 °C) due to a slower rate of reactions. A substantial reduction in

activation polarisation was observed at higher temperature, given that charge and gases are

easily transferred, leading to faster reaction rates.

On the other hand, the concentration polarisation was rather similar regardless of operating

temperature as it is mostly related to the mass transport resistance at both electrodes and

electrode/electrolyte interface. Since this is mainly affected by the transport of reactants and

products within the cell electrode, the optimisation of an electrode’s micro-structural design is

of marked importance. During operation, the reactant and products need to be supplied and

removed continuously through the porous electrode. A multi-channel design with hierarchical

micro-channels are beneficial in enhancing the mass transfer rate and minimising the

concentration polarisation. The presence of such unique structure reduces the transport

diffusion restrictions of reactants and products, to or away from the fuel electrode/electrolyte

interface, respectively. Such region is where most of the active TPB located, i.e. sponge-like

region close to the electrode/electrolyte interface. Moreover, the thin sponge-like region

between each channel and the exterior surface functions like a short-path, thus eases the gas

transport. It could be anticipated that the H2 concentration within this thin sponge-like region

would be comparable to the bulk H2 concentration in the channel.

Chapter 6

184

The excellent performance in H2–fuelled system achieved in this work, which to the best of our

knowledge is the highest maximum power density ever achieved for micro-tubular system

employing the conventional Ni-YSZ/YSZ-based system.

6.3.3.2 SOEC-SOFC dual-operation analysis

One attractive feature of the solid oxide electrochemical cell is that it could be operated in

reversible mode. Herein, the reversibility of the developed cell has been investigated. The

electrochemical performance test was performed using a voltage range between 1.8 and 0.6 V,

from 700 to 800 °C by supplying the fuel electrode with 40 ml min−1 of CO2-CO (90-10 vol.

%) gas mixture and the oxygen electrode is exposed to ambient air. Such voltage range was

chosen considering that thermo-neutral voltage (Vtn) for CO2 electrolysis is about 1.5 V.

Thermo-neutral voltage is a potential at which the generated Joule heat in the electrolysis cell

is equal to the heat consumption for the electrolysis reaction.

In this work, good reproducibility has been observed for three repeats at 750 °C (Cell 1-3) as

can be seen in Figure 6.8.

Chapter 6

185

Figure 6.8: j-V curves for three identical cells operated at 750 °C with CO2-CO (90-10 vol. %)

gas input.

From Figure 6.9, the OCVs were 0.91, 0.88 and 0.85 V at 700, 750 and 800 °C, respectively.

These values are comparable to the predicted values from Nernst equation, assuring good

electrolyte properties. Small deviations, up to 10 mV were observed from three identical cells

which might be due to minor leakage through ceramic sealant causing minimal changes in

partial pressure. Another possible reason was due to non-uniform gas distribution causing

minor fluctuation in the gas composition at the electrodes [2].

Chapter 6

186

Figure 6.9: j-V curves for SOEC-SOFC dual-mode operation with a feed mixture of CO2-CO

(90-10 vol. %) at various operating temperatures.

During the investigation of cells performance, the voltage was applied from OCV to 1.8 V for

electrolysis mode and from OCV to 0.4 V in fuel cell mode. The turning point between each

mode occurred at OCV, at which the current density (j) was zero. From j–V curves for R-SOER

(Figure 6.9), the negative current density at cell voltage above OCV implies the energy input

to electrochemically convert CO2 to CO during electrolysis mode, whereas the positive current

density at cell voltage lower than OCV corresponds to electricity generation in fuel cell mode.

Near OCVs, the j–V curves were relatively linear, signifying no substantial change gradient of

the curves when switching from one mode to the other. The smooth curves obtained signified

that the cell behaviour is fully reversible in the range of temperatures investigated. A common

trade-off between electrochemical performances in either mode can be observed when using

similar gas feed; better performance in electrolysis mode corresponds to lower performance in

fuel cell mode.

Chapter 6

187

The thermo-neutral voltage (Vtn) is about 1.464 V for CO2 electrolysis at 750 °C [5]. Ideally,

operating at Vtn is preferable since there is no net heat flux to the cell. This is because when the

cell potential is any higher than that, some of the applied current will be transformed to thermal

energy and wasted. During electrolysis experiments, the maximum potential voltage was

limited at 1.8 V to prevent parasitic loss which occurs at higher potential voltage, specifically

above 2.3 V. Moreover, the electronic conductivity of YSZ is negligible at a voltage of less

than 1.5 V but becomes more significant beyond 1.8 V [7]. It has been reported that irreversible

damage towards YSZ electrolyte would occur if a voltage above 2.3 V was applied [24]. This

is largely due to the reductive decomposition of zirconium oxide from Zr4+ to Zr3+ leading to

rapid decline in cell performance.

For CO2 electrolysis, the current density reached at a certain applied voltage is the indication

of cell performance. In Figure 6.9, the current densities of −1.92 (−1.03) A cm−2, −3.48 (−1.60)

A cm−2 and −6.26 (−3.10) A cm−2 were achieved at 1.8 V (1.5 V) for 700, 750 and 800 °C,

respectively. The current density obtained at 800 °C is almost double than that of 750 °C,

showing that temperature has a substantial effect on cell performance. Such outstanding

performance stems from lower overall cell polarisation, as proven by the EIS analysis. From

Figure 6.10, it can be estimated that the activation polarisation is reduced with the increase of

operating temperature, while concentration polarisation is comparable at all operating

conditions and is trivial compared to other resistances, as it is small at approximately 0.05 Ω

cm2, as estimated from the spectra as shown in Figure 6.10.

Chapter 6

188

Figure 6.10: Electrical impedance spectra (EIS) for operation with CO2-CO (90-10 vol. %) at

different temperatures.

The electrochemical performances obtained with this novel cell design have surpassed those

reported in the literatures, considering similar cell materials. For instance, at 800 °C with

similar gas feed composition of CO2-CO (90-10 vol. %), cell developed in this study achieved

a very high current density of −6.3 (−3.1) A cm−2 at 1.8 (1.5) V. Such significant increase could

be due to improved cell’s micro-structural design which greatly minimised the cell overall

polarisation, particularly the concentration polarisation. As can be seen from Table 6.3, the

current densities achieved in this work have surpassed other work with cells of similar

materials.

Chapter 6

189

Figure 6.11: j-V and j-P curves for a system operated with CO2-CO (90-10 vol. %) gas mixture

at different operating temperature in fuel cell mode.

Fuel cell performance with CO2-CO (90-10 vol. %) gas mixture as reactant is shown in Figure

6.11. The maximum power densities were 0.18, 0.27 and 0.33 W cm−2 at 700, 750 and 800 °C,

respectively. Such values were much lower than those obtained during operation fuelled by

pure H2, since the gas feed to the fuel electrode contains low amount of fuel (10 vol. % of CO);

thus, less fuel would generate low power densities. Nevertheless, the maximum power densities

reported here were higher than those obtained by single-channel micro-tubular system fed

using similar gas feed composition; 0.18 and 0.33 W cm−2 at 700 and 800 °C, respectively,

against 0.08 and 0.12 W cm−2 under similar conditions [2]. Fuel cell performance could be

simply enhanced by having higher concentration of CO in the feed. However, extra precaution

is required when using gas mixture with more CO content due to the inclination of carbon

formation. Higher CO concentration would promote the carbon formation through Boudouard

reaction, which could degrade cell performance. It is worth noting that the CO2-CO

compositions used throughout this work before and during electrolysis operation were below

Chapter 6

190

the CO equilibrium fraction for carbon formation via Boudouard equation (Appendix A),

approximately 0.6, 0.8 and 0.9 at 700, 750 and 800 °C, respectively which has also been

described in previous work [2, 28]. Therefore, coking was unlikely to occur throughout the

operation.

6.3.3.3 Short-term SOEC stability study

Performance stability is one of the key requirements of practical operation. The system (Cell

2) was operated under electrolysis mode with applied current density of 1.25 A corresponding

to the Vtn (~1.5V) for 20 hours. Figure 6.12 shows that the system was relatively stable for 20

hours dwelling time with negligible sign of cell degradation. The operation was restricted to

20 hours due to the limitation of the system and safety concerns.

Figure 6.12: Short-term stability test under CO2-CO (90-10 vol. %) electrolysis operation.

The SEM-EDX analysis (OXFORD Instruments) post-testing indicated insignificant carbon

formation as can be observed from Figure 6.13. The chemical mapping and sum spectrum

showed that only a small amount of carbon (C) element was detected on the fuel electrode

Chapter 6

191

surface. As explained earlier, the gas mixture used in this study was not within the carbon

formation regime which agrees well with SEM-EDX analysis.

Figure 6.13: Post-test SEM-EDX analysis for two different fuel electrode regions, (a) and (b),

where (i), (ii) and (iii) represent the SEM images of the mapped area, carbon (C) element

distribution and sum spectra, respectively.

Chapter 6

192

6.3.3.4 Cell reversibility

It has been reported that the reversible operation could extend the lifetime of the

electrochemical cell [26]. Additionally, operating the cell under electrolysis/fuel cell cyclic

mode could minimise the degradation of the oxygen-electrode, as often detected for a cell

operated under long-term electrolysis mode [27]. Herein, the reversibility of cell was examined

under a short-term test by operating the cell under two modes cyclically for one hour each

where the cell was operated under electrolysis (at 1.5 V) and fuel cell mode (at 0.7 V) using a

feed gas of CO2-CO (90-10 vol. %).

Figure 6.14: SOEC-SOFC cyclic operation for a period of 10 hours.

For this test, Cell 3 was used and the current in different modes was monitored over a period

of 10 hours. After 10 hours of cyclic operation, cell performance was observed to be rather

stable in each mode. From Figure 6.14, at constant 1.5 V, the current was observed to be

relatively stable at about −1.25 A while under 0.7 V, the current was about 0.25 A. However,

earlier study [2] has reported that using the same gas compositions for both modes is

Chapter 6

193

inadvisable as each mode requires different feed; electrolysis mode needs high CO2/CO ratio

whereas fuel cell requires high CO/CO2 ratio in order to deliver good performance. If the same

gas composition is applied, it will cause at least one mode to underperform and increase

degradation rates. In addition, longer stability test (> 1000 hours) is suggested for future work

to further confirm the stability of the system.

6.4 Conclusions

In summary, improvements of the micro-structural design for better mechanical robustness and

high performance SOER could be tailored via phase inversion and sintering technique.

Outstanding electrochemical performances have been achieved via this novel cell design, with

excellent performance in both fuel cell and electrolysis modes observed. Under fuel cell

operation with pure H2, the 6-channel cell reached a maximum power density (Pmax) of 1.62 W

cm−2 at 800 °C, which by far, the highest reported values achieved by Ni-YSZ/YSZ/YSZ-LSM

micro-tubular system. This corresponds to about 27.6 % performance improvement compared

to the 7-channel cell design reported in Chapter 5 (Pmax 1.62 vs. 1.27 W cm−2 at 800 °C).

Cell performance for CO2 electrolysis also showed that this novel cell design gave outstanding

performance at 800 °C, achieving −6.26 (−3.10) A cm−2 at 1.8 (1.5) V. In addition, excellent

reversibility has been demonstrated, as cell degradation could be minimised by cycling between

fuel cell and electrolysis modes. Such findings indicate that the realisation of sustainable

energy system could be possible soon. The robust and high-performance reversible (R)-SOER

is a unique device that could facilitate both supplying energy during periods of higher demand

and converting surplus renewable energy for energy storage.

Chapter 6

194

Table 6.3: Studies on CO2-CO electrolysis using similar cell materials.

No.

Cell configuration Feed composition

Temp

(°C)

Voltage

(V)

Current

density

(A cm−2)

Ref.

Fuel electrode Electrolyte Oxygen electrode CO2 (%) CO (%)

1 Ni-YSZ YSZ YSZ-LSM/LSM 90 10 700 1.80 −1.92 This work

750 −3.48

800 −6.26

90 10 700 1.50 −1.03

750 −1.60

800 −3.10

2 Ni-YSZ YSZ YSZ-LSM/LSM 90 10 800 1.80 −1.00 [2]

3 Ni-YSZ YSZ YSZ-LSM 50 50 750 1.09 −0.25 [4]

70 30 750 1.06 −0.25

50 50 850 1.05 −0.25

70 30 850 1.26 −0.90

Chapter 6

195

No.

Cell configuration Feed composition

Temp

(°C)

Voltage

(V)

Current

density

(A cm−2)

Ref.

Fuel electrode Electrolyte Oxygen electrode CO2 (%) CO (%)

70 30 850 1.01 −0.25

4 Ni-YSZ YSZ YSZ-LSM 90 10 718

809

1.50

−0.35

−0.78

[28]

5 Ni-YSZ YSZ YSZ-LSM 90 10 707

752

788

1.50 −0.82

−1.4

−3.3

[15]

6 Ni-YSZ YSZ YSZ-LSM 50 50 850 1.26 −0.87 [29]

7 Ni-YSZ YSZ YSZ-LSM/LSM 50 50 800 1.12 −0.25 [30]

70 30 800 1.10 −0.25

50 50 850 1.02 −0.25

70 30 850 1.03 −0.25

8 Ni-YSZ YSZ YSZ-LSM 70 30 950 1.29 −1.50 [31]

Chapter 6

196

6.5 References

1. Zhan, Z. and L. Zhao, Electrochemical reduction of CO2 in solid oxide electrolysis

cells. Journal of Power Sources, 2010. 195(21): p. 7250-7254.

2. Kleiminger, L., et al., CO2 splitting into CO and O2 in micro-tubular solid oxide

electrolysers. RSC Advances, 2014. 4(91): p. 50003-50016.

3. Lu, J., et al., Efficient CO2 electrolysis with scandium doped titanate cathode.


4. Ebbesen, S.D. and M. Mogensen, Electrolysis of carbon dioxide in Solid Oxide

Electrolysis Cells. Journal of Power Sources, 2009. 193(1): p. 349-358.

5. Hansen, J.B., Solid oxide electrolysis – a key enabling technology for sustainable

energy scenarios. Faraday Discussions, 2015. 182(0): p. 9-48.

6. Zhang, L., et al., Electrochemical reduction of CO2 in solid oxide electrolysis cells.

Journal of Energy Chemistry, 2017. 26(4): p. 593-601.

7. Cheng, C.-Y., G.H. Kelsall, and L. Kleiminger, Reduction of CO2 to CO at Cu–ceria-

gadolinia (CGO) cathode in solid oxide electrolyser. Journal of Applied

Electrochemistry, 2013. 43(11): p. 1131-1144.

8. Tan, X., Y. Liu, and K. Li, Mixed conducting ceramic hollow-fiber membranes for air

separation. AIChE Journal, 2005. 51(7): p. 1991-2000.

9. Li, T., Z. Wu, and K. Li, A dual-structured anode/Ni-mesh current collector hollow

fibre for micro-tubular solid oxide fuel cells (SOFCs). Journal of Power Sources, 2014.

251: p. 145-151.




Chapter 6

197

11. Lee, M., et al., Micro-structured alumina multi-channel capillary tubes and monoliths.

Journal of Membrane Science, 2015. 489: p. 64-72.



768-772.

13. Chi, Y., et al., Morphology, performance and stability of multi-bore capillary

La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen transport membranes. Journal of Membrane Science,

2017. 529: p. 224-233.

14. Wang, H., Z. Gao, and S.A. Barnett, Anode-Supported Solid Oxide Fuel Cells

Fabricated by Single Step Reduced-Temperature Co-Firing. Journal of The

Electrochemical Society, 2016. 163(3): p. F196-F201.

15. Farandos, N.M., T. Li, and G.H. Kelsall, 3-D inkjet-printed solid oxide electrochemical

reactors. II. LSM - YSZ electrodes. Electrochimica Acta, 2018. 270: p. 264-273.

16. Koh, J.-H., et al., Carbon deposition and cell performance of Ni-YSZ anode support

SOFC with methane fuel. Solid State Ionics, 2002. 149(3–4): p. 157-166.

17. Cronin, J.S., et al., Three-dimensional reconstruction and analysis of an entire solid

oxide fuel cell by full-field transmission X-ray microscopy. Journal of Power Sources,

2013. 233: p. 174-179.

18. Campana, R., et al., Fabrication, electrochemical characterization and thermal cycling

of anode supported microtubular solid oxide fuel cells. Journal of Power Sources, 2009.

192(1): p. 120-125.



84.

Chapter 6

198

20. Droushiotis, N., A. Hankin, and G.H. Kelsall, New Fabrication Techniques for Micro-

Tubular Hollow Fiber Solid Oxide Fuel Cells. ECS Transactions, 2013. 50(45): p. 105-

114.

21. Yang, C., et al., Fabrication and characterization of an anode-supported hollow fiber

SOFC. Journal of Power Sources, 2009. 187(1): p. 90-92.


gas treatment and clean energy generation. Energy & Environmental Science, 2016.

9(12): p. 3682-3686.

23. Lu, X., et al., The application of hierarchical structures in energy devices: new insights

into the design of solid oxide fuel cells with enhanced mass transport. Energy &


24. Weppner, W., Formation of intermetallic Pt-Zr compounds between Pt electrodes and

ZrO2-based electrolytes, and the decomposition voltage of yttria-doped ZrO2. Journal

of Electroanalytical Chemistry and Interfacial Electrochemistry, 1977. 84(2): p. 339-

350.


Reviews, 2004. 104(10): p. 4845-4866.

26. Graves, C., et al., Eliminating degradation in solid oxide electrochemical cells by

reversible operation. Nature Materials, 2014. 14: p. 239.

27. Schefold, J., A. Brisse, and H. Poepke, 23,000 h steam electrolysis with an electrolyte

supported solid oxide cell. International Journal of Hydrogen Energy, 2017. 42(19): p.

13415-13426.

28. Farandos, N.M., et al., Three-dimensional Inkjet Printed Solid Oxide Electrochemical

Reactors. I. Yttria-stabilized Zirconia Electrolyte. Electrochimica Acta, 2016. 213: p.

324-331.

Chapter 6

199

29. Graves, C., S.D. Ebbesen, and M. Mogensen, Co-electrolysis of CO2 and H2O in solid

oxide cells: Performance and durability. Solid State Ionics, 2011. 192(1): p. 398-403.

30. Ferrero, D., et al., Reversible operation of solid oxide cells under electrolysis and fuel

cell modes: Experimental study and model validation. Chemical Engineering Journal,

2015. 274: p. 143-155.

31. Jensen, S.H., P.H. Larsen, and M. Mogensen, Hydrogen and synthetic fuel production

from renewable energy sources. International Journal of Hydrogen Energy, 2007.

32(15): p. 3253-3257.

Chapter 7

200

7 CHAPTER 7

General conclusion and future works

7.1 General conclusion

Solid oxide fuel cells (SOFCs) have attracted considerable attention due to certain remarkable

features such as the ability to be operated with different fuels including hydrogen,

hydrocarbons, syngas, and biofuels. Additionally, it is known that SOFC to have the flexibility

in operation by allowing the reversible process using the same cell. This reversible

electrochemical device is capable of operating in both fuel cell and electrolysis operating

modes. Ideally, it could facilitate both delivering energy during periods of higher demand and

converting surplus renewable energy for energy storage. Geometrically, micro-tubular (MT)-

SOFCs offer several additional benefits, such as better thermal shock resistance, higher power

to volumetric density and portable characteristic.

This thesis has focused on designing micro-structured micro-tubes/micro-monoliths using

phase-inversion process functioning as a suitable support for the development of micro-tubular

and micro-monolithic SOFCs. The phase-inversion process has an advantage in that different

morphologies for the micro-tube can be formed. The fabrication processes of micro-tubes

(Chapter 3 and 4) and micro-monoliths (Chapter 5 and 6) are similar, both comprising three

main steps: (1) preparation of the dope suspension; (2) extrusion (spinning process) of the

precursor using a custom-design spinneret; and (3) sintering process to consolidate the ceramic

materials. Following that, each support was developed into a complete single cell. A series of

characterisations have also been performed, such as micro- and macro-structure, gas

permeability, anode porosity, electrolyte gas-tightness, mechanical strength and

electrochemical test.

Chapter 7

201

Materials available for practical use in the high-temperature SOFCs remain limited. Although

alternatives such as those with both mixed ionic electric conductivity (MIEC) including ceria-

based and perovskites have been recommended, their widespread use is still constrained due to

their high costs and low stability for long-term operation. While there has been considerable

progress in material selection for producing SOFC, commercial SOFC products are largely still

based on the conventional Ni-cermet anode. Mass transport in the electrodes is one of the

critical problems limiting electrochemical performance. With these constraints, one strategy to

achieve high-performance SOFC is micro-structural improvement. Understanding the

dependence of electrochemical performance on the micro-structure is crucial in order to

optimise the electrode fabrication process. For that reason, a proper fabrication procedure must

be developed to obtain a structure that is beneficial for SOFC applications.

Chapter 3 introduces an innovative application of hierarchical structure in MT-SOFC

fabricated using a phase-inversion technique with the aim to improve the mass transport within

the cell. A common anode material, nickel-yttria stabilised zirconia (Ni-YSZ) cermet, is used

for the fabrication of micro-structured anode for MT-SOFC. Two different types of anode

design fabricated by phase-inversion technique using different bore fluids. The conventional

anode design is composed of a short conical-shaped micro-channel and large fraction of

sponge-like region. In contrast, the anode substrate derived from solvent-based bore fluid has

a distinct structure, composed of long micro-channels with larger pore entrance and thin

sponge-like region at the outer section. Solvent-based bore fluid functions to delay the

precipitation of polymer phase allowing micro-channels originating from the external surface

to penetrate through the whole cross section, forming open entrances to the inner surface. It

was expected that the combination of a longer micro-channels with large pore entrance and the

thin sponge-like region would lead to improved electrochemical performance. Compared to the

conventional design, the new anode structure gave better cell performance when fuelled with

Chapter 7

202

pure hydrogen (H2), achieving up to 31.8% higher power density due to much lower

concentration polarisation. The finding shows the effectiveness of micro-channels with larger

pore entrance in enhancing mass transport, thereby mitigating concentration polarisation in the

MT-SOFC.

SOFCs’ main advantage of fuel flexibility appears to be an interesting subject for further

investigation. In this light, the benefits of the new anode design reported in Chapter 3 have

been further explored by utilising the electrolyte material yttria-stabilised zirconia (YSZ). This

unique design could be applied for the fabrication of integrated electrolyte/anode structure

(YSZ scaffold) using a phase-inversion and sintering process with vacuum-assisted

impregnation. The YSZ structure consists of a dense outer skin layer (approximately 10 µm

thick) functioning as suitable electrolyte, with a plurality of micro-channels with a large

entrance for the infiltration of anode materials. Such a design allows the use of copper (Cu)-

based anode, which has a lower melting point than the required temperature for the YSZ micro-

tube sintering process, to be applied. The Cu-CeO2 composite anode is recognised to have a

greater resistance towards carbon formation when hydrocarbon fuel such as methane (CH4) is

used. It was incorporated through vacuum-assisted co-impregnation which was uniformly

distributed onto the inner surface as well as inside the micro-channels. The findings showed

that the cell operated with dry CH4 reached a power density of 0.16 W cm−2 at 750 °C, which

is one of the highest ever reported for Cu-CeO2 anode. Post-test analysis following 30 hours’

operation with dry CH4 showed only trivial carbon formation and negligible performance

degradation. It is noted that other types of anode materials could also be incorporated to the

micro-structured YSZ micro-tube by the same impregnation procedure. Nevertheless,

improvements in the impregnation process should be further studied to make it less complicated

to be feasible for a large-scale cell production.

Chapter 7

203

Chapter 3 and 4 deal with the fabrication of a single-channel micro-tube for MT-SOFC.

Notwithstanding the great potential of MT-SOFC, inherent problems such as the low

mechanical robustness of the small micro-tubes must be addressed. This is a major concern if

the system is exposed to a situation in which mechanical shocks, vibration or habitual start-ups

or shut-down exist. To overcome this shortcoming, a study on the improvement of the

geometrical features of the cells was attempted and described in Chapter 5. A multi-channel

design or termed as micro-monolithic design has been proposed not only to increase cell

robustness but also for performance improvement. A holistic cell design has been developed

by introducing the micro-monolith geometry in the anode support for SOFC. As this was the

first attempt to prepare anode micro-monolith for the preparation of SOFC, a common anode

material, Ni-YSZ cermet, was selected. Anodes with various designs, 3, 4, and 7 channels,

were fabricated via a phase-inversion assisted-extrusion process. It is noteworthy that the bore

fluid applied was de-ionised (DI) water instead of the solvent-based bore fluid. For the multi-

channel design, as the wall between each channel is thin, the use of solvent-based bore fluid

seems to be inappropriate as it could partially dissolves the interior surface, causing the

neighbouring channels to merge. It may also partly eliminate readily formed micro-channels

when diffusing outward into the external coagulation bath.

Such a micro-monolithic design has been demonstrated to be highly-effective in improving

mechanical properties. The bending loads which the anode substrate could withstand increases

of 4-8 times, as the number of channels increases whilst bending strength remains comparable.

During the fabrication process, the split of bore fluid (internal coagulant) greatly affects the

precipitate rate and thus the cross-section morphology, whereby micro-channels formed from

multiple direction could be observed. The distinction between the micro-structure of the micro-

monolith and the single-channel micro-tube is the non-uniform sponge thickness between the

channels and outer section. Due to this important feature, the micro-monolith design displayed

Chapter 7

204

better gas transport properties, as shown by the higher nitrogen (N2) gas permeation tests, while

the mean flow pore size remains the same. Such improvement results in the delivery of

excellent electrochemical performances, with all three micro-monolithic designs achieved

much higher maximum power density than the single-channel counterpart. The cell with 7-

channel anode achieved the best performance of 1.27 W cm−2 at 800 °C and 200 hours stability

test showed trivial performance degradation. The impedance spectroscopy further proved the

effectiveness in achieving more efficient gas transport by showing much smaller concentration

polarisation than the 7-channel design.

Recognising the superior performance of the cell with multi-channel design, further

improvement of such a design were explored. In Chapter 6, a novel micro-monolithic cell with

6-channel NiO-YSZ substrate was fabricated via phase-inversion and sintering technique. Its

unique characteristics were then studied and described. The fabrication process of this new 6-

channel anode design was similar to the work reported in Chapter 5, with some changes of

fabrication parameters, e.g. type of bore fluid and spinning process with no air gap. It is

noteworthy that the 6-channel anode did not exhibit a circular channel as that observed for the

multi-channel anode reported in Chapter 5, instead displaying a rather interesting triangular-

shaped channels. This could be explained by the effect of hydraulic pressure of the bore fluid.

In addition to preventing the buckling of inner contour, hydraulic pressure was adjusted to

change the membrane geometry. For each stream, hydraulic pressure in certain directions was

counterbalanced by the pressure from the adjacent streams of bore fluid, resulting in a non-

uniform expansion of the lumen. This led to the revolution from circular to a triangular shape.

Furthermore, the applied water-ethanol mixture as a bore fluid was effective in reducing the

precipitation rate of the polymer binder, allowing enough time to acquire the unique-shaped

channel geometry.

Chapter 7

205

The mechanical strength of the 6-channel design is comparable to the other multi-channels

design, proving the greater robustness of such design than the single-channel counterpart.

Based on the earlier findings, a much better electrochemical performance achieved by the cell

with multi-channel design could be attributed to the better structural design and lower cell

polarisation. The excellent performance of the 6-channel design could be associated with the

larger proportion of thin sponge-like region between each channel and the outer section due to

the unique non-circular channel compared to the other multi-channel design. The

improvements of the micro-structural design not only lead to better mechanical robustness, but

also to improved electrochemical performance for both fuel cell and electrolysis modes. During

fuel cell test with pure H2, the cell achieved 1.62 W cm−2, representing performance

improvement of about 27.6 % as compared to the 7-channel cell (the best performing cell)

described in Chapter 5. Similarly, high performance for CO2 electrolysis has been

demonstrated as the cell gave current densities of −6.26 (−3.10) A cm−2 at 1.8 (1.5) V at 800

°C. Short-term stability test presented stable performance with negligible performance

degradation and minor carbon formation. Cycling between fuel cell and electrolysis modes

presented good reversibility of the cell, thus helping minimise cell degradation.

Overall, the findings from this work have demonstrated that the structure of micro-tube can be

optimised and controlled to develop a high-performance SOFC by adjusting fabrication

parameters such as dope suspension or bore fluid compositions, and operating parameters of

the extrusion/sintering process. The flexibility of micro-structure tailoring via phase-inversion

assisted process allows the development of SOFC with both decent mechanical property and

great electrochemical performances. The findings have been used as a guidance to further

improve the performance of SOFC. Eventually, a high-performance SOFC was demonstrated,

achieving performance which rates among the best for a Ni-YSZ-based system. While the

experimental works of material choice and fabrication are focused on the preparation of micro-

Chapter 7

206

tubes and micro-monoliths for making SOFC, they could also applicable to the development

of micro-tubes/micro-monoliths for other applications of great importance.

7.2 Future works

7.2.1 Hydrocarbon-fuelled system

SOFCs’ main advantage of fuel flexibility appears to be an interesting subject for further

exploration. The use of hydrocarbons as a fuel for Ni-based SOFCs is desirable, provided that

certain strategies and extra precautions are to be taken to ensure stable performance. High-

temperature SOFC operation enables internal reforming to convert hydrocarbon into hydrogen

(H2) and carbon monoxide (CO) by adding reforming agents, for example, steam and CO2 to

the fuel stream. Such a process would be useful to minimise carbon formation, an inherent

problem associated with an Ni-based anode. Our preliminary work using an Ni-based cell

fuelled by a mixture of methane (CH4)-CO2 for internal dry reforming has demonstrated

superior cell performance under a proper operating condition. Further investigation is required

to obtain not only excellent and stable performance, but also good cell reproducibility.

7.2.2 Cell with dual-layer electrolyte

The attractiveness (CeO2)-based SOFC using nickel-gadolinium doped ceria (Ni-

GDC)/GDC/LSCF as reported earlier by our group has been demonstrated whereby some

outstanding performances could be achieved [1]. However, issues surround “current leakage”

as well as its durability for long-term operation remains as major challenges. The application

of dual-layer electrolyte (YSZ/GDC) is proposed to overcome this problem and it allows the

use of more active cathode material such as La1−xSrxCo1−yFeyO3−δ (LSCF) which has been

reported to be incompatible with YSZ. Therefore, cells with common Ni-YSZ anode and dual-

layer electrolyte (GDC layer on top of conventional YSZ electrolyte) could be investigated for

their electrochemical performances.

Chapter 7

207

7.2.3 New materials for the development of MT-SOFC

The high temperature requirements of SOFC results in limited choices for suitable materials to

make the cell. Therefore, finding materials other than Ni-cermet for the development of MT-

SOFC is desirable. Recent progress in lowering SOFC working temperatures largely involves

the use of proton-conducting electrolytes. The proton-conducting SOFCs, sometimes termed

as protonic ceramic fuel cells (PCFCs, have drawn huge interest by demonstrating the

capability of power output at intermediate temperatures, ranging from 400 to 600 °C [2-4].

Reduced operating temperature would help increase the stability of the components and allow

the use of less expensive interconnector materials such as stainless steel.

7.2.4 Long-term durability test

In addition to achieving superior performance in terms of power density, such cells should also

offer good stability in prolonged operation to be commercially viable. While the outstanding

performance, particularly with a micro-monolithic design, of the H2-fuelled system has been

demonstrated, study on its long-term operation (>1000 hours) is desirable.

7.3 References

1. Othman, M.H.D., et al., High-Performance, Anode-Supported, Microtubular SOFC

Prepared from Single-Step-Fabricated, Dual-Layer Hollow Fibers. Advanced Materials,

2011. 23(21): p. 2480-2483.

2. Duan, C., et al., Readily processed protonic ceramic fuel cells with high performance at

low temperatures. Science, 2015. 349(6254): p. 1321-1326.

3. Choi, S., et al., Exceptional power density and stability at intermediate temperatures in

protonic ceramic fuel cells. Nature Energy, 2018. 3(3): p. 202-210.

4. Shim, J.H., Ceramics breakthrough. Nature Energy, 2018. 3(3): p. 168-169.

208

List of publications

1. Li, T., Rabuni, M. F., Kleminger, L., Wang, B., Kelsall, G. H., Hartley, U., and Li, K., A

highly-robust solid oxide fuel cell (SOFC): Simultaneous greenhouse gas treatment and

clean energy generation, Energy & Environmental Science, 2016. 9(12): p. 3682-3686.

2. Rabuni, M.F., Li, T., Punmeechao, P., and Li, K. Electrode design for direct-methane

micro-tubular solid oxide fuel cell (MT-SOFC), Journal of Power Sources, 2018. 384: p.

287-294.

3. Lu, X., Li, T., Bertei, A., Cho, J. I. S., Heenan, T. M. M., Rabuni, M. F., Li, K., Brett, J.

L., and Shearing, P. R., The application of hierarchical structures in energy devices: new

insights into the design of solid oxide fuel cells with enhanced mass transport. Energy &


4. Li, T., Heenan, T. M. M., Rabuni, M. F., Wang, B., Farandos, N. M., Kelsall, G. H.,

Matras, D., Tan, C., Lu, X., Jacques S. D. M., Brett, D. J. L., Shearing P. R., Michiel, M.

D., Beale, A. M., Vamvakeros, A. and Li K., Next-generation Ceramic Fuel Cells: from

New Design to Real-life Characterization with Synchrotron X-ray Diffraction Computed

Tomography. Jan 2019. Nature Communications, 2019. 10(1): p. 1497.

5. Rabuni, M. F., Li, T., Vatcharasuwan, N., and Li, K., High-performance micro-monolithic

solid oxide fuel cell (SOFC) for extended application: CO2 electrolysis. To be submitted to

Journal of Power Sources.

Oral conference presentation

1. Rabuni, M. F., Li, T., Punmeechao, P., and Li, K., Micro-structured Hollow Fibres for

Micro-tubular SOFCs. Presented at the PERMEA and MELPRO 2016, 15–19 May 2016,

Prague, Czech Republic.

209

2. Rabuni, M. F., Li, T., and Li, K., Multi-channel Hollow Fibres for Micro-tubular Solid

Oxide Fuel Cells (MT-SOFCs). Presented at the 1st National Congress on Membrane

Technology (NATCOM) 2016, 24 – 25 August 2016, UTM Johor Bahru, Malaysia.

3. Rabuni, M. F., Li, T., and Li, K., A Novel Micro-Monolithic Solid Oxide Fuel Cells.

Presented at the MELPRO 2018, 13 – 16 May 2018, Prague, Czech Republic.

4. Rabuni, M. F., Li, T., and Li, K., High-performance Micro-Monolith Solid Oxide Fuel

Cells (SOFC). Presented at the 15th International Conference on Inorganic Membranes

(ICIM), 18 – 22 June 2018, Dresden, Germany.

5. Rabuni, M. F., Li, T., and Li, K., Electrode Design for Direct-methane Micro-tubular

Solid Oxide Fuel Cell (MT-SOFC). Presented at the Euromembrane 2018, 9 – 13 July 2018,

Valencia, Spain.

Poster conference presentation

1. Rabuni, M. F., Li, T., and Li, K., Micro-structured, Multi-channel Hollow Fibres for

Micro-tubular Solid Oxide Fuel Cells (MT-SOFCs). Presented at the 12th European Fuel

Cell Forum 2016, 5 – 8 July 2016, Lucerne, Switzerland.

2. Rabuni, M. F., Li, T., and Li, K., Multi-channel Hollow Fibres for Micro-Tubular Solid

Oxide Fuel Cells (MT-SOFCs). Presented at the FCH2 2016, 25 – 26 May 2016,

Birmingham, UK.

3. Rabuni, M. F., Li, T., and Li, K., Micro-structured Micro-tubular Solid Oxide Fuel Cell

(MT-SOFC) for Direct Methane Utilisation. Presented at the Malaysia-Singapore Research

Conference, 25 March 2017. University of Cambridge, UK.

4. Rabuni, M. F., Li, T., and Li, K., Multi-channel micro-tubular solid oxide fuel cell (MT-

SOFC) for simultaneous clean energy generation and greenhouse gas treatment. Presented

at the FCH2 2017. 31 May – 1 June 2017. Birmingham, UK.

Appendices

210

Appendix A

The effects of temperature on thermo-neutral voltage for carbon dioxide (CO2) electrolysis is

shown in Figure A1 with Vtn is approximately 1.5 V. Data for ΔH, ∆G and ∆S at different

temperature (T) is obtained from: Huang, K. and Goodenough, J.B. (2009) Solid Oxide Fuel

Cell Technology: Principles, Performance and Operations. Elsevier Science.

Figure A1: Thermo-neutral voltage (Vtn) for CO2 electrolysis at various temperature.

Figure A2: Schematic diagram of the carbon monoxide (CO) equilibrium fraction of the

Boudouard reaction (at 1 atm) to determine the carbon deposition.

Boudouard reaction: 2CO ↔ CO2 + C (Eq. A1)

Appendices

211

Appendix B

Copyright permission

Within the thesis, the following journal articles by the author have been reproduced;

• Chapter 3 is partly based on: Lu, X., et al., The application of hierarchical structures in energy devices: new insights into the design of solid

oxide fuel cells with enhanced mass transport. Energy & Environmental Science, 2018. 11(9): p. 2390-2403.

• Chapter 4 is reproduced from: Rabuni, M.F., et al., Electrode design for direct-methane micro-tubular solid oxide fuel cell (MT-SOFC). Journal

of Power Sources, 2018. 384: p. 287-294.

• Chapter 5 is partly based on: Li, T., et al., Design of next-generation ceramic fuel cells and real-time characterization with synchrotron X-ray diffraction

computed tomography. Nature Communications, 2019. 10(1): p. 1497.

*Note that all journal articles are under “Open Access” licence (http://creativecommons.org/licenses/by/4.0/)

Table A1: Permission to re-use content from the literature.

Page Type of

work

Name of work Source of work Copyright

holder

Permission

requested

Permission

license no.

34 Figure Fig. 3. TPB regions for (a) pure

electronic conductor anode, (b)

mixed electronic ionic conductor

anode and (c) electronic–ionic

composite anodes

Properties and development of Ni/YSZ

as an anode material in solid oxide fuel

cell: A review, Renewable and

Sustainable Energy Reviews, 2014. 36:

p. 149-179.

© Elsevier Yes 4505870745687

http://creativecommons.org/licenses/by/4.0/

Appendices

212

36 Figure Fig. 1. Conductivity of yttria and

scandia stabilized zirconia in air at

1000 °C.

Electrolytes for solid oxide fuel cell,

Journal of Power Sources, 2006. 162(1):

p. 30-40.

© Elsevier Yes 4505870975103

41 Figure Fig. 1. Typical SOFC designs. Recent fabrication techniques for micro-

tubular solid oxide fuel cell support: A

review, Journal of the European

Ceramic Society, 2015. 35(1): p. 1-22

© Elsevier Yes 4505871401723

44 Figure Fig. 1. Single cell fabrication

process.

Fabrication and characterization of

micro tubular SOFCs for operation in

the intermediate temperature. Journal of

Power Sources, 2006. 160(1): p. 73-77.

© Elsevier Yes 4510680401451

45 Figure Fig. 1. The external view of the co-

extruder.

Design and characterisation of a co-

extruder to produce trilayer ceramic

tubes semi-continuously, Journal of the

European Ceramic Society, 2001. 21(7):

p. 883-892.

© Elsevier Yes 4505880290374

62 Figure Fig. 1. Operating RSOFC

principles: SOFC and SOEC

modes.

Current developments in reversible solid

oxide fuel cells. Renewable and

Sustainable Energy Reviews, 2016. 61:

p. 155-174.

© Elsevier Yes 4513730977862

64 Figure Fig. 3. ΔH, ΔG and TΔS as a

function of temperature for water

splitting and CO2 electrolysis.

Solid oxide electrolysis – a key enabling

technology for sustainable energy

scenarios. Faraday Discussions, 2015.

182(9): p. 9-48.

© Royal

Society of

Chemistry

Yes 4513740501420

MICRO-TUBULAR AND MICRO-MONOLITHIC SOLID OXIDE FUEL … · for their financial support under...

Documents

Transcript of MICRO-TUBULAR AND MICRO-MONOLITHIC SOLID OXIDE FUEL … · for their financial support under...