Fabrication of High Quality One Material Anode

i

Fabrication of High Quality One Material Anode and Cathode

for Water Electrolysis in Alkaline Solution

By

Jingshu JIA

A Thesis Submitted to

The Hong Kong University of Science and Technology

in Partial Fulfilment of the Requirements for

the Degree of Master of Philosophy

in the Environmental Engineering Program

School of Engineering

December 2008, Hong Kong

Copyright © by Jingshu JIA 2008

iv

Acknowledgements

I would like to express my deepest appreciation to my supervisor Prof. Guohua Chen, for introducing this exciting research topic to me. During my whole master studies, he gave me excellent guidance, invaluable suggestions, enthusiastic encouragement. I always feel inspired by his dedication to excellence, and careful attention to details. My overseas experience to Tokyo Institute of Technology would not be possible without his support. I feel lucky to have Prof. Guohua Chen as my supervisor. I wish to thank Prof. Xijun Hu and Prof. Chii Shang for their serving on the committee and providing constructive comments. I would like to thank my group mates, Dr. Wei Wang, Dr. Caideng Yuan, Dr. Liang Guo, Dr. Minyong Xiong, Mr. Huanjun Zhang, Mr. Xusong Qin, Mr. Bin Yang, Mr. Jing He, Miss Shanshan Wang, Miss Mo Lin and Miss Molenda for their friendship assistance, discussion and encouragement during my study. It is fortune for me to have many friends in HKUST, Dr. Ke Yao, Dr. Junde Lv, Mr. Qilei Zhang, Mr. Feng Zhou, Dr. Huan Wang, Miss Wenjing Zhang, Miss Min Zuo, Ms. Rujie Wang, Miss Yan Geng, Miss Wuman Luo and Mr. Zhiyan Li. Miss Yan Geng, Miss Wenjing Zhang，Miss Shanshan Wang and Mr. Jing He have lent their kind help during my thesis writing and revision. Without their help, I cannot imagine how I can complete this work in less than a month. I would like to express my thanks to the technicians in the Department of Chemical and Bimolecular Engineering, Civil and Environmental Engineering, and MCPF, especially Mr. Ho, Mr. Huang, and Ms. Cheung, for their helpful assistance in my research. I would like to thank my friends outside HKUST, including Mr. Guanghui Wang, Miss Jiahua Huang, Miss Xuesong Du. They always provide moral support to me at the needed time. Finally, I would like to thank my parents for their unconditional love, understanding and support. Without their encouragement and care, this work could not have been finished in such a short time.

v

Table of Contents

Title i

Authorization ii

Signature iii

Acknowledgements iv

Table of Contents v

List of Figures viii

List of Tables xi

Abstract xii

Chapter 1 INTRODUCTION 1

1.1 Background 1

1.2 Research Objectives 4

Chapter 2 LITERATURE REVIEW 5

2.1 Introduction of Electrochemistry 5

2.2 Electrode Fabrication Methods 6

2.2.1 Thermal Decomposition Method 7

2.2.2 Sol-gel Method 7

2.2.3 Electrostatic Spray Deposition (ESD) 8

2.2.4 High-energy Ball Milling (HEBM) 9

2.2.5 Chemical Vapor Deposition (CVD) 9

2.3 Application of Electrochemistry 10

2.4 Electrochemical Technology in Environmental

Application

10

2.5 Electroflotation 12

2.5.1 Overview 12

2.5.2 Electrodes for O2 Evolution 14

2.5.2.1 Graphite, PbO2 and Pt 14

2.5.2.2 DSAs 14

2.5.3 Electrodes for H2 Evolution 17

vi

2.5.3.1 Metals and Alloys 17

2.5.3.2 DSAs 18

2.6 Summary 20

Chapter 3 EXPERIMENTAL SET UP 22 3.1 Chemicals and Materials 22

3.2 Preparation of Ti/metal Oxide Electrodes 23

3.2.1 Pretreatment of Ti Substrate 23

3.2.2 Precursor Solution Preparation 23

3.2.3 Electrode Preparation 23

3.3 Accelerated Service Life Tests 24

3.4 Physiochemical Characterization 25

3.4.1 Scan Electron Microscopy (SEM) 25

3.4.2 X-ray Diffraction (XRD) 26

3.4.3 X-ray Photoelectron Spectroscopy (XPS) 26

3.4.4 Inductively Coupled Plasma Mass

Spectrometry (ICP-MS)

26

3.4.5 Coating Resistivity 26

3.5 Electrochemical Characterization 26

Chapter 4 SEARCHING FOR PROPER ELECTRODE MATERIALS

29

4.1 Introduction 29

4.2 Experimental Results 31

4.2.1 Ir-based Electrodes 31

4.2.2 Other Metals-based Electrodes 34

4.2.3 Ru-based Electrodes 37

4.2.4 Co-based Electrodes 43

4.3 Summary 52

Chapter 5 STUDIES OF Ti/CuxCo3-xO4 ELECTRODES FOR BOTH OXYGEN AND HYDROGEN EVOLUTION IN ALKALINE SOLUTION

53

5.1 Introduction 53

5.2 Molar Ratio Optimization 54

vii

5.2.1 Accelerated Service Life Test 54

5.2.2 EDX 56

5.3 Electrolyte Temperature Effect on Accelerated

Service Life

56

5.4 Physicochemical Characterization 57

5.4.1 SEM 57

5.4.2 XRD 59

5.4.3 XPS 61

5.4.4 ICP 63

5.4.5 Coating Resistivity 63

5.5 Electrochemical Behavior 64

5.5.1 Cyclic Voltammetry 64

5.5.2 Polarization Curves for Hydrogen Evolution 65

5.6 Electrode Surface Changing and Mechanism of

Oxygen and Hydrogen Evolution in Alkaline Solution

69

5.6.1 Fixed Current Direction Electrolysis Process 69

5.6.2 Alternating Current Direction Electrolysis

Process

72

5.7 Summary 88

Chapter 6 CONCLUSIONS 89 6.1 Conclusions of Present Study 89

6.2 Recommendations for Future Work 90

Reference 92

viii

List of Figures

Figure 2.1 Schematic diagram of the experimental apparatus

used for electrostatic spray deposition

8

Figure 3.1 The process of Ti substrate pretreatment 23

Figure 3.2 Experimental setup for accelerate service life test 25

Figure 3.3 Schematic diagram of a three-electrode cell 27

Figure 4.1 SEM picture of Ti/IrO2-Sb2O5-SnO2 electrode 32

Figure 4.2 SEM picture of Ti/PdO-Sb2O5-SnO2 electrode 33

Figure 4.3 SEM picture of Ti/IrO2-Sb2O5-SnO2 with PdO-Sb2O5-

SnO2 electrode

33

Figure 4.4 SEM picture of Ti/RhOx-Sb2O5-SnO2 electrode 34

Figure 4.5 SEM picture of Ti/PtOx-Sb2O5-SnO2 electrode 35

Figure 4.6 SEM picture of Ti/Au electrode 36

Figure 4.7 SEM picture of Ti/Au-Sb2O5-SnO2 electrode 37

Figure 4.8 SEM picture of Ti/ RuO2-Sb2O5-SnO2 electrode 38

Figure 4.9 SEM picture of Ti/RhOx-RuO2-Sb2O5-SnO2 electrode 38

Figure 4.10 SEM picture of Ti/PdO-RuOx-Sb2O5-SnO2 electrode 40

Figure 4.11 SEM picture of Ti/ RuO2-TiO2 electrode 41

Figure 4.12 SEM picture of Ti/RuO2-TiO2-CeO2 electrode 41

Figure 4.13 SEM picture of Ti/RuO2-ZrO2 electrode 43

Figure 4.14 SEM picture of Ti/Ru0.9Co2.1O4 electrode 44

Figure 4.15 SEM picture of Ti/Rh0.1Co2.9O4 electrode 45

Figure 4.16 SEM picture of Ti/Ir0.9Co2.1O4 electrode 47

Figure 4.17 SEM picture of Ti/Ni1.5Co1.5O4 electrode 48

Figure 4.18 SEM picture of Ti/Cu0.3Ni1.35Co1.35O4 electrode 49

Figure 4.19 SEM picture of Ti/Cu1.5Co1.5O4 electrode 50

Figure 4.20 Cathode accelerated service life in acid electrolyte

with fixed current direction electrolysis process

51

Figure 4.21 Cathode accelerated service life in alkaline electrolyte

with alternating current direction electrolysis process

51

ix

Figure 5.1 Temperature effect on accelerated service life 57

Figure 5.2 SEM images of oxide coating surfaces. (a) Co3O4;

(b) Cu0.1Co2.9O4; (c) Cu0.3Co2.7O4; (d) Cu0.6Co2.7O4;

(e) Cu0.9Co2.1O4; (f) Cu1.2Co1.8O4; (g) Cu1.5Co1.5O4

59

Figure 5.3 XRD patterns for Ti/CuxCo3-xO4 electrodes (x=0, 0.3,

1.5), pure titanium substrate and Ti/CuO electrode

60

Figure 5.4 XPS spectra of Ti/Cu0.3Co2.7O4 electrode 62

Figure 5.5 Cyclic votammograms for Ti/Cu0.3Co2.7O4 in 1M

NaOH obtained at a sweep rate of 50 mV/s. The

electrodes had an area of 2.55 cm2

65

Figure 5.6 Potential versus current density curves for hydrogen

evolution from 1 M NaOH on a Ti/Cu0.3Co2.7O4

electrode. (1) First run; (2) second run. Arrows

indicate the direction of potential variation

67

Figure 5.7 Application of Equation 5.2 in the text to the current-

potential data (2nd run, reverse direction) of Figure

5.6 to derive ohmic drop and Tafel slopes. Insert:

enlargement of initial part of the main plot to give

evidence to the intercept

68

Figure 5.8 Experimental data corrected for IR drop 68

Figure 5.9 XRD patterns of Ti/Cu0.3Co2.7O4 virgin electrode 70

Figure 5.10 XRD patterns of fixed current direction electrodes.

(a) anode, (b) cathode

71

Figure 5.11 XRD patterns of the first pair (one day using) of

electrodes. (a) anode (E 1), (b) cathode (E 2)

74

Figure 5.12 SEM pictures of the first pair (one day using) of


75

Figure 5.13 SEM pictures of the second pair (two days using) of


75

Figure 5.14 XRD patterns of the second pair (two days using) of


76

Figure 5.15 XRD patterns of Ti substrate 78

x

Figure 5.16 XRD patterns of the third pair (ten days using) of


79

Figure 5.17 SEM pictures of the third pair (ten days using) of


80

Figure 5.18 SEM pictures of the fourth pair (twenty days using) of


80

Figure 5.19 XRD patterns of the fourth pair (twenty days using) of


81

Figure 5.20 XRD patterns of the fifth pair (thirty days using) of


83

Figure 5.21 SEM pictures of the fifth pair (thirty days using) of


84

Figure 5.22 SEM pictures of the sixth pair (forty days using) of


84

Figure 5.23 XRD patterns of the sixth pair (forty days using) of


85

Figure 5.24 XRD patterns of the used out electrodes. (a) anode (E

1), (b) cathode (E 2)

86

Figure 5.25 SEM pictures of the used out electrodes. (a) anode (E

1), (b) cathode (E 2)

87

xi

List of Tables

Table 2.1 Economic parameters in treating oily effluents 12

Table 4.1 Accelerated service life of Ti/RuO2-ZrO2 electrode 42

Table 4.2 Accelerated service life of Ti/Ru0.9Co2.1O4 electrode 44

Table 4.3 Accelerated service life of Ti/Rh0.1Co2.9O4 electrode 45

Table 4.4 Accelerated service life of Ti/Ir0.9Co2.1O4 electrode 46

Table 4.5 Accelerated service life of Ti/Ni1.5Co1.5O4 electrode 48

Table 4.6 Accelerated service life of Ti/Cu0.3Ni1.35Co1.35O4

electrode

49

Table 4.7 Accelerated service life of Ti/Cu1.5Co1.5O4 electrode 50

Table 5.1 Accelerate service life of Ti/CuxCo3-xO4 electrodes in

1 M NaOH solution under 10,000 Am-2 at 35 °C

55

Table 5.2 Atomic ratios for Ti/CuxCo3-xO4 electrodes nominal

and experimental obtained by EDX techniques

56

Table 5.3 Surface compositions of Cu0.3Co2.7O4 films measured

by XPS

63

Table 5.4 Effect of Cu content on coating resistivity 64

xii

Fabrication of High Quality One Material Anode and Cathode

for Water Electrolysis in Alkaline Solution

By

Jingshu JIA

Environmental Engineering Program

School of Engineering

The Hong Kong University of Science and Technology

Abstract

Electroflotation is receiving more and more attention for wastewater treatment.

This electrochemical process employs the oxygen and hydrogen gas bubbles

evolved from the surfaces of anode and cathode during water electrolysis. The

lack of ideal electrodes that can service as both anode and cathode has been a

problem since the very beginning of the study of this process. This dissertation

reports a kind of unique electrode that can partly solve this problem.

xiii

After careful examination of 16 different materials including metal oxides of Ir,

Ru, Rh, Pt, Sb, Sn, Ti, Cu, Pd, Ni and Co, the CuxCo3-xO4 combination was

found to be the candidate for the one material electrode. The Ti/CuxCo3-xO4

electrode (x = 0~1.5) developed in the present work was much more stable

compared with the other electrodes. All the electrodes were prepared by

thermal decomposition at 550 °C from metal chloride salts precursors which

were dissolved in a mixture of 37 % HCl and isopropanol solution. The stability

of Ti/Cu0.3Co2.7O4 electrode was found to be the best, lasting for 1080 h under

daily alternative anodic and cathodic polarization in 1 M NaOH electrolyte.

The Ti/CuxCo3-xO4 electrodes were characterized by different techniques such

as scanning electron microscopy connected with energy-dispersive X-ray (SEM-

EDX), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS),

cyclic voltammetry and quasi-steady state polarization curves. The effect of

crystallographic properties and surface morphology have been analyzed

systemically before and after this one material electrodes used in water

electrolysis process. Under hydrogen evolution condition, cobalt oxide could be

reduced to amorphous metal Co and Co(OH)2. The formation of the amorphous

Co is the key process for this kind of electrode to be served as both anode and

cathode under alternative current. It is also the main reason leading to the

failure of the electrode eventually.

1

CHAPTER 1 INTRODUCTION

1.1 Background

Since the early 16th and 17th centuries, electrochemistry went through several

evolutions involving complex theories such as conductivity, electrical charge and

mathematical methods, till late 19th century when the term electrochemistry was

used to describe electrical phenomena. Nowadays, electrochemistry has

become an area of current research, including research in batteries and fuel

cells, corrosion prevision of metals, and techniques in electrolysis cells

improvement (Wikipedia). Scientists and engineers working in diverse areas

need to locate and use electrochemical technology (Bard, 2006).

For decades electrochemical technologies have contributed successfully to

environmental protection (Janssen and Koene, 2002) mainly dealing with water

and wastewater, wastes and soils treatment. A particular focus is given to

electrocoagulation (EC), electrodeposition (ED), electroflotation (EF),

electrokinetic (EK), electrooxidation (EO), electrochemical disinfection (ECD)

and photoelectrooxidation (PEO) processes. These technologies are effective in

improving the treatment quality of industrial wastes, wastewater and drinking

water on integration into a treatment plant or replacement of conventional

processes that are found to be less effective to eliminate specific organic and

inorganic pollutants (Patrick et al., 2007). All these technologies are conducted in

electrolysis cells, which have three basic components: an electrolyte and two

electrodes. The performance of electrode could restrict efficiencies of all these

electrochemical processes (Macdonald and Schmuki, 2007).

Electroflotation, one of the environmental protection wastewater treatment

technologies, is an attractive method at present (Chen et al., 2002), because of

its high efficiency, compact facility, and easy operation (Bockris and Khan, 1993;

Sequeira, 1994) and could be considered as the most effective process in

2

removing colloidal particles, oil & grease, as well as organic pollutants (Chen,

2004).

In an electrofloatation (EF) process, pollutants adhered onto tiny bubbles of

hydrogen and oxygen generated from electrolysis of water. The electrochemical

reactions at the cathode and anode are hydrogen and oxygen evolution,

respectively. Considering the increasing importance of EF as a tool for

environmental application (Casqueir et al., 2006), many electrodes have been

fabricated for electroflotation, with Dimensional Stable Anodes (DSAs) being the

most widely used. For example, Ti/IrO2-Sb2O5-SnO2 (Chen et al., 2001) should

be considered as one of the best DSAs which could last for 1600 h in 3 M H2SO4

solution under a current density of 1 Acm-2 at 35 °C. Besides Ti/IrO2-Sb2O5-SnO2

electrode, Chen and Chen (2005) also developed Ti/RuO2-Sb2O5-SnO2

electrode which had a good performance with accelerated service life of 307 h in

3 M H2SO4 solution under a current density of 0.5 Acm−2 at 25 °C for anode which

is believed to be stable enough for applications in which low current densities

(<0.02 A cm-2) are required.

In real electroflotation application, DSAs and metal such as stainless steel are

employed as anode and cathode, respectively. After a certain time of working, a

lot of substances or produced polymers would attach onto the surfaces of both

electrodes which may severely decrease the current efficiency of the

electroflotation. If chloride and heavy metal ions contained wastewater is treated

by electroflotation process, the metal cathode can suffer from poisoning effect by

chloride corrosion and metal deposits which also influence the electroflotation

efficiency (Nidola and Schira, 1990).

To solve these problems, it is proposed to find a DSA-based electrode that can

be utilized as anode and also cathode. The main benefits of using one material

for both anode and cathode are listed below:

3

• Self-cleaning. After certain operating time of electroflotation, the anode

and cathode may have some foreign materials deposited. Such a

deposition would hinder the performance of the electrodes. If one material

electrodes are installed in the electrolysis cell, the deposited materials

may be removed by changing the polarity of the electrodes. This

operation would lead to the self-cleaning of the electrode surfaces

automatically.

• Tolerating chlorides and metal ions. The service life of platinum or

nickel cathode is influenced greatly by the concentration of chloride and

metal ions in solution, which could easily deactivate the cathode surface.

As a consequence of the surface chemistry of oxides, compared to the

respective metals, metal oxides may be less sensitive to the poisoning by

chloride and metal deposits (Nidola and Schira, 1990).

• Reducing the cost from mal-operation. Anode material is usually more

expensive than the cathode materials in an electroflotation system.

However, as discussed before, a quick loss of stability may be resulted

when there is a human error in the installation and operation of the

system, if the material can only function as an anode. One material

electrode would eliminate such a problem since the electrode is

insensitive to the polar connection.

• Saving electricity cost. The most conventional cathode materials used

today are mild steel, stainless steel and nickel exhibiting overvoltages

ranging from 300 to 400 mV. These overvoltages are much higher than

the anode overvoltage with a level normally lower than 50 mV (Cornell

and Simonsson, 1993). The one material electrodes made of metal oxides

similar to those Dimension Stable Anodes (DSAs) could decrease the

cathode overvoltage dramatically to 70 mV (Houda et al., 2003). This

would proportionally reduce the electrical energy consumption.

4

1.2 Research Objectives

With the background of research as introduced above, the research objectives of

this study are therefore

• to fabricate highly dimensional stable electrodes to be applied as both

anode and cathode with long service life, good activity and low cost,

• to investigate physicochemical and electrochemical characteristics of

the target electrodes,

• to evaluate the performance of the novel electrode for oxygen and

hydrogen evolutions, and,

• to explore the mechanism of the electrode failure.

5

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction of Electrochemistry

Electrochemistry is the branch of chemistry concerned with the interrelation of

electrical and chemical effects. A large part of this field deals with the study of

chemical changes caused by the passage of an electric current and the

production of electrical energy by chemical reactions (Bard, 2006).

Modern electrochemistry presents itself as a strongly diversified field, which one

would call “applied electrochemistry”, ranging from electrochemical processes

for synthesis of inorganic or organic compounds, energy conversion, sensor

development, electro-analytical devices, electrolytic anti-corrosion to solid-state

ionic, electrode fabrication methods and any kind of system where charge

transfer is involved at an electrified interface between electronic and ionic

conductors. Research is more and more specialized on very particular problems,

to find proper electrode material for different electrochemical processes for

example (Macdonald and Schmuki, 2007).

Applied electrochemistry can provide valuable cost efficient and environmentally

friendly contributions to industry process development with a minimum of waste

or toxic material production. Examples are the implementations of

electrochemical effluent treatment, such as the removal of suspended solids

from solutions, destruction of organic pollutants, or abatement of toxic gases

(Macdonald and Schmuki, 2007).

The main advantages of electrochemical processes are as follows (Macdonald

and Schmuki, 2007):

• Versatility: Direct or indirect oxidation and reduction, phase separation,

concentration or dilution, biocide functionality, applicability to a variety of

6

media and pollutants in gases, liquids, and solids, and treatment of small

to large volumes from microliters up to millions of liters.

• Energy efficiency: Lower temperature requirements than their

nonelectrochemical counterparts, for example, anodic destruction of

organic pollutants instead of thermal incineration; power losses caused by

inhomogeneous current distribution, voltage drop, and side reactions

being minimized by optimization of electrode structure and cell design.

• Amenability to automation: The system inherent variables of

electrochemical processes, for example, electrode potential and cell

current, are particularly suitable for facilitating process automation and

control.

• Cost effectiveness: Cell constructions and peripheral equipment are

generally simple and, if properly designed, also inexpensive.

Since electrochemical reactions take place at the interface of an electronic

conductor, the electrode, and an ion conductor, the electrolyte, electrochemical

processes are heterogeneous in nature. This implies that, despite the

advantages mentioned above, the performance of electrochemical processes

suffers from the performance of electrode and the size of the limited electrode

area. Another crucial point is also concern with the stability of cell components

which always in contact with aggressive media; in particular, the durability and

long-term stability of the electrode material and catalyst (Parsons, 1961).

2.2 Electrode Fabrication Methods Electrodes can be fabricated in many ways. For pure metal electrodes, they are

relatively easy to fabricate. For example, current industrial carbon electrodes are

typically manufactured by blending petroleum coke particles (the filler) with

molten coal tar pitch (the binder) and extruding the resultant mix to form the

“green electrode”. This is then baked under controlled conditions. In case of

usage as anodes in steel electric furnaces (or as other carbon and graphite

products), the electrodes could undergo further processing like pitch

7

impregnation or graphitization (Adumbrava et al., 1999). In the following text,

some typical electrode fabrication methods in these days’ research and

application will be reviewed.

2.2.1 Thermal Decomposition Method

The thermal decomposition method is commonly used nowadays, due to its easy

condition and low equipment cost (Beer, 1972, 1973, 1980, 1982; Novak et al.,

1982). The detailed schematic diagram of thermal decomposition method for

metal oxide electrode producing will be shown in next chapter and all electrodes

fabricated in this project are applied thermal decomposition method. When apply

this method, normally the corresponding metal chloride salts and metal nitrate

salts are used as the respective metal sources, which are dissolved into mixture

of isopropanol and hydrochloric acid to form precursor solution (Fachinotti et al.,

2006). The main principle of thermal decomposition method is oxidation

reactions which happened on electrode surface in high temperature furnace. By

changing brush cycles, calcination temperature and precursor compositions,

electrochemical and physicochemical properties of electrodes could be easily

controlled (Chen, 2002).

2.2.2 Sol-gel Method

The sol-gel process is a wet-chemical technique (Chemical Solution Deposition)

for the fabrication of materials starting either from a chemical solution (sol short

for solution) or colloidal particles (sol for nanoscale particle) to produce an

integrated network surface morphology (Wikipedia). Sol-gel synthesis is widely

used for making transition-metal oxide solids with fine-scaled microstructures

(Anderson and Bard 1995; Antonelli and Ying 1995; Lakshmi et al., 1997).

Sol-gel preparation, which involves the formation of a sol followed by formation

of a gel, typically uses either colloidal dispersions or inorganic precursors as the

starting material. With an alkoxide (M(OR)n) as a precursor solution, sol-gel

chemistry can be described in terms of two classes of reactions (Ward and Ko,

1995):

8

Hydrolysis: MOR + H2O → MOH + ROH (2.3)

Condensation: MOH + ROM → MOM + ROH (2.4)

or MOH + HOM → MOM + H2O (2.5)

Sol-gel research grew to be so important that in the 1990s more than 35,000

papers were published worldwide on the process (Wikipedia).

2.2.3 Electrostatic Spray Deposition (ESD)

The ESD technique has shown many advantages over several conventional

deposition techniques, such as a simple set-up, inexpensive and nontoxic

precursors, high deposition efficiency and, in particular, easy control of the

surface morphology of the deposited layers by changing various parameters,

such as precursor concentration, deposition temperature, deposition time and

liquid flow rate. This technique is widely used in fabrication cathode for energy

conversion using solid oxide fuel cells (SOFCs) because of its capability of

providing un-agglomerated mono-disperse particles in the nanometer diameter

range (Taniguchi et al., 2002).

Figure 2.1 Schematic diagram of the experimental apparatus used for electrostatic spray deposition. 1.Micro feeder, 2.Srainless steel nozzle,

3.Test section, 4.Substrate, 5.Heating element, 6.D.C. high voltage supply, 7.CCDcamera and monitor, 8.Light source

9

2.2.4 High-energy Ball Milling (HEBM)

HEMB is known as an effective means of preparing nonequilibrium and

metastable phases, amorphous alloys etc. In summary, high-energy mechanical

alloying by ball-milling consists of a powder mixture by repeated mechanical

deformations. During milling, numerous structural defects are created which

increase the interatomic diffusion, thereby allowing a solid-state reaction to

proceed. Also strain is introduced in the lattice during milling, which results in

crystal breaking into smaller pieces down to the nanometer range. Metal oxides

is milled together, added water and polytetraflorthylene to form slurry, then slurry

is painted on substrate and dried under a nitrogen flow. HEMB method could

resist precipitation kinetics of two (or more) dissimilar metallic ions (Gaudet et al.,

2005). But the adhesion effect between substrate and metal oxide film is poor.

2.2.5 Chemical Vapor Deposition (CVD)

CVD has been a powerful technique in synthesizing film-structured materials with

a variety of morphologies and properties such as metal oxide semiconductors,

diamond-related materials, carbon nanotubes, etc. In a typical CVD process, the

substrate is exposed to one or more volatile precursors, which react and/or

decompose on the surface of the substrate to produce the desired deposit.

Frequently, volatile byproducts are also produced, which are removed from the

reaction region by gas flow though the reaction chamber (Zhang et al., in press).

Particularly in the fabrication of film-structured diamond electrodes, some new

techniques have been incorporated on the basis of the traditional CVD and such

new methods as hot-filament vapor deposition (HFCVD). Boron-doped

diamond-film-coated titanium (Ti/BDD) has fabricated by HFCVD, the

accelerated working lifetime was significantly increased to 804 h for the 2-T

electrode, compared with 244 h for the diamond-film electrode fabricated under

the one-temperature (1-T)-stage method (Guo and Chen, 2007).

10

2.3 Application of Electrochemistry

Electrochemistry principle involves redox reactions (shorthand for

reduction-oxidation reactions) which describe all chemical reactions where an

electron is transferred to or from a molecule or ion changing its oxidation state.

This reaction can occur through the application of an external voltage or though

the release of chemical energy. Redox reactions are the foundation of

electrochemical cells (Milos, 1990; 1996). Electrochemical reactions have many applications, but those electrochemical

reactions which take place in cells have unique applications because cell

reactions can involve electricity flowing through metallic conductors. For this

reason, electrochemical cells can serve as sources of electrical power such as in

a fuel cell. The same processes are also responsible for the deleterious

corrosion of metals. Electrochemical reactions which consume electrical power

can have useful effects, as in the processes of industrial electrolysis (Plambeck,

1998).

Electrolytic industrial processes for metals include the production of metals

themselves from their compounds, which is called the electrowinning of metals;

the electrolytic purification of metals; and the deposition or electroplating of

metals on conducting surfaces. In all these types of electrolytic process, the

reactions are reduction of ions of the metal in solution in some carefully selected

electrolyte. While most of the electrolytic processes that involve metals are

reductions, most of the electrolytic processes that involve nonmetals are

oxidations. Among them, electrolysis of water process uses both the anodic

oxidation and cathodic reduction processes (Plambeck, 1998).

2.4 Electrochemical Technology in Environmental Application

Electrochemical technology has contributed successfully to environmental

protection for over a century (Chen, 2004; Janssen and Koene, 2002).

11

Electrocoagulation (EC) can be used for clarifying water or to eliminate organic,

inorganic and microbial pollutants from wastewater. The technology delivers in

situ coagulant by anodic dissolution of sacrificial electrodes (aluminium or iron)

and is finding wider applications with the possible improvements in the reduction

of energy consumption and metallic sludge production. Electrodeposition (ED) is

a well-established process for toxic metal removal whose effectiveness can be

greatly improved by selecting suitable electrode material (type and geometry of

the electrode). Electroflotation (EF) technology is effective in removing

suspended solids, colloidal particles and oil & grease suspension. Electrokinetic

(EK) technology is a promising method to remove toxic metals from the matter

with low hydraulic permeability or to enhance dewaterability of wastes including

soils and sludge. The development of new, more stable and active catalytic

electrodes has led to a renewed interest in Electrooxidation (EO) for degrading

toxic or biorefractory pollutants (Chen, 2004; Patrick et al., 2007).

Electrodisinfection (ECD) is a process in an electrolytic system equipped with

electrodes on which electric current is applied and it can be considered as

effective method because of its primary advantage of in situ production of

disinfectants in the treatment system (Fang et al., 2006).

The performance of an electroflotation system is affected by the pollutant

removal efficiency and the power and/or chemical consumptions. The pollutant

removal efficiency is largely dependent on the size of the bubbles formed. For

the power consumption, it relates to the cell design, electrode materials as well

as the operating conditions such as current density, water conductivity, etc.

(Chen, 2004). Table 2.1 summaries the comparison of different flotation

processes for treating oily wastewater (Il’in and Sedashova, 1999).

12

Table 2.1 Economic parameters in treating oily effluents

Treatment type EF DAF IF Settling

Bubble size (μm) 1-30 50–100 0.5–2

Specific electricity consumption

(W/m3)

30–50 50–60 100–150 50–100

Air consumption (m3/ m3) water 0.02–0.06

1

Chemical conditioning IC

OC +F OC IC + F

Treatment time (min) 10–20

30–40 30–40 100–120

Sludge volume as percent of

treated water

0.05–0.1

0.3–0.4 3–5 7–10

Oil removal efficiency (%) 99–99.5

85–95 60–80 50–70

Suspended solids removal

efficiency (%)

99–99. 5

90–95 85–90 90–95

Low cost and easy operation characters are the advantages of electroflotation

process, since the objective of this project is searching one material as both

anode and cathode, more details of electroflotation analysis will be given in next

section.

2.5 Electroflotation

2.5.1 Overview

In wastewater treatment, flotation is an indispensable process for the separation

of oil and low-density suspended solids from wastewater (Lafrance and Grasso,

1995; Chen and Horan, 1998; Huang and Liu, 1999; Vaughan et al., 2000;

Manjunath et al., 2000). Electroflotation was first proposed by Elmore (1905) for

flotation of valuable minerals from ores. This process is essentially an

electrochemical version of flotation. It differs from dissolved air flotation which is

13

the traditional source for flotation mainly in the mechanism of bubble generation

(Chen, 2002).

Electroflotation has been receiving more and more attention because of its

important features of high separating efficiency, compact facility, and easy

operation. It can be used to treat palm oil mill effluent that about 40% of the COD

of the dissolved substances could be anodically destroyed together with 86% of

suspended particles, and then floated off (Ho and Chan, 1986) When apply to

oil-water emulsion which the oil separation reached 65% at optimum conditions;

75% in the presence of NaCl (3.5% by wt. of solution); and 92% with the

presence of NaCl and at optimum concentration of flocculant agent (Hosny,

1996). For mining wastewater for the condition that wastewater of an opencast

mine containing about 50 mg/L copper ions is purified (Alexandrova et al., 1994).

Groundwater that the electroflotation device was used successfully to remove Ni,

Zn, Pb, Cu/CN in a polluted groundwater obtained from directly under a

contaminated site, meeting the pretreatment standards of the pollutants for the

local POWT sewer (Poon, 1997). In restaurant wastewater treatment, the

removal efficiencies of oil and grease, COD, and SS are high, being 99, 88, and

98%, respectively (Chen et al., 2000). And crude oil wastewater remediation

could reach to 40% COD removal after 12 h (Santo et al., 2006).

In electroflotation, the useful small O2 and H2 bubbles are generated on an anode

and a cathode, respectively, as below:

OH- – 2e- → ½ O2 + ½ H2O on an anode (2.1)

H2O + 2e- → ½ H2 + OH- on a cathode (2.2)

High stability, high electrical efficiency electrodes for electroflotation play a

crucial role in the whole process. High stability means long working life time with

stable working performance and little contamination to the environment coming

from the dissolution of the metal oxides. High electrical efficiency is closely

related to the electrocatalysis performance. However, the performance of

14

electrolytic cell is not dictated only by the quality of its anodic and cathodic

electrocatalysts but also by the resistance of the electrolyte between the two

electrodes, which must be minimized, and by the rates of mass transport of

reactants and products to and from the two electrodes (Macdonald and Schmuki,

2007). Detailed information about electrodes applied in electrochemistry industry

will be reviewed subsequently.

2.5.2 Electrodes for O2 Evolution

The selection of anode material, on the other hand, is difficult because severe

electrochemical corrosion always occur when common metals and alloys are

used as anodes. Actually, stable anode materials are quite limited in

electrochemistry (Chen, 2002).

2.5.2.1 Graphite, PbO2 and Pt

The most common non-consumable anode materials are normally Graphite,

PbO2 and Pt. Graphite and PbO2 anodes are cheap and easily available, and thus

have been widely investigated for O2 evolution in electroflotation (Ho and Chan,

1986; Hosny, 1996; Burns et al., 1997). However, the durability of graphite is

poor; the service life of graphite is only 6-24 month due in part to the oxidation of

C to CO2 and to physical wear arising from gas evolution (Novak et al., 1982).

The stability of PbO2 is also poor. Ho et al. (1986) investigated the corrosion of

PbO2 anodes and found that the concentrations of Pb2+ ions present in solutions

ranged from 0.06 to 0.68 mgL-1 after 24 hours of electrolysis. Therefore, graphite

and PbO2 are not good O2 candidates for electroflotation. A few researchers

reported the use of Pt or Pt-plated meshes as anodes for electroflotation (Ketkar

et al., 1991; Poon, 1997). They are much more stable than graphite and PbO2.

However, their known high costs make large-scale industrial applications

impracticable (Chen, 2002).

2.5.2.2 DSAs

The DSAs are the most important anodes nowadays in electrochemical

engineering, invented by Beer in the late 1960s which stands for Dimensional

15

Stable Anodes. Due to the high stability under high current density loading and

high electrical efficiency for chlorine gas production, mixed oxide of ruthenium

and titanium on titanium substrate has become the most popular electrode in

industrial application (Beer, 1980). Usually, DSAs use conductive precious metal

oxides (RuO2, IrO2, etc.) as electrocatalysts and non-conductive metal oxides

(TiO2, Ta2O5, ZrO2, Nb2O5, etc.) as dispersing or stabilizing agents, which are

coated on valve metal substrates (Ti, Ta, Zr, W, Nb, Bi).

a) Ruthenium oxide based electrode (RuOx)

RuO2 is the most widely used electrocatalyst in DSAs. This oxide exhibited

excellent activity for both Cl2 and O2 evolution. The Tafel slope for O2 evolution is

only 0.031-0.066 Vdec-1 (Yeo et al., 1981; Melsheimer and Ziegler, 1988).

Unfortunately, RuO2 is not stable in acidic environments, and the O2 evolution

overpotential increases strictly with time (Burke et al., 1977; Loucka, 1977). The

electrochemical stability can be significantly improved by incorporating other

components into RuO2. Burke and McCarthy (1984) reported that the addition

of 20 molar percent of ZrO2 to the RuO2 layer increased the service life of the

electrode for O2 evolution. Iwakura and Sakamoto (1985) studied the effect of

addition of SnO2 to RuO2 on the service life, and found that the optimal molar

ratio of Ru:Sn was 30:70. The electrodes with a molar ratio of Ru:Sn = 30:70 had

a service life about 12 h, four times higher than the pure RuO2 coated electrode,

under the accelerated life test conditions (5,000 Am-2, 0.5 M H2SO4, 30 oC).

Despite significant improvement in electrochemical stability by introducing

various components, the durability of RuO2-based DSAs is still insufficient for

industrial application. Chen et al. (2005) discovered ternary metal oxide

Ti/RuO2–Sb2O5–SnO2 had a service life of 307 h in 3 M H2SO4 solution under a

current density of 0.5 Acm-2 at 25 °C, which was a great improvement for

ruthenium oxide based electrode’s service life.

16

b) Iridium oxide based electrode (IrO2)

In the last two decades, IrO2-based DSAs have received more and more

attention. When adding the same amount of IrO2 and RuO2, the former presents

a service life about 20 times longer than the latter (Alves et al., 1998).

Nevertheless, IrO2 is much more expensive than RuO2 and its activity is slightly

lower. To save cost and to improve the coating property, other components are

usually added. For optimizing the higher activity and lower cost, Stucki and

Muller (1980) researched on combined IrO2 with RuO2. Subsequently, Hutchings

et al. (1984) developed a ternary IrO2-RuO2-SnO2 catalyst containing 25 mol %

IrO2 and 25 mol % RuO2, leading to a further electrode cost reduction and

durability improvement. In 1991, Balko and Nguyen investigated IrO2-SnO2 over

a composition range of 5 to 100 mol % IrO2. It was found that 80 mol % IrO2 can

be replaced by SnO2 without noticeable deterioration of the coating property.

Ti/IrO2-Ta2O5 electrodes prepared by thermal decomposition of the respective

metal chlorides were successfully employed as oxygen evolving electrodes for

electroflotation of waste water contaminated with dispersed peptides and oils.

Service lives and rates of dissolution of the Ti/IrO2-Ta2O5 electrodes were

measured by means of accelerated life tests, electrolysis in the condition of 0.5

M H2SO4 at 25°C and current density was 2 Acm–2. The steady-state rate of

dissolution of the IrO2 active layer was reached after 600-700 h (R. Mráz and

J. Krýsa, 1994). Chen et al. (2001) fabricated a excellent anode named

Ti/IrOx-Sb2O5-SnO2 electrode had a service life of 1600 h in 3 M H2SO4 solution

under a current density of 1 Acm-2 at 35 °C, which could last more than 20 years

for real electroflotation process and save money by introducing only 10 mol % of

IrOx nominally in the coating.

c) Cobalt oxide based electrode (Co3O4)

Electrode based on Co3O4 have been studied for a long time, a spinel oxide,

shows excellent physicochemical properties that make its application possible as

a noble metal in the preparation of DSAs (Liu et al., 1999; Grupioni and Lassali,

17

2001). Generally, when the research is focus on using Co3O4 as an oxygen

anode, efforts are directed at improving both electrode stability and

electrocatalytic activity. The presence of small amounts of Rh in the Ti/Co3O4

spinel coating increases the oxide electrocatalytic activity towards the oxygen

evolution reaction, acting on the geometric factors and decreasing the ohmic

drop which is an advantage with respect to the performance of the Ti/Co3O4

electrode (Nunes et al., 2005). The IrO2+Co3O4 binary oxide mixed electrodes

were prepared as a candidate for capacitor devices. The oxide layer shows an

amorphous nature made up of a mixture of crystalline and hydrous amorphous

oxides with an inhomogeneous surface composition and segregation of the

component of larger atomic mass. These characteristics lead to a significant

increase in the surface area of the oxide coating (Grupioni and Lassali, 2001).

Accelerated service life tests of the Ti/CuxCo3-xO4 electrodes were performed by

anodic polarization in 1 M NaOH solution showing the best performance after 25

deposition steps and service life efficiency reached to 184 Ahmg-1 (Rosa-Toro et

al., 2006).

2.5.3 Electrodes for H2 Evolution

2.5.3.1 Metals and Alloys

There are many metals and alloys that can be used as cathodes. Stainless steel,

for example, is a good cathode material for electroflotation (Hosny, 1996; Poon,

1997; Chen et al., 2000). But the over potential on the steel cathode today is

about 500 mV compared to about 40 mV on the anode side (Colman, 1981). If

the former could be lowered, to say 100 mV, it would lead to a cut in production

costs of electricity by 10-15%. The benefits of an activated, non-corroding

cathode is obvious (Cornell and Simonsson, 1993)

Another material usually used for cathode in water electrolysis is Ni. It resists to

the strongly alkaline environment, however, it is not very stable and its activity

decreases progressively with time (Tavares and Trasatti, 2000).

18

2.5.3.2 DSAs

As the name shown for DSAs, dimensionally stable anodes which normally used

for anode application. Nowadays, more and more chemists pay their attention on

H2 evolution with DSAs electrodes. However, the most important thing made

metal oxides receiving more and more interested for cathode material

application. As a consequence of the different surface chemistry, oxides may be

less sensitive to poisoning by metal deposits compared with other pure metals or

alloys (Kotz and Stucki, 1987).

a) Ruthenium oxide based electrodes (RuOx)

RuO2 was received no attention to be used as a cathode material for a long time.

The first exception is that Kotz and Stucki (1987) demonstrated that metallic

oxides like RuO2 were insensitive towards poisoning by metal deposition during

hydrogen evolution in acid media. In addition, the catalytic activity of RuO2 for H2

evolution fell between those of platinum and nickel. The exchange current density

of 6×10-5 Acm-2 in combination with a Tafel slope of 40 mV per decade resulted in

an overpotential for hydrogen evolution reaction which was about 50 mV higher

than that of platinum at a current density of 0.1 Acm-2. After that a lot of research

conducted on RuO2 hydrogen evolution electrode. Ni/ (Ni + RuO2) electrodes are

prepared by simultaneous electrodeposition method from a suspension

electrolyte (Watts bath) of RuO2 particles under vigorous stirring condition and

they are found to have excellent characteristics as an active cathode for

hydrogen evolution in hot concentrated alkaline solution (Iwakura et al., 2001).

Ruthenium dioxide as electrocatalyst on an activated cathode can also be used

for chlorate production which is investigated with respect to its activity towards

hydrogen evolution, hypochlorite reduction, and chlorate reduction, respectively.

Ruthenium dioxide is an active electrocatalyst for hydrogen evolution in chlorate

electrolyte with about 300 mV (depending on coating thickness) lower

overvoltages than corroded iron at the technical current density 3 Kam-2 (Cornell

and Sinomsson, 1993). Cathodic activation of the RuO2 (1 1 0) surface is

interpreted as the formation of metallic ruthenium sites which can be re-oxidized

19

to ruthenium dioxide. This process is reversible in the early stages of activation

when no significant surface corrugation is produced. Irreversible surface

roughening can be produced with activation at very negative potentials. On the

other hand, the activation of the RuO2 (1 0 0) surface leads, under even very mild

conditions, to an irreversible increase in surface pseudocapacitance and the

appearance of new voltammetric features. This behavior is attributed to

irreversible morphological changes following hydrogen intercalation into the

subsurface region (Lister et al., 2003).

b) Iridium oxide based electrode (IrOx)

A renewed interest has occurred in the field with the publication a few years ago

of a number of papers reporting that conducting ceramic oxide materials could be

suitable electrocatalysts for hydrogen evolution in both acidic and basic media. In

this respect, IrO2 have been shown to be promising candidates (Chen and Guay,

1996). Hydrous iridium oxide films are highly resistant to reduction under cathodic,

hydrogen gas evolution, conditions in aqueous acid or base. Such behavior is not

in agreement with simple thermodynamic (Pourbaix) data based on the

assumption that the system behaves in a reversible manner (Burke et al., 2006).

A lot of researchers pay their attention on mixture RuxIr1–x Oxide for cathodic

polarization and hydrogen evolution. The CV spectra of mixed Ru-Ir oxide

electrodes possessed both pure RuO2 and IrO2 electrode characteristics,

whereas their electrochemical behavior was close to that exhibited by a pure

IrO2 electrode. Wen and Hu’s study (1992) showed that the maximum

electrocatalytic activity (rated in terms of q*) was demonstrated by the

Ru0.3Ir0.7O2 electrode.

c) Rhodium oxide based electrode (RhOx)

Ti/RhOx electrode shows higher catalytic performance for hydrogen evolution

during cathodic polarization more active than RuO2 as well as IrO2 (Morimitsu et

al., 2000; Campari et al., 2002; Hussanova et al., 2003). The main outcome of

20

hydrogen evolution on RhOx in acid solution is that the oxide is unstable toward

cathodic reduction for calcination temperatures lower than 550°C. While the

apparent electrocatalytic activity increases with decreasing calcination

temperature, the opposite is the case for the true electrocatalytic activity. This

suggests that, below 550°C, the cathodic reduction of the oxide gives rise to a

finely subdivided metal surface whose activity is essentially related to an

increase of surface area (Campari et al., 2002).

d) Cobalt oxide based electrode (Co3O4)

Amorphous metallic alloys (AMAs) have interesting mechanical, magnetic and

electrical properties. Co3O4 is considered as one of AMAs sources applied in

electrochemical systems, the AMAs have been the object of several studies

because of their enhanced corrosion resistance and the possibility that they may

present a good catalytic activity toward complex electrochemical reactions like

the hydrogen evolution reaction (HER) (Raj and Vasu, 1992). The hydrogen

evolution reaction was conducted on nickel-cobalt-molybdenum electrodes in a

30 wt% KOH solution at temperatures from 298 to 353 K. It showed good

adhesion to the substrate, high surface roughness and they enhanced the HER

in water electrolysis compared with nickel, cobalt, nickel-molybdenum and

cobalt-molybdenum deposited electrodes (Fan et al., 1994). Electrodes made

from Co-Pd mixtures were found to be more porous and were more efficient

catalysts compared to the individual metal powders and alloys in HER that

conducted in 6 M KOH electrolyte (Elumalai et al., 2002). Doping Co3O4 with

RuO2 increases the electrocatalytic activity for hydrogen evolution in alkaline

solutions by about two orders of magnitude as the doping content is only 10

mol% RuO2. In this composition range the surface of the mixed oxides is

dramatically enriched with Ru (Krstajic and Trasatti, 1997).

2.6 Summary

Based on the above literature review, it is clear that DSAs remains a promising

21

anode for electroflotation application. Especially, Ti/IrOx-Sb2O5-SnO2 electrode

has a service life of 1600 h in 3 M H2SO4 solution under a current density of 1

Acm-2 at 35 °C, which could last more than 20 years for real electroflotation

process. DSAs have also been tested as cathode for hydrogen evolution in

recent years, however, most of authors pay their attention to electrochemical

properties, and for example, Tafel slope, reaction order and cyclic

voltammograms etc. parameters with their service lives seldom reported.

It is possible for one DSA material to be suitable to serve as an anode and also a

cathode. For the cathode DSA materials, the test of service life is requested to fill

the need for industrial design and operation plan. It is well known that an

electrode is eventually worn out or failed after certain time operation. In order to

prolong the service life of an electrode, it is essential to understand the

mechanism of its failure so that one can prevent the failure based on theory.

These research demands therefore become the objectives of the present study

and some interesting results are obtained as seen subsequently.

22

CHAPTER 3 EXPERIMENTAL SET UP

3.1 Chemicals and Materials

The major chemicals and materials used are listed as follows:

A. CeCl3 (99.99+%, Aldrish, USA)

B. CoCl2 (97%, Aldrish, United Kingdom)

C. CuCl2⋅ 6H2O (99%, AnalaR, England)

D. H2SO4 (98%, Acros, NJ, USA)

E. Hydrochloric acid (37%, Riedel-deHaen, Seelze, German)

F. IrCl4⋅ x H2O (53.89% Ir, STREM Chemicals, USA)

G. Iso-propanol (99.7 %, Lab-scan, Bangkok, Thailand)

H. KCl (99%, AnalaR, England)

I. NaOH (99%, AnalaR, England)

J. Na2SO4 (99+%, Aldrich, WI, USA)

K. NiCl2⋅6H2O (99.9%, Aldrish, USA)

L. PdCl2 (99.9+%, Sigma-Aldrish, USA)

M. PtCl4 (98%, Aldrish, USA)

N. RhCl3 (98%, Aldrich, USA)

O. RuCl3⋅ x H2O ( ReagentPlus, Sigma-Aldrish, USA)

P. SbCl3 (99+%, Acros, NJ, USA)

Q. SnCl4⋅5H2O (98+%, Acros, NJ, USA)

R. Platinum wire (EG&G, USA)

S. TiCl4 (solution, Sicherheits-Flasche, Hohenbrunn, Germany)

T. Titanium rods, 5 mm in diameter (98%, EnFeiTaiYe, Shenzhen,

China)

U. ZrCl4 (99.5+% metals basis, Aldrish, USA)

23

3.2 Preparation of Ti/metal Oxide Electrodes

3.2.1 Pretreatment of Ti Substrate

Titanium substrate was treated by sandblasting, tap water washing, ultrasonic

cleaning, acid etching and ultrasonic cleaning again. The specific process is

addressed in Figure 3.1.

Sandblasting

Tap water washing

10 mins ultrasonic cleaning in DDI water

2 mins of etching in boiling 37% hydrochloric acid

10 mins ultrasonic cleaning in DDI water

Figure 3.1 The process of Ti substrate pretreatment

3.2.2 Precursor Solution Preparation

The metal chlorides were dissolved in isopropanol and hydrochloric acid with in a

volume ratio of 7:1.The metal salts were added in the required molar ratio into

the solvents keeping a total concentration of 0.5 molL-1. This precursor solution

underwent an ultrasonic to make sure all salts are dissolved; and for the hard

dissolving cases, the precursors were under ultrasonic from very beginning until

the last brushing cycle. Normally precursor solutions are fresh prepared just

before electrode fabrication procedure.

3.2.3 Electrode Preparation

Titanium was used as substrate in the shape of cylinder of 5 mm in diameter and

30 mm in length, half of the substrate (15 mm length) was machined into screw

24

shape for the electrical connection. Thus the effective surface area was 2.55 cm2.

All electrodes were prepared by a thermal decomposition method. After

pretreatment, the titanium substrates were first brushed at a room temperature

with the precursor solution, dried at 80 oC for 5 minutes to allow the solvents to

vaporize, and then calcinated at 550 oC for another 5 minutes. This procedure

was repeated until a total oxide coating loading reached to at least 20 gm-2,

usually the whole procedure need 18 to 20 rounds. Finally, the electrodes were

annealed for an hour at the calcination temperatures for the sake of stabilizing

the active layer.

3.3 Accelerated Service Life Tests

Generally, when focus on fabricating electrodes for electrolysis, efforts are

directed at improvement electrode stability. In order to evaluate the stability of

targeted metal oxide electrodes, the accelerated life test was applied. The tests

were conducted in a standard thee-electrode cell. Both anode and cathode

made in the same material were mounted on Teflon holders as shown in Figure

3.2, and Ag/AgCl, KCl (sat.) served as a reference electrode. The

electrochemical reactor was in form of 80 cm in diameter and 110 cm in length.

The electrolyte were fresh prepared 3 M H2SO4 or 1 M NaOH and the volume

was kept as 400 ml by adding DDI water every day, the working temperature

was settled at 35 oC using a water bath (Model 3028H, Fisher Scientific, USA). A

DC power supply (HY 3005-2, SHENZHEN MASTECH, China) was used to

provide a constant current density of 10,000 Am-2. Both cell potential and

potential of anode were periodically monitored. Due to generation of a large

amount of bubbles, use of a Luggin capillary was impossible. However, the

reference electrode was placed as close as possible to the working electrode.

Because the ohmic drop from the solution was not compensated, the true

potential could be a bit smaller than the measured value. The service life test

was end at the time point when the potential between anode and reference

electrode suddenly rise up.

25

Figure 3.2 Experimental setup for accelerate service life test. 1 & 2. one

material electrodes performed as anode and cathode, 3. Reference electrode (Ag/AgCl, KCl), 4. Electrolyte (3 M H2SO4 or 1 M NaOH), 5. Water

bath (normally settled at 35 °C), 6. Signal connection between DC power

and computer

3.4 Physicochemical Characterization

3.4.1 Scan Electron Microscopy (SEM)

Surface morphology of metal oxide electrode samples are checked by SEM

(JEOL 6300 or JEOL 6300F). The samples are mounted on a circular copper

holder by conductive carbon adhesion. Both SEM machines connected with an

Energy-dispersive X-ray (EDX), which can analyze sample surface elements

26

composition. The acceleration voltage is settled at 15 KV or 20 KV.

3.4.2 X-ray Diffraction (XRD)

Coating microstructure and morphology are analyzed by X-Ray Diffraction. XRD

analysis is conducted by a Philps PW 1830 powder X-ray diffraction system with

a primary target of Cu Kα (wavelength of 1.54 A). The X-ray power is 1.6 KW

and diffractograms are recorded with a 2 Theta range of 20 → 80° with a step of

0.05° and scan speed of 0.025°/min.

3.4.3 X-ray Photoelectron Spectroscopy (XPS)

Surface composition was measured by X-ray Photoelectron Spectroscopy (XPS,

PHI 5600, Physical Electronics, USA) equipped with an Al monochomatic X-ray

source.

3.4.4 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Electrode surface coating metal concentration is measured by ICP-MS (Optima

2000, VARIAN, USA). It is based on coupling together an inductively coupled

plasma as a method of producing ions (ionization) with a mass spectrometer as

a method of separating and detecting the ions.

3.4.5 Coating Resistivity

Electrode surface coating resistivity is measured using resistivity/Hall

measurement system (HL5500PC, Bio-Rad).

3.5 Electrochemical Characterization

Electrochemical characterization was performed with a potentiostat/galvanostat

(PGSTAT 100, Autolab, The Netherlands) and a standard thee-electrode cell

(RDE0018, EG&G) as shown in Figure 3.3. Pt wire was used as a

counter-electrode, and saturated Ag/AgCl, KCl (0.197 V vs. NHE) with a Luggin

capillary as a reference-electrode. Electrode potentials are quoted with respect

27

Figure 3.3 Schematic diagram of a three-electrode cell

28

to normal hydrogen electrode (NHE). The resistances between a working

electrode and the Luggin capillary were measured using the frequency response

analyzer of the potentiostat/galvanostat. The ohmic drops of the solutions were

compensated. Solutions were purged with nitrogen gas before electrochemical

experiments for about 30 min. Open-circuit potential was examined in 1 M NaOH

solution (Krstajic and Trasatti, 1997; Fachinotti et al., 2006). In chlor-alkali and

bleaching industries, high concentration alkaline solution is needed like 1 M

NaOH, thus cyclic voltammetry was carried out in 1 M NaOH solution with 50

mVs-1 as a settled scan rate. Quasisteady state polarization curves were

recorded by first holding the electrode at -150 mV vs Ag/AgCl for 10 min (this is

close to the open-circuit potential), then at -1.0 V for 5 min and stepping the

potential by 10 mV and reading the current after 90 second up to a potential that

current density was reach to 100 mA. The direction of potential variation was

then reversed and the run continued until the current became anodic.

29

CHAPTER 4 SEARCHING FOR PROPER

ELECTRODE MATERIALS

4.1 Introduction

Looking back at the past decades of electrochemistry development, one can

witness the expansion of electrocatalysis in both the range of electrode reactions

and the range of electrode materials pushed by science or drawn by the industry

demands. Electrode materials is definitely more striking then electrode reactions

from both the experimental and the theoretical point of view; normally electrode

reactions are essentially confined to H2 and O2 evolution (water electrolysis), O2

reduction, H2 ionization and organic oxidation (fuel cell and air batteries).

Electrocatalysis has benefited from the improvement in the preparation and the

control of electrode materials quality, as well as from a better understanding the

structure of electrode surfaces (Trasatti, 2000). This is certainly the case of oxide

electrodes, world-wide known as DSAs (Dimensionally Stable Anodes). These

electrodes have met in electrochemistry with a great success and could be

considered as one of the greatest technological breakthrough of the past 50

years of electrochemistry with the advantages of excellent electrocatalytic

properties and long service life (Trasatti, 2000). DSAs are mostly applied in chlor-alkali industry mainly for Cl2 evolution, organic

material oxidation process (electrooxidation) and water electrolysis process

(electroflotation) etc. Among them, electroflotation received more and more

attention due to its high separating efficiency, simple operation, and few

accessories required (Chen et al., 2002; Chen, 2003). Chen et al. (2001) had

already fabricated a promising anode, Ti/IrO2-Sb2O5-SnO2, which could last 1600

h in 3 M H2SO4 solution under a current density of 1 A cm-2 at 35 °C. Even

though Ti/IrO2-Sb2O5-SnO2 has achieved such a success; the lack of ideal

electrode material for both O2 and H2 evolution troubled engineers a lot. The

30

aspiration for searching a better material as both anode and cathode electrode is

continuing.

If one material DSA-based anode and cathode can be developed for

electrofloation industry, which means both anode and cathode are made of the

same DSA material, the electrodes can perform self-cleaning with current

direction alternating. The reasons for seldom researching of metal oxides as a

cathode material are obvious. Firstly, oxides in general are non-conducting or

semiconducting which makes them less attractive as cathode electrode material.

Secondly, noble metal oxides are expected to be reduced to the metal phase by

hydrogen in which will lower their long-term stability (Kotz and Stucki, 1987). If

we can decrease the reduction rate of metal oxide in cathode, balance anode

and cathode service life, the profound goal ⎯ one material electrodes as both

anode and cathode for water electrolysis could achieve. By the way, as a

consequence of the different surface chemistry of oxide, compared to the

respective metals, oxides may be less sensitive to poisoning by metal deposits.

For metals like platinum or nickel, poisoning by metal deposits is well known

(Kotz and Stucki, 1987).

In this chapter, electrodes prepared in different prescriptions are checked by

SEM for surface morphology and service tested life under accelerated condition.

Normally, the same material electrodes conducting in electrolysis process in acid

solution, the anode service life is much longer than cathode. Thus the first step

for this project was optimizing a good material for cathode.

31

4.2 Experimental Results

4.2.1 Ir-based Electrodes

a) Ti/IrO2-Sb2O5-SnO2

In our group’s former work, titanium anodes coated with ternary iridium, antimony,

and tin oxide mixture were investigated for oxygen evolution. In the active oxide

coating, SnO2 serves as a dispersing agent, Sb2O5 as a dopant, and IrO2 as a

catalyst. Experimental results showed that the Ti/IrOx-Sb2O5-SnO2 electrode

containing only 10 mol % of IrOx nominally in the coating had a service life of

1600 h in 3 M H2SO4 solution under a current density of 1 Acm-2 at 35 °C (Chen

et al., 2001). IrO2-Sb2O5-SnO2 was chosen as the starting material for this

project. The substrates were brushed with the precursor solution with a molar

ratio of Ir:Sb:Sn = 10:10:80 for 20 times. Similar result was obtained for

accelerated service life in 3 M H2SO4 solution under a current density of 1 Acm-2

at 35 °C; Ti/IrO2-Sb2O5-SnO2 anode lasted for 1430 h. But when it is applied as a

cathode, the total service life was severely decreased to 5 h 20 min. If changing

the electrolyte to 1 M NaOH, the situations were significantly different;

Ti/IrOx-Sb2O5-SnO2 anode accelerated service life was 355 h and

Ti/IrOx-Sb2O5-SnO2 cathode service life was 99 h. SEM picture (Figure 4.1)

shows the smooth and compact coating of newly made Ti/IrO2-Sb2O5-SnO2

electrode with only a few cracks. The generation of cracks may be associated

with the rod shape substrates.

32

Figure 4.1 SEM picture of Ti/IrO2-Sb2O5-SnO2 electrode

b) Ti/IrO2-Sb2O5-SnO2 with PdO-Sb2O5-SnO2

Palladium exists mainly as PdO, which is semiconducting in nature (Shivastava

and Moats, 2008). Addition of palladium to ruthenium–titanium-based coatings

shows improved corrosion resistance (Kawashima et al., 1984). Thus Pd is

chosen as electrode catalyst replacing Ir. A precursor solution with a molar ratio

of Pd:Sb:Sn = 1:1:8 was brushed on titanium rod substrates. The cathode

accelerated service life (9 h) was even longer than anode accelerated service

life (3 h 15 min).

Since IrO2-Sb2O5-SnO2 anode could last more than 1,000 h in acid solution. It is

anticipated that if Ir/Sb/Sn and Pd/Sb/Sn precursors were alternately brushed on

substrates with each precursor brushed 10 times, the Ir and Pd mixture could

have synergetic effect. However, this kind of cathode could only last for 3 h 50

min, considered as the shortest of all. SEM pictures reveal that alternative

procedure induces cracks on electrode surface illustrating by Figure 4.3 and

Ti/PdO-Sb2O5-SnO2 was flat with nearly no cracks on electrode surface, Figure

4.2.

33

Figure 4.2 SEM picture of Ti/PdO-Sb2O5-SnO2 electrode

Figure 4.3 SEM picture of Ti/IrO2-Sb2O5-SnO2 with PdO-Sb2O5-SnO2 electrode

34

4.2.2 Other Metals-based Electrodes

a) Ti/RhOx-Sb2O5-SnO2

RhOx electrode is one kind of gas evolution electrodes, which shows similar

electrocatalytic activity for oxygen gas evolution with IrO2, but lower stability than

IrO2 during anodic polarization (Hussanova et al., 2004). However, it shows

higher catalytic performance for hydrogen evolution during cathodic polarization

(Morimitsu et al., 2000; Campari et al., 2002). RhCl3, SbCl5 and SnCl3 were used

as precursor solution metal sources with a molar ratio of Rh:Sb:Sn = 10:2.5:87.5.

The cathode accelerated service life was 45 h 24 min and the anode accelerated

service life was far more than two times under acid electrolyte of a current

density of 1 Acm-2. The surface morphology (Figure 4.4) is different from that of

Ti/IrO2-Sb2O5-SnO2 electrode of Figure 4.1, with bigger and wider cracks.

Figure 4.4 SEM picture of Ti/RhOx-Sb2O5-SnO2 electrode

35

b) Ti/PtOx-Sb2O5-SnO2

Pt is a kind of inert metal that shows good electrocatalytic property. A reasonable

explanation for the improved electrode kinetics by adding Pt into electrode

composition is based on the electron tunneling between Pt particles on the

surface of the oxide film and the underlying metal electrode. It was proposed in

earlier work that the resonance tunneling is responsible for the enhanced

electron-transfer rate on the Pt-doped passive oxide electrodes (Schmickler and

Stimming, 1981). The result found in the present work was not good with anode

accelerated service life which lasted for 12 h and cathode accelerated service

life could only reach 5 h 56 min in acid electrolyte. It can be observed from the

SEM picture, Figure 4.5, the coating shows large cracks severely with

introducing 3 molar % Pt.

Figure 4.5 SEM picture of Ti/PtOx-Sb2O5-SnO2 electrode

c) Ti/Au and Ti/Au-Sb2O5-SnO2

Among the various metals, Gold is regarded as an anti-corrosion metal due to its

low chemical reactivity. It can be hardly oxidized in most conditions. In view of

this good feature, gold could be a qualified electrode material. Some

prescriptions with Au were tested. The Ti substrate was coated with pure AuCl3

36

and AuCl3 combined with SbCl5 and SnCl3, because incorporation of Sb2O5 and

SnO2 metal oxides improves the coating structure of DSA electrodes (Chen and

Chen, 2005). Pure gold coated titanium cathode performed relatively long: the

deactivation took place after 49 hours under cathodic H2 evolution. But in anode

application the deactivation effect was observed after 4 minutes only. This is due

to the oxidization reaction “Au+ + e- → Au” with a standard potential of E = +1.83

Volt in acidic aqueous solution ([H+] = 1 molL-1), and the operating voltage

between anode and reference electrode was 3.46 Volt (read from the DC power

screen) to keep current density settled as 1 Acm-2, that is one of the possible

reason that metal gold was oxidized quickly. The coating surface of Ti/Au

exhibits a very rough and non-compact structure, Figure 4.6. Ti/Au-Sb2O5-SnO2

electrodes performed badly either as anode or cathode. After 6 min, 90% of the

cathode coating was dissolved. Ti/Au-Sb2O5-SnO2 is, as expected, more

compact but covered with cracks, Figure 4.7.

Figure 4.6 SEM picture of Ti/Au electrode

37

Figure 4.7 SEM picture of Ti/Au-Sb2O5-SnO2 electrode

4.2.3 Ru-based Electrodes

a) Ti/RuO2-Sb2O5-SnO2

RuO2–Sb2O5–SnO2 is a promising electrocatalyst for O2 evolution. It has a

compact microstructure with its metal oxides existing in a solid solution. The

accelerated life test showing that the Ti/RuO2-Sb2O5-SnO2 electrode containing

12.2 molar percent of RuO2 nominally in the coating had a service life of 307 h in

a 3 M H2SO4 solution under a current density of 0.5 Acm−2 at 25 °C, over 15

times longer than other typical RuO2-based electrodes. Ti/RuO2–Sb2O5–SnO2

electrodes are believed to be stable enough for applications in which low current

densities (<0.02 Acm−2) (Chen and Chen, 2005). A precursor solution of RuCl3,

SbCl5 and SnCl3 with molar ratio of Ru:Sb:Sn = 3:1:7 was brushed on substrates

in this study. The result was much better than the former results, anode

accelerated service life was far more than 51 h 25 min and cathode accelerated

service life was 26 h 25 min. Figure 4.8 presents a typical SEM image of a

Ti/RuO2-Sb2O5-SnO2 electrode. The film had fewer cracks which may indicate

that incorporation of Ru could improve coating surface morphology.

38

Figure 4.8 SEM picture of Ti/ RuO2-Sb2O5-SnO2 electrode

Figure 4.9 SEM picture of Ti/RhOx-RuO2-Sb2O5-SnO2 electrode

39

b) Ti/RhOx-RuO2-Sb2O5-SnO2

A combination of RhOx and RuO2 was chosen to serve as electrode catalyst;

SnO2 and Sb2O5 serve as a dispersing agent and a dopant, respectively. The

molar ratio of Rh:Ru:Sb:Sn was 5:5:10:80. However, the mixture did not prolong

the service life but made it even shorter than RuO2 performed as catalyst alone.

Ti/RhOx-RuO2-Sb2O5-SnO2 cathode accelerated service life was 10 h 5 min

under acid condition testing. SEM image, Figure 4.9, shows a comparable flat

surface with no cracks and pores.

c) Ti/PdO-RuOx-Sb2O5-SnO2

RuO2-Sb2O5-SnO2 combination was discovered to be good as cathode when

tested in 3 M H2SO4 solution which was used before for the accelerated life test

of PdO containing electrodes. The solution showed light yellow color from the

dissolved Pd2+. When conducting the service life test of Ti/RuO2-Sb2O5-SnO2 in

this Pd containing 3 M H2SO4 solution, the color of electrolyte disappeared

gradually and both cathode and anode worked considerably longer than in fresh

3 M H2SO4 solution. Consequently Pd2+ was adsorbed on the oxide coating

surface, which had a positive effect on the accelerated service life.

Ti/PdO-RuOx-Sb2O5-SnO2 was tested with a molar ratio in precursor solution of

Pd:Ru:Sb:Sn = 5:30:10:70 and 10:30:10:70 fabricated by thermal decomposition

method. The former cathode accelerated service life could reach 40 h and the

latter cathode accelerated service life could last even longer — 48 h. In contrary,

Ti/RuO2-Sb2O5-SnO2 cathode represent a cathode accelerated service life time

of 18 h only. All the facts illustrated that with incorporation of Pd, cathode

accelerated service life was obviously improved in acid solution. The possible

explanation is that Pd2+ is easier reduced than Ru4+, thus introducing Pd content

into RuO2-Sb2O5-SnO2 could protect cathode from reduction deactivation. Figure

4.10 shows the SEM image of a Ti/PdO-RuOx-Sb2O5-SnO2 electrode, which

presents a typical crack-mud structure.

40

Figure 4.10 SEM picture of Ti/PdO-RuOx-Sb2O5-SnO2 electrode

d) Ti/RuO2-TiO2 and Ti/RuO2-TiO2-CeO2

Roginskaya et al. (1982) studied the structure of RuxTi1-xO2 (x=0.1-0.9) and

found that a solid solution with a rutile structure exists in those coatings. The

properties of (RuO2 + TiO2) based industrial electrodes (DSA) are usually

modified by the addition of a third component (or even more) to optimize activity,

selectivity, and stability (Trasatti and Lodi, 1981). The apparent activity increased

with CeO2 content, whereas the true electrocatalytic properties, estimated by

normalizing the current to unit surface charge, decreased in the presence of

CeO2 with a minimum of 10% CeO2. The presence of CeO2 also makes the

oxide layers more prone to mechanical erosion by the evolving gas bubbles

(Faria et al., 1997). Mixed oxide layers on a Ti support were prepared by thermal

decomposition of RuCl3, TiCl4 (liquid) and CeCl3 under an O2 atmosphere. The

molar ratio of Ru:Ti and Ru:Ti:Ce were 3:7 and 3:7:1, respectively. Under acid

condition, Ti/RuO2-TiO2 anode accelerated service life was 31 h 25 min and

cathode accelerated service life was 142 h 35 min. However, the anode and

cathode accelerated service life of Ti/RuO2-TiO2-CeO2 were 13 h 35 min and 105

h. Compared with the previous results, cathode accelerated service life was

improved a lot.

41

Figure 4.11 SEM picture of Ti/ RuO2-TiO2 electrode

Figure 4.12 SEM picture of Ti/RuO2-TiO2-CeO2 electrode

42

In 1 M NaOH solution, situations were changed a lot that anode accelerated

service life was 76 h 30 min and cathode accelerated service life was 176 h 30

min with the prescription of RuO2-TiO2. And for Ti/RuO2-TiO2-CeO2, anode and

cathode accelerated service life were 8 h 30 min and 76 h 30 min, respectively. It

can be simply concluded that replacement of TiO2 with CeO2 bring benefit in

increasing in the electrocatalytic activity for oxygen evolution while the layer

becomes more prone to mechanical corrosion (lower accelerated service life).

Comparing the surface morphologies (Figure 4.11 and 4.12) indicate few

differences between these two prescriptions.

e) Ti/RuO2-ZrO2

One of the major components of DSAs is RuO2, which imparts its electrocatalytic

activity, and TiO2, which brings in its chemical inertness. In the search for new

components with varied surface properties, ZrO2 has been proposed as a

candidate to replace TiO2 in strongly aggressive environment (Burke and

McCarthy, 1984; Comninellis and Vercesi, 1991; Subramanian and Guruvish,

1986). ZrO2 is an inert oxide with interest in the fields of catalysis (Tanabe, 1985)

and surface chemistry (Ardizzone and Bassi, 1990; Randon et al., 1991). Ti/

RuO2-ZrO2 had been studied with a molar ratio of Ru:Zr = 3:7. This time service

life test was under switching condition. Electrodes 1 and 2 were immersed into 3

M H2SO4, thus electrode 1 performed as anode and electrode 2 performed as

cathode in the first 5 h, then current direction was changed in the following 5 h,

namely electrode 1 performed as the cathode but electrode 2 performed as the

anode.

Table 4.1 Accelerated service life of Ti/RuO2-ZrO2 electrode

Electrode Name

Working Condition

Total Service Life

As Anode Service Life

As Cathode Service Life

1 3M H2SO4 10h* 5h* 5h*

2 3M H2SO4 10h* 5h* 5h*

3 1M NaOH 144h* 70h15min* 74h45min*

4 1M NaOH 144h* 74h45min* 70h15min*

(* means the service life test was under switching condition)

43

Figure 4.13 SEM picture of Ti/RuO2-ZrO2 electrode

In alkaline solution, the same switching current condition was conducted;

electrode accelerated service life was much longer than in acid solution, so 24 h

was chosen as alternating current direction rate. SEM picture shows special

cloudy like porous structure, Figure 4.13.

4.2.4 Co-based Electrodes

a) Ti/Ru0.9Co2.1O4

Ru in the rutile structure can be demonstrated on Ru–Co–O ternary system

which was studied previously (Jirkovsky et al., 2006; Silva et al., 2007). The

problems in synthesis of a single phase Ru–Co–O oxide with rutile structure can

be overcome by the implementation of sol-gel synthesis which can produce

phase pure materials, whose chemistry differs from that of RuO2 or Co3O4. Such

single phase materials do exist, however, only in a limited composition interval

(Jirkovsky et al., 2006). The activity of these electrode materials differs from that

of the un-doped RuO2. The activity towards oxygen evolution in acid chloride

free media, in contrast to pure RuO2, is affected in opposite way by the variation

in particle size (Makarova et al., 2007). The origin of such a behavior was

44

tentatively attributed to a specific effect of cobalt; a conclusive explanation of this

effect remains, however, unknown (Jirkovsky et al., 2006). A precursor of RuCl3

and CoCl3 was brushed on titanium substrate with a molar ratio of Ru:Co = 3:7.

Table 4.2 Accelerated service life of Ti/Ru0.9Co2.1O4 electrode

Electrode Name

Working Condition

Total Service Life



1 3M H2SO4 17h25min 17h25min /

2 3M H2SO4 27h55min / 27h55min

3 1M NaOH 366h* 235h30min* 130h30min*

4 1M NaOH 366h* 130h30min* 235h30min*


SEM picture is shown below indicating a crack structure with a lot of layers

overlapped, Figure 4.14.

Figure 4.14 SEM picture of Ti/Ru0.9Co2.1O4 electrode

45

b) Ti/Rh0.1Co2.9O4

A single-phase oxide coating with the nominal composition of Rh0.1Co2.9O4, was

stable in alkaline solutions under oxygen evolution (Pereira et al., 2001). One

reason for the choice of rhodium as a substitute of cobalt is the high stability of

Rh3+ and its large preference to occupy the octahedral sites in the spinel

structure (Nunes et al., 2005).

Table 4.3 Accelerated service life of Ti/Rh0.1Co2.9O4 electrode

Electrode Name

Working Condition

Total Service Life



1 3M H2SO4 1h 1h /

2 3M H2SO4 1h / 1h

3 1M NaOH 366h* 235h30min* 130h30min*

4 1M NaOH 366h* 130h30min* 235h30min*


Figure 4.15 SEM picture of Ti/Rh0.1Co2.9O4 electrode

46

The cationic distribution commonly accepted in the literature for the Co3O4 spinel

is represented as Co2+[Co23+]O4, where the Co2+ and Co3+ species occupy the

tetrahedral and octahedral sites, respectively, in the spinel structure (Miyatami et

al., 1966). Once Rh3+ is replacing Co3+ it should stay in the octahedral sites in

the spinel structure and the cationic distribution should be Co2+[Co1.953+

Rh0.053+]O4 (Nunes et al., 2005). The accelerated service life in alkaline solution

was much longer than in acid solution, demonstrated in Table 4.3. It is clear to

see that the surface morphology is affected by the presence of Rh. The

Rh-containing coatings show a cracked dried-mud-like morphology, Figure 4.15.

c) Ti/Ir0.9Co2.1O4

The IrO2-Co3O4 binary oxide mixture shows characteristics that suggest high

potential as an electrode material for electrochemical capacitors. The oxide layer

shows an amorphous nature made up of a mixture of crystalline and hydrous

amorphous oxides with an inhomogeneous surface composition and segregation

of the component of larger atomic mass. These characteristics lead to a

significant increase in the surface area of the oxide coating (Grupioni and Lassali,

2001). The molar ratio of Ir:Co was 3:7. Compared with Ti/IrO2-Sb2O5-SnO2

electrode, accelerated service lives in alkaline solution were greatly improved,

however, in acid solution were decreased. The surface morphology of

Ti/Ir0.9Co2.1O4 was similar to that of Ti/ IrO2-Sb2O5-SnO2 electrode (Figure 4.1)

but with more cracks on surface, Figure 4.16.

Table 4.4 Accelerated service life of Ti/Ir0.9Co2.1O4 electrode

Electrode Name

Working Condition

Total Service Life



1 3M H2SO4 6h 6h /

2 3M H2SO4 6h / 6h

3 1M NaOH 334h* 188h* 146h*

4 1M NaOH 334h* 146h* 188h*


47

Figure 4.16 SEM picture of Ti/Ir0.9Co2.1O4 electrode

d) Ti/Ni1.5Co1.5O4

Ni and Co oxides exhibit interesting electrocatalytic activities, which make them

attractive for O2 evolution (Schumacher et al., 1990; King and Tseung, 1974).

For instance, in cobaltite spinels (MCo2O4), the mixed valences of the cations

are helpful in the reversible adsorption of oxygen by providing donor–acceptor

sites for chemisorptions. Co3O4 and NiCo2O4 has long been known to be an

active and stable bi-functional catalyst for oxygen evolution and reduction in

alkaline media, as well as a catalyst in alcohol and amine electro oxidation

(Singh et al., 1990; Rashkova et al., 2002). Hence the investigation of the

electrocatalytic synergism of Ni+Co mixed oxides has attracted considerable

attention in view of their potential applications in electrocatalysis (Gang et al.,

2004). To study Ni-Co combination electrode accelerated service life, a

precursor solution with a molar ratio of Ni:Co=1:1 was brushed on substrates.

Compared with accelerated service life tested in alkaline condition, it could only

last for 10 min in acid solution. Thus, introducing Co content as one of electrode

materials, the electrode is severely unstable. And from SEM pictures, there are a

lot of crystals presenting on Ti/Ni1.5Co1.5O4 electrode surface.

48

Table 4.5 Accelerated service life of Ti/Ni1.5Co1.5O4 electrode

Electrode Name

Working Condition

Total Service Life



1 3M H2SO4 10min 10min /

2 3M H2SO4 10min / 10min

3 1M NaOH 374h* 227h* 147h*

4 1M NaOH 374h* 147h* 227h*


Figure 4.17 SEM picture of Ti/Ni1.5Co1.5O4 electrode

e) Ti/Cu0.3Ni1.35Co1.35O4

Since both NixCo3-xO4 and CuxCo3-xO4 showed excellent result in alkaline

solution, Co+Ni+Cu ternary oxides would be investigated as well. The active

sites of Co+Ni+Cu ternary oxides for oxygen evolution could mainly result from

the contribution of Co3+ and Ni2+ without Cu2+ on the surface of electrodes. The

intercalation of Cu2+ in Co3O4/NiCo2O4 matrixes did cause an inhomogeneous

distribution of the cations which increases q* and i, depending on the amount of

Cu spices (Wen and Kang, 1997). A precursor solution of Cu:Ni:Co=2:9:9 was

prepared for electrode making. Even though the electrochemical properties

probably improved by adding Cu, compared with Ti/Cu1.5Co1.5O4 electrode, the

accelerated service life did not prolong significantly.

49

Table 4.6 Accelerated service life of Ti/Cu0.3Ni1.35Co1.35O4 electrode

Electrode Name

Working Condition

Total Service Life





3 1M NaOH 443h30min* 224h15min* 219h15min*

4 1M NaOH 443h30min* 219h15min* 224h15min*


Figure 4.18 SEM picture of Ti/Cu0.3Ni1.35Co1.35O4 electrode

f) Ti/Cu1.5Co1.5O4

Substitution of another foreign divalent metal ion into cobalt ions would result in

spinel structures with better catalytic activity and stability. Among them,

copper-substituted cobaltite spinels combine advantageously high stability and

activity with low cost and availability (Tavares et al., 1999; Singh et al., 2000). A

nominal molar ratio of Cu:Co=1:1 was chosen for electrode preparation. Cu-Co

combination electrode shows mesoscopic structure which is a lot different from

all above electrode surfaces and the accelerated service life in alkaline solution

was the longest of all, also.

50

Table 4.7 Accelerated service life of Ti/Cu1.5Co1.5O4 electrode

Electrode Name

Working Condition

Total Service Life





3 1M NaOH 460h* 217h10min* 242h50min*

4 1M NaOH 460h* 242h50min* 217h10min*


Figure 4.19 SEM picture of Ti/ Cu1.5Co1.5O4 electrode

51

IrSbSn

IrSbSn+PdSbSn

IrSbSn+RuSbSn

PtSbSn

RhRuSbSn

NiRuSbSn

RuSbSn

PdRuSbSn

Au

AuSbSn

RuTi

RuTiCe

0 20 40 60 80 100 120 140

Service life (h)

Elec

trode

com

posi

tion

Figure 4.20 Cathode accelerated service life in acid electrolyte with fixed current direction electrolysis process

RuZr

RuCo

RhCo

IrCo

NiCo

CuCo

CuNiCo

0 100 200 300 400 500

Service life (h)

Ele

ctro

de c

ompo

sitio

n

Figure 4.21 Cathode accelerated service life in alkaline electrolyte with alternating current direction electrolysis process

52

Figures 4.20 (in acid solution) and 4.21 (in alkaline solution) show the cathode

accelerated service life time of all tested materials in different electrolytes,

respectively. Figure 4.20 illustrates that most of the cathode materials did not

perform for longer than 15 hours under acid condition. The deactivation process

of all cathodes exhibits a change of the color of the solution or a black powder on

the bottom of the beaker within a short time after life test started. The

detachment of the oxide coating from the substrate could be observed.

It is important to see that the cathodes service lives in alkaline solution are much

longer than in acid solution. Cu-Co combination shows the best accelerated

service life and is therefore chosen as the candidate for more in-depth

investigation in the next stage of this project.

4.3 Summary

From all cathodes accelerated service life time data which were tested in acid

solution, it is obvious to know that acid surrounding is not an appropriate choice

for one material anode and cathode application. Reduction reactions happened

quickly for most cathode electrodes. The Co-based anodes were influenced by

deactivation effect which could induce the sharp increase of potential between

anode and reference electrodes. Thus, the whole cell potential would rise up

accordingly.

Ti/RuO2-TiO2 and Ti/RuO2-TiO2-CeO2 performed well in acid electrolyte as

cathode, but their accelerated service life as anodes are short. Hence this

material cannot be considered as a candidate for one material electrode

fabrication. In contrast, Ti/CuxCo3-xO4 showed a remarkably high stability when it

is employed as a cathode. Such a material has been proven in reported literature

to be an ideal anode. Therefore, it can be considered as a candidate for one

material electrode testing. A systemic study on this material is reported in next

chapter.

53

CHAPTER 5 STUDIES OF Ti/CuxCo3-xO4

ELECTRODES FOR BOTH OXYGEN AND

HYDROGEN EVOLUTION IN ALKALINE

SOLUTION

5.1 Introduction

Extensive use of the electrocatalytic processes in practice drives the

development of cheap and easily available electrode materials having high

activity and reasonable selectivity (Trasatti, 1980). Along with the general pattern

of development of heterogeneous chemical catalysis (Koreskov, 1977), in which

ever increasing importance is acquired by complex compounds as catalysts,

investigation of the electrode properties of non-metals and semiconducting

compounds have become the main trend in the selection of new types of

electrocatalysts. The widest class of non-metal substances investigated in

electrocatalysis is the oxide system which have used for a long time in

heterogeneous catalysis (Trasatti, 1980). Amongst the oxide catalysts,

electrodes based on Co3O4 which is a complex oxide with spinel structure have

been studied for a long time due to its various properties and practical

applications (Burke and McCarthy, 1988; Kinoshita, 1992; Trasatti, 1994; Trasatti

and Lodi, 1980; Hamdani et al., 1988). The interest in this kind of material is

mainly due to their low cost, wide disposability, stability in alkaline solution and

good electrocatalytic properties (Silva et al., 2000). However, Co3O4 spinel

oxides behave the inconvenience of being anodically unstable in acid solution,

low specific surface and relatively high electric resistance, making its application

is mainly restricted to alkaline solutions.

The solution of the above problems requires the substitution of new materials

54

into Co3O4 spinel oxides (Trasatti, 1980). Substituting another metal ion,

especially divalent metal ion, could enhance both catalytic activity and stability of

the spinel thin film, which is widely known as the “synergetic effect”. For this

reason, the morphology, crystallinity, composition, and electrochemical

properties of binary spinel oxides of the type MxCo3-xO4 (with M = Ni, Cu, Mn,

etc.) (Singh et al., 2000; Rios et al., 2000; Marco et al., 2001) and of ternary

spinel oxides involving Ni-Cu-Co (Tavares et al., 1998; Tavares et al., 1999) or

Cu-Zn-Co (Lee et al., 1996) have been the subject of comprehensive research in

an attempt to establish clearly defined composition-structure-properties

correlations. Among them, copper-substituted cobaltite spinels combine

advantageously high stability and activity with low cost and availability (Tavares

et al., 1999, Singh et al., 2000, Gautier et al., 1997).

In this chapter, Cu-Co combination will be chosen as the candidate material for

study. The Cu/Co molar ratio optimization will be found out first to get the best

performing candidate electrode. The effects of the precursor composition

variation on the stability and activity of the electrodes will also be investigated.

The electrochemical behavior and the kinetics of H2 evolution on the optimal

composition electrode will be examined. Finally, the physicochemical properties

of the coatings will be measured for all anodes and cathodes conducting water

electrolysis in alkaline solution. The electrodes would be examined daily until

used out condition to study the mechanism of electrodes failure.

5.2 Molar Ratio Optimization

5.2.1 Accelerated Service Life Test

Table 5.1 shows the accelerated service life results under current density of

10,000 Am-2 at 35 °C in 1 M NaOH solution. The effect of Cu content on the

electrochemical stability of Ti/CuxCo3-xO4 electrode is clearly illustrated. When

the Cu content is zero, the coating is a single oxide Co3O4. The Ti/Co3O4

electrode has a service life of 406 hours only. However, when 3 molar percent of

Cu was added in the precursor, the service life of the Ti/Cu0.1Co2.9O4 electrode

55

increased by 81% to 735 hours. This reveals that Cu plays a very important role

in enhancing electrode stability. The maximum service life achieved was that

having 10 percent of Cu in the Cu-Co combination. Further increase of the Cu

content resulted in a slight decrease in the service life instead.

Table 5.1 Accelerate service life of Ti/CuxCo3-xO4 electrodes in 1 M NaOH

solution under 10,000 Am-2 at 35 °C

Electrode composition

Total service life

As anode Service life

As cathode Service life

Co3O4 (1) 406h 215h15min 190h45min

Co3O4 (2) 382h 190h45min 191h15min

Cu0.1Co2.9O4 (1) 735h 383h50min 351h10min

Cu0.1Co2.9O4 (2) 699h 351h10min 347h50min

Cu0.3Co2.7O4 (1) 1080h 561h35min 518h25min

Cu0.3Co2.7O4 (2) 1008h 518h25min 489h35min

Cu0.6Co2.4O4 (1) 891h30min 473h55min 417h35min








Ti/Cu0.3Co2.7O4 service life of 1080 hr equals to 252.66 Ahmg-1 in the form of

service life efficiency (Ahmg-1 means the ratio of the total charge passing to the

oxide loading), that is much longer than the results reported by Rosa-Toro et al.

(2006). In Rosa-Toro’s report Ti/Co3O4 had shown the best service life efficiency

of 184 Ahmg-1.

56

5.2.2 EDX

The O/M values (O stands for the oxygen and M stands for the total amount of

metal) determined by EDX also listed in Table 5.2. It is clear that the amount of

oxygen is lower than the theoretical one (4/3=1.333) for all spinel coatings

including pure Co3O4, except Cu0.3Co2.7O4. It was reported previously (Li et al.,

1990) that oxygen excess could lead to structural improvement involved in the

electrical conduction. This may be one of the reasons that Ti/Cu0.3Co2.7O4

electrode lasts longer than the other Ti/CuxCo3-xO4 electrodes. The higher O/M

value for this particular electrode suggests that the morphology of this

combination is different from others. In order to find out why this electrode gives

the longest service life, more characterizations are necessary as seen

subsequently.

Table 5.2 Atomic ratios for Ti/CuxCo3-xO4 electrodes nominal and

experimental obtained by EDX techniques

Electrode Composition

Nominal Cu/Co

Precursor Cu/Co

Cu/Co EDX

O/M EDX

Co3O4 / / / 1.213

Cu0.1Co2.9O4 0.035 0.037 0.013 0.965

Cu0.3Co2.7O4 0.111 0.113 0.065 1.471

Cu0.6Co2.4O4 0.250 0.252 0.103 1.176

Cu0.9Co2.1O4 0.429 0.431 0.221 0.942

Cu1.2Co1.8O4 0.667 0.661 0.519 0.985

Cu1.5Co1.5O4 1 1.001 0.938 0.861

5.3 Electrolyte Temperature Effect on Accelerated Service Life

Since Ti/Cu0.3Co2.7O4 prescription shows the best performance in 1 M NaOH

electrolyte at 35 °C, its stability should be checked under various temperatures.

Figure 5.1 shows the accelerated service life variation with changing of

57

electrolyte temperature. When the temperature is increased from 35 °C to 70 °C,

the accelerated service life is accordingly decreased to 456 h. It is worth to

re-emphasize that the service life was measured with a daily switching of the

polarity under accelerated life test condition mentioned in Chapter 3. It should be

noted that an accelerated service life of 456 h can be estimated to be 15 years

when the current density is 300 Acm-2. This tolerance of high temperature by this

electrode makes it possible to be applied in electrochemistry industries where

high temperature operation is usually encountered.

40 50 60 70400

500

600

700

800

900

1000

1100

Acc

eler

ated

ser

vice

life

tim

e (h

)

Electrolyte temperature ( oC )

Figure 5.1 Temperature effect on accelerated service life

5.4 Physicochemical Characterization

5.4.1 SEM

Figures 5.2(a)-(g) show the typical SEM images of Ti/CuxCo3-xO4 coatings with

different precursor compositions. Upon increasing the amount of incorporated

Cu, the surface became progressively more porous and rougher; some even

showed mesoscopic structure, such as Figure 5.2(e)-(g). At low Cu doping levels

58

(Figures 5.2(a)-(d)), the surfaces maintained the smooth and compact texture.

Even though the difference between Figure 5.2(a) and (b) was not obvious, the

service lives changed a lot. The CuxCo3-xO4 coating prepared using a Cu content

of 10 mol% in the precursor (Figure 5.2 (c)) is the most compact, with few pores

and cracks detected; suggesting that proper incorporation of Cu component can

improve the coating structure effectively.

It is necessary to notice that the physical structure of the Ti/Cu0.3Co2.7O4

electrode is distinctively different from others. Its compact structure certainly

favor stability (Chen et al., 2002), consistent with that shown in Table 5.1.

(a) (b)

(c) (d)

59

(e) (f)

(g)

5.4.2 XRD

The X-ray diffraction patterns of Ti/CuxCo3-xO4 electrodes with different levels of

Cu doping (x=0, 0.3, 1.5) are shown in Figure 5.3. For comparison purposes, the

figure also includes the Ti substrate and a Ti/CuO electrode that were fabricated

by the same experimental procedure as that for copper-cobalt electrodes.

Figure 5.2 SEM images of oxide coating surfaces. (a) Co3O4;

(b) Cu0.1Co2.9O4;

(c) Cu0.3Co2.7O4;

(d) Cu0.6Co2.4O4; (e) Cu0.9Co2.1O4;

(f) Cu1.2Co1.8O4; (g) Cu1.5Co1.5O4.

60

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Ti/Cu1.5Co1.5O4

Ti/Cu0.3Co2.7O4

Ti/Co3O4

Ti substrate

Ti/CuO

2 Theta

Figure 5.3 XRD patterns for Ti/CuxCo3-xO4 electrodes (x=0, 0.3, 1.5), pure titanium substrate and Ti/CuO electrode

The XRD of sample Ti/Co3O4, Ti/Co0.3Cu2.7O4 and Ti/Co1.5Cu1.5O4 show 2θ =

31.271, 36.852, 38.541, 44.808, 59.357, 65.236 and 77.338° (International

Centre for Diffraction Data) which are indexed to a cubic spinel lattice. The two

peaks at 2θ = 35.543 and 38.708° (International Centre for Diffraction Data) from

CuO are not obvious for Ti/Co0.3Cu2.7O4 case. Comparing the five spectra, one

can conclude that the titanium substrate has been well covered by the coating.

The differences between Ti substrate and Ti/Co3O4 are that the main Ti peak at

2θ = 40.170° (International Centre for Diffraction Data) is visibly decreased and

the width of 2θ = 70.661, 76.218 and 77.368° (International Centre for Diffraction

Data) are narrowed. In addition, the crystal morphology of Ti/Co0.3Cu2.7O4

61

resembles that of Ti/Co3O4 indicating that Cu is replacing Co to have spinel

structures, whereas Ti/Co1.5Cu1.5O4 electrode shows the formation of CuO in

addition to Co3O4 spinel structure. The XRD patterns for Ti/Co1.5Cu1.5O4 show

clearly the peaks of Co3O4 and CuO are mixture, not in a solid solution.

Rosa-Toro et al. (2006) showed that peaks of CuO cannot detected with x up to

1.5, attributing to the formation of too small size or amorphous CuO.

5.4.3 XPS

Quantitative analyses of Ti/CuxCo3-xO4 electrodes coating compositions were

analyzed by XPS. Figure 5.4 shows the XPS spectra of the coating prepared

from a precursor with Cu:Co = 1:9 in terms of the emitted electron intensity

against binding energy.

Table 5.3 shows the atomic concentrations of the Cu0.3Co2.7O4 coatings

measured by XPS based on the peaks of Co 2p, Cu 2p3, O 1s and C 1s. The

molar ratios of O/M = 47.81/ (23.47+4.7) = 1.70 which is agreeable with the EDX

result. Since there is no Cl observed, 550 °C is therefore high enough to oxidize

the precursor solution to metal oxides. The maximum sampling depth of XPS is

only 5-8 nm, thus no Ti was detected which implies that the substrate is fully

covered by metal oxide thin film, consistent with the XRD results. The C is from

instrument chamber and N is perhaps sourced from a contaminant, such as

nitrogen gas adsorbed on the coating surface. The two shake-up peaks shown in

Figure 5.4 beside Cu 2p3/2 and Cu2p1/2 peaks demonstrate that the copper is

indeed existed as Cu2+ state. It is surprising to note that Cu/Co is 0.186 much

bigger than 1:9 or 0.111, indicating more Cu exist on the surface layer.

62

0 200 400 600 800 1000 1200 14000

10000

20000

30000

40000

50000

60000

70000

80000

C 1

s

N 1

s

Na

KLL

O 1

s

Co

LMM

Co

2p3

Co

LMM

Cu

2p3

Cu

LMM

O K

LL

Na

1s

C K

LL

Cou

nts

Binding energy (eV)

760 770 780 790 800 810 8204000

6000

8000

10000

12000

14000

16000

18000

20000

22000

Co 2p1/2

Co 2p3/2

Cou

nts

Binding energy (eV)

930 940 950 960 970

11500

12000

12500

13000

13500

14000

14500

shake-upshake-up Cu 2p1/2

Cu 2p3/2

Cou

nts

Binding energy (eV)

Figure 5.4 XPS spectra of Ti/Cu0.3Co2.7O4 electrode

63

Table 5.3 Surface compositions of Cu0.3Co2.7O4 films measured by XPS

Element C N O Co Cu

Atomic concentration

in film, % 22.43 0.15 47.81 23.47 4.7

5.4.4 ICP

To further check the exact Cu/Co molar ration in the electrode coating, a fresh

Ti/Cu0.3Co2.7O4 electrode was immersed into a mixture of 20 ml 68%-70% HNO3

and 20 ml 37% HCl under ultrasonic waves at 80 °C for about 10 h. The 40 ml

high concentration acid solution was then diluted into 200 ml after filtration

treatment. ICP result shows that 2.369 mg/L of Ti, 31.22 mg/L of Co and 3.354

mg/L of Cu from the 200 ml solution. The high concentration of Ti indicates the

complete dissolution of the coating. A real Cu/Co molar ratio is 0.113 which is

close to the precursor solution of 3/27=0.111. According to Co3O4 and CuO

mixture, the total electrode coating which dissolved in Nitric acid and HCl

solution is 8.50 mg (Co3O4) plus 0.84 mg (CuO) = 9.34 mg a bit less than the

oxide loading of 9.7 mg. This may be because of oxygen excess in electrode

coating which is consistent with the EDX result. The results of EDX, XPS and ICP shown above suggest that Cu is accumulated

on top surface of the coating film.

5.4.5 Coating Resistivity

Co3O4 is a semiconductor with high resistivity at room temperature. However, its

conductivity can be improved significantly by doping as mentioned previously.

Table 5.4 exhibits the effect of the Cu content on the resistivity of the

64

Ti/CuxCo3-xO4 electrodes. When Cu = 0, the coating resistivity was as high as

17.4 ohm*cm. Once 3.3 % of Cu was added, the coating resistivity decreased to

0.267 ohm*cm, nearly 60 times lower. Obviously, Cu plays a very important role

in improving the conductivity of Ti/CuxCo3-xO4. Further increase in Cu shows little

additional effect.

Table 5.4 Effect of Cu content on coating resistivity

Electrode composition Electrode coating resistivity (ohm*cm)

Co3O4 17.4

Cu0.1Co2.9O4 0.267

Cu0.3Co2.7O4 0.246

Cu1.5Co1.5O4 0.294

5.5 Electrochemical Behavior

5.5.1 Cyclic Voltammetry

Figure 5.5 shows the cyclic votammograms of Ti/Cu0.3Co2.7O4 conducted in 1 M

NaOH at a sweep rate of 50 mVs-1. It is found that the CVs of the later scans

display some changes with respect to the first scan. At the first scan, reduction

peaks are different from others with the inset of C1 much earlier and without the

appearance. After the fifteen cycles, the voltammogram curves keep almost

identical. Since the electrode under goes negative and positive polarization, the

CV curves showing in Figure 5.5 are different from those reported where only

positive polarization were studied (Wen and Kang, 1997; Nunes et al., 2005;

Grupioni amd Lassali, 2001; Rosa-Toro et al., 2006). Since no reference is now

available the peaks notified may be assigned to the following reactions. Because

there is no counter-part for C2 peak, it means that this reaction 5.2 is irreversible.

65

-1.5 -1.0 -0.5 0.0 0.5 1.0-0.004

-0.003

-0.002

-0.001

0.000

0.001

0.002

0.003

C2

C1

A1

j/ A

cm-2

E/V (vs. Ag/Ag/Cl)

The first cycle The fifth cycle The tenth cycle The fifteenth cycle

Figure 5.5 Cyclic votammograms for Ti/Cu0.3Co2.7O4 in 1M NaOH obtained

at a sweep rate of 50 mVs-1. The electrodes have an area of 2.55cm2

A1/C1 can be considered as a reversible couple, peaks associate with Co3+/Co4+

CoOOH+OH- ↔ CoO2+H2O+e- (5.1)

C2 irreversible peaks relate with the reduction process

Co3O4+2H2O+2H++2e- → 3Co(OH)2 (5.2)

Thus, one of the results of using it as a cathode is the consumption of Co3O4.

Hence, it explains partly why alternating current direction process gives a longer

life.

5.5.2 Polarization Curves for Hydrogen Evolution

The first run of the polarization curves shows a marked hysteresis, irrespective

of electrode composition. In the second run the hysteresis disappears, as

66

evident in Figure 5.6. Hysteresis has been observed in pure Co3O4 (Veggetti et

al., 1992) that the surface turns out to be “prepared” to hydrogen evolution by

some sort of reduction and/or site rearrangement. Since hydrogen is evolved on

Ti/Cu0.3Co2.7O4 electrode surface, significant kinetic data are only those taken in

the backward direction of potential variation. Furthermore, to attain more stable

surface conditions, the whole kinetic analysis has been carried out on the data

recorded in the backward direction during the second run.

All polarization curves show deviations from a Tafel line at higher current

densities. They are related to uncompensated ohmic drops, but they can also

affect Tafel slope. Uncompensated ohmic drops include electrode, as well as

electrolyte, resistance. The resistance tends to increase with increasing current

density due to the presence of gas bubbles in the electrolyte. As a rule, the

eventual presence of gas bubbles on the electrode surface reduces the effective

surface area. Both effects attribute to increased deviations from the actual Tafel

line with increasing overpotential.

With the aim of distinguishing between ohmic effects and change in Tafel slope,

the experimental data were treated by assuming that the electrode potential, at

each current, is given by the equation:

E = a + b log10(I)+ RI (5.3)

Where a and b are Tafel constant and slope, respectively, and R is the

uncompensated resistance. In Equation 5.3 R is assumed to be a constant

independent of current. Differentiating with respect to current:

dE/dI = b/I + R (5.4)

Thus, plotting dE/dI (where dE is the potential variation between two consecutive

experimental points and dI is the related current variation) against 1/I should

result in a straight line whose slope is b and whose intercept is R. The value of

67

1/I was taken in the middle of the dI interval. The resulting R was then used to

correct the experimental polarization curve.

Figures 5.7 and 5.8 combination shows a typical analysis of the kinetic data. In

Figure 5.7, it is evident that two Tafel lines are hidden in the nonlinear sections of

the polarization curves at high current densities. Figure 5.8 shows the

experimental data when corrected using the value of R extrapolated from Figure

5.7. In the case of single Tafel slope, a value of R increases with current density.

Therefore, the observed shape of the plot can only be attributed to a change in

Tafel slope to a higher value with increasing current density. R value is

calculated as 2.4 Ω and may be quantitatively in error up to 20% (Krstajic and

Trasatti, 1997). Figure 5.8 shows that subtraction of IR values from the

experimental potential, E, results in a reasonably linear plot for j > 0.25 mAcm-2

showing a Tafel slope is 101 mVdec-1.

-0.5 0.0 0.5 1.0 1.5-1.4

-1.3

-1.2

-1.1

-1.0

2

1

E vs

Ag/

AgC

l / V

j / mA cm-2

Figure 5.6 Potential versus current density curves for hydrogen evolution from 1 M NaOH on a Ti/Cu0.3Co2.7O4 electrode. (1) First run; (2) second run.

Arrows indicate the direction of potential variation

68

0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

0.02 0.04 0.06 0.08 0.10

4

6

8

10

12

j-1 / cm2 mA-1

(dE/

dI) /

ohm

cm

2

(dE

/dI)

/ ohm

cm

2

j-1 / cm2 mA-1

Figure 5.7 Application of Equation 5.2 in the text to the current-potential data (2nd run, reverse direction) of Figure 5.6 to derive ohmic drop and Tafel slopes. Insert: enlargement of initial part of the main plot to give

evidence to the intercept

-0.5 0.0 0.5 1.0 1.5 2.0-1.45

-1.40

-1.35

-1.30

-1.25

-1.20

-1.15

-1.10

-1.05

-1.00

Experimental data

Corrected for IR drop

E vs

Ag/

AgC

l / V

j / mA cm-2

Figure 5.8 Experimental data corrected for IR drop

69

5.6 Electrode Surface Changing and Mechanism of Oxygen and

Hydrogen Evolution in Alkaline Solution

The detailed mechanism of electrode failure will be disclosed in this section by

examining the electrode material changing from fresh virgin condition to used out

condition in 1 M NaOH electrolyte. Discussion will be divided into two parts; one

part is on fixed current direction electrolysis process and the second part is one

material electrodes for anode and cathode by alternating current direction in

electrolysis process. Working conditions were as follows: surrounding

temperature was kept constantly at 35 °C, current density was 10,000 Am-2,

alternating anodic and cathodic polarization was conducted every 24 h. All the

electrodes were examined by techniques such as XRD and SEM, etc.

5.6.1 Fixed Current Direction Electrolysis Process

Fixed polarization electrolysis procedure was conducted for finding the trend of

electrode material changing. The accelerated service life was only 696 h.

Figure 5.9 shows an enlarged version of XRD result of Ti/Cu0.3Co2.7O4 electrode

freshly prepared without any treatment. As having seen in Figure 5.3 no Cu

peaks can be found, although XPS, EDS and ICP results, all show the existence

of Cu. This explains that Cu does not exist separately as CuO, it is highly

possible to stay in the crystal lattice of the spinel. Pure Co3O4 and Ti could be

seen clearly and no Cu related crystal could be found. SEM picture of virgin

Ti/Cu0.3Co2.7O4 electrode is shown in Figure 5.2 (c), and the surface morphology

was very flat and with rare cracks on it.

70

20 25 30 35 40 45 50 55 60 65 70 75 800

50

100

150

200

250

TiTiTi

Ti Co3O4

Co3O4

Co3O4

Co3O

4

Co3O4

Co3O

4

Co3O4

Cou

nts

2 Theta

Figure 5.9 XRD patterns of Ti/Cu0.3Co2.7O4 virgin electrode

From the anode XRD result, it is clear to see, compared with virgin

Ti/Cu0.3Co2.7O4 electrode, in addition to Co3O4 crystal peaks the Ti and TiO2

peaks are also found. This implies that oxygen evolution does not change

electrode surface composition significantly and anode deactivation is due to the

consumption of the coating material, and detachment of oxides leaving the

substrate surface exposed. For the cathode, only amorphous Co can be

detected, which demonstrated that cathodic polarization is the key step in the

formation of Co. The results have are consistent to those reported before for the

alternating current cases.

71

20 25 30 35 40 45 50 55 60 65 70 75 800

50

100

150

200

250

Co3O4

TiO2

Ti

Ti

Ti

Co3O4Co3O4

Ti

Co3O4

Ti

TiO2

Co3O4

Co3O4

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

10

20

30

40

50Co

Cou

nts

2 Theta

(b)

Figure 5.10 XRD patterns of fixed current direction electrodes. (a) anode, (b) cathode

72

5.6.2 Alternating Current Direction Electrolysis Process

Seven pairs of Ti/Cu0.3Co2.7O4 electrodes were placed in electrolysis cells at the

same time. The first pair of electrodes worked for only one day without changing

current direction. The second pair worked for two days with one time current

direction change. The third, fourth, fifth and sixth pairs worked for ten days,

twenty days, thirty days and forty days, respectively. The last pair of electrodes

was tested until both anode and cathode used out in alkaline solution. In addition,

electrode 1 and electrode 2 from every pair are represented as E 1 and E 2.

After one day using in 1 M NaOH electrolyte, from XRD results, the anode (E 1)

is nearly unchanged compared with the original one, except that Ti peaks

become stronger. But the cathode (E 2) changed significantly with some metal

Co peaks showing up. This is maybe because reduction reaction happened on

cathode electrode (E 2) surface, Con+ + ne → Co (n=2, 3) and under cathodic

polarization effect, electrode material begins to change from crystal to

amorphous phase indicate by the large number of noisy peaks showing up. The

above phenomena are consistent with the fixed current direction electrolysis

process discussed above.

SEM pictures, Figure 5.12, show some tiny cracks randomly distributed on the

surface of both anode (E 1) and cathode (E 2). This should be because of the

generation of oxygen and hydrogen gases with these gas bubbles intensively

bumped electrodes surface.

After first 24 h electrolysis, anode and cathode were reversed by changing

current direction which means at the beginning E 1 was employed as anode and

E 2 was employed as cathode, after one day operation, E 1 was became as

cathode and E 2 performed as anode for the following 24 h electrolysis. After two

days electrolysis, the anode (E 2) from the second pair shows similar result to

the cathode (E 2) from the first pair of electrodes in terms of the XRD peaks. The

metal Co still can be found in metal oxides surface and seems not oxidized

under anodic polarization, consistent with the CV curves seen in Figure 5.5.

73

Such a phenomenon may be explained by the trapping of elemental Co inside

the “cage” structure of spinel crystals and oxygen evolution effect would not

change electrode surface crystal phase significantly. Hence metal Co is not

dissolving into the solution and does not be re-oxidized to Co2O3 as well. For the

cathode (E 1) part, Co(OH)2 peaks are detected on the electrode surface. It

should be a reduction process:

Co3O4 + 2H2O + 2e- → 3Co(OH)2 (5.5)

The same behavior was observed from cyclic voltammograms (Figure 5.5). Thus,

there are two main reduction products, Co and Co(OH)2, found from two day

operation electrode (E 1) surface as detected by XRD.

74

20 25 30 35 40 45 50 55 60 65 70 75 800

50

100

150

200

250

Ti Co3O4

Ti

Ti

Co3O4

Co3O4

Co3O4

Co3O4

Co3O4

Co3O4

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

10

20

30

40

50

Co

CoCo

Ti

Ti

TiCo3O4

Co3O

4

Co3O4

Cou

nts

2 Theta

(b)

Figure 5.11 XRD patterns of the first pair (one day using) of electrodes. (a) anode (E 1), (b) cathode (E 2)

75

(a) (b)

Figure 5.12 SEM pictures of the first pair (one day using) of electrodes. (a) anode (E 1), (b) cathode (E 2)

(a) (b) Figure 5.13 SEM pictures of the second pair (two days using) of electrodes.

(a) anode (E 2), (b) cathode (E 1)

76

20 25 30 35 40 45 50 55 60 65 70 75 800

10

20

30

40

50

60

Co

Co3O4

Co

Co

Ti Ti

Ti

Co3O4

Co3O4

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

Co

Co

Co

Ti

TiCo(OH)2

Ti

Co(OH)2

Co(OH)2

Cou

nts

2 Theta

(b)

Figure 5.14 XRD patterns of the second pair (two days using) of electrodes.


77

Figure 5.13 shows the SEM pictures of the second pair of electrodes after two

days application. Compared with Figure 5.12, the cracks are more obvious which

is related with gas evolution effect on electrodes surface directly. EDX data show

48.84% Co and 5.66% Cu with little Ti and Na (probably came from sodium

hydroxyl solution) detected, illustrating the electrode surfaces were fully covered

by Cu0.3Co2.7O4 metal oxide thin film. The molar ratio of Cu/Co is 0.116, slightly

bigger than the theoretical value of 0.111.

Besides, peaks of Co3O4 are no longer discovered from the cathode (E 1)

surface (Figure 5.14 (b)). It could be concluded that two kinds of reactions

happened to the original Co3O4-Cu spinel material at the same time; one is

reduction reactions and the other one is the crystal Co3O4 changing into

amorphous Co3O4 and Co.

After ten days using under alternative anodic and cathodic polarization process;

Co3O4 crystals could no longer be observed by XRD from both anode (E 2) and

cathode (E 1) surfaces. Although EDX results (34.61% Co and 5.75% Cu) show

that there is plenty of Co and Cu remaining. This result reveals that Co and Cu

contents exist in metal oxide film in an amorphous phase. For the anode (E 2),

some TiN was found as well whose source could be from Ti substrate, which can

be verified by XRD peaks of a Ti substrate after pretreatment work.

Comparing Figures 5.15 and 5.16, it is obvious to see that the intensity of TiN

peaks is turning stronger; the generation of extra TiN may be from electrode

preparing procedure in 550 °C furnace and exist firmly between substrate

surface and metal oxide thin film.

78

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

120

140

160

180

200

TiTiN

TiN

TiTi

Ti

Ti

TiTi

Cou

nts

2 Theta

Figure 5.15 XRD patterns of Ti substrate

From SEM pictures (Figure 5.17) of ten-days-using anode (E 2) and cathode (E

1), it is clear to see big cracks appearing at the surfaces, the gaps became

bigger under day by day gas evolution effect and the lower layer of metal oxide

could be seen clearly. After twenty days electrolysis of water in alkaline condition,

the upper layer detached from substrate as shown by Figure 5.18. The

roughness of the surface increased along with the formation of cavities.

The XRD patterns of the fourth pair of anode (E 2) illustrate apparently the same

outcome with the anode (E 2) of the third pair (10 days using). The only

difference is that small amount of TiO2 could be found from electrode surfaces

after twenty days operation. For the cathode (E 1), there are three differences: (a)

TiO2 appeared on twenty days cathode (E 1); (b) TiN appeared on twenty days

cathode (E 1); (c) No Co(OH)2 can be found from twenty days cathode (E 1)

surface.

79

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

120

140

TiNTiN

TiN Ti TiTi

Ti

TiTiN

Co

Co

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

5

10

15

20

25

30

35

40

45

Co

TiTi

Co(OH)2

Co(OH)2Co

Co

Ti

Co(OH)2

Cou

nts

2 Theta

(b)

Figure 5.16 XRD patterns of the third pair (ten day using) of electrodes.


80

(a) (b)

Figure 5.17 SEM pictures of the third pair (ten days using) of electrodes. (a) anode (E 2), (b) cathode (E 1)

(a) (b)

Figure 5.18 SEM pictures of the fourth pair (twenty days using) of electrodes. (a) anode (E 2), (b) cathode (E 1)

81

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

120

TiO2

TiO2TiO2

TiNTiN

TiN

TiNTiTi

Ti

Ti

Ti

Ti

CoCo

Co

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

10

20

30

40

50

60

70

80

Co

TiN Ti

TiTiNTi

Co

Co

TiN

Ti

Ti

TiN

TiO2

Cou

nts

2 Theta

(b)

Figure 5.19 XRD patterns of the fourth pair (twenty day using) of electrodes.


82

After thirty days electrolysis, a lot of noise could be found from the XRD spectra

of the cathode (E 1) surface and it is hard to determine whether TiN or TiO2 exist,

Figure 5.20. There is a possibility that the noise is due to amorphous Co and Cu.

The anode (E 2), Figure 5.20 (a), gives similar XRD spectra to those from

cathode after twenty days operation cathode (E 1), Figure 5.19 (b).

SEM pictures of Figures 5.21 and 5.22 show that the morphology of the

electrodes after thirty days operation became rougher and with deeper cavities

generated at more locations.

After forty days electrolysis, it seems that there is no distinctive difference

between the anode (E 2) and cathode (E 1) in term of the XRD peaks. However,

more and more black powders could be seen at the bottom of the electrolysis cell

which was derived from metal oxide thin films of both electrodes. It could be

preliminarily concluded that the deactivation mechanism of one material

Ti/Cu0.3Co2.7O4 electrodes performed as both anode and cathode for electrolysis

application is a combination of metal oxide film detachment from electrode

surface and generation of TiO2 inner layer. The similar XRD results from Figure

5.23 (a) (b) do not mean the anode (E 2) and the cathode (E 1) had become the

same material, but because a lot amorphous or small sized Cu and Co oxides

cannot be distinguished by XRD technique.

83

20 25 30 35 40 45 50 55 60 65 70 75 800

10

20

30

40

50

60

70

80

Co

TiN

Co

Co

TiN

TiO2

Ti

Ti

TiTiN

TiN

Ti

TiO2

TiO2

TiTi

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

5

10

15

20

25

30

35

40

45

Ti

Ti

Ti

Co

CoCo

Cou

nts

2 Theta

(b)

Figure 5.20 XRD patterns of the fifth pair (thirty days using) of electrodes. (a) anode (E 2), (b) cathode (E 1)

84

(a) (b) Figure 5.21 SEM pictures of the fifth pair (thirty days using) of electrodes.


(a) (b) Figure 5.22 SEM pictures of the sixth pair (forty days using) of electrodes.


85

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

120

140

CoCo

CoTiO2

TiN

TiN

Ti TiTiO2

TiO2TiN

TiNTi

Ti

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

CoCo

Co

TiN

TiO2

Ti

Ti

TiNTiN

TiTiO2

TiO2TiO2

Ti Ti

Ti

Cou

nts

2 Theta

(b)

Figure 5.23 XRD patterns of the sixth pair (forty days using) of electrodes.


86

20 25 30 35 40 45 50 55 60 65 70 75 800

50

100

150

200

250

TiN

TiN

TiTi

Ti

TiO2

Ti TiN TiN

TiN

Ti

Ti

Cou

nts

2 Theta

(a)

20 25 30 35 40 45 50 55 60 65 70 75 800

20

40

60

80

100

120

Ti

TiN

TiNTiN

Cu Cu

Cu

TiN

Ti

Ti

Cou

nts

2 Theta

(b)

Figure 5.24 XRD patterns of the used out electrodes.


87

(a) (b)

Figure 5.25 SEM pictures of the used out electrodes.


After forty-two days using, one of the seventh pair electrode (E 2) deactivated

and cannot be applied as an anode any more with the anode voltage increased

sharply exceeding 15 volt. Hence, the current direction was reversed until the

other electrode (E 1) cannot perform as an anode, either. The total electrolysis

procedure lasted forty five days from the very beginning until both electrodes

deactivated. At the end, no Co could be detected from Figure 5.24 (a) (b) but

metal Cu showed up finally which may be related to Cu deposition on the

cathode surface. EDX result for the used out cathode (E 2) shows 11.79% Cu

and 8.15% Co.

Figure 5.25 shows the surface of the electrodes after 1000 hours acceleration

test. These figures are dramatically different from those seen for the virgin

electrodes. They are also very much different from those shown in Figure 5.22

where anode and cathode were similar. The rough surface of the anode in Figure

5.25 may be the TiO2 formed while the deposits on the cathode could be Cu and

Co. In any case, the thin film of Cu0.3Co2.7O4 should have been used up.

88

5.7 Summary

Incorporation of Cu into Co3O4 can significantly improve the stability of

Ti/CuxCo3-xO4. The optimum Cu content is 10 molar percentages in Cu-Co

combination. XPS analysis demonstrated that titanium substrate was fully

covered by Cu0.3Co2.7O4 thin film with no Ti detected. SEM analysis reveals that

Cu0.3Co2.7O4 films are very compact, with few pores or cracks. With application in

electrolysis, surface morphology of the electrodes shows a trend of no cracks →

tiny cracks → big cracks → top surface film detachment from substrate →

development of cavity on surface → many holes randomly distributed. EDX

analysis together with SEM images demonstrate all the electrodes covered with

cobalt and copper oxide thin film even after 1080 h using under alternative

anodic and cathodic polarization process. XRD patterns show that freshly

prepared Ti/Cu0.3Co2.7O4 mainly exhibit pure Co3O4 peaks without obvious Cu or

CuO peaks. Eight pairs parallel experiments combined with no switching current

electrolysis in alkaline solution reveal how crystal structure changing after 1 d, 2

d, 10 d, 20 d, 30 d, 40 d and until electrodes used out in 45 d.

It seems oxidation reaction did not change electrode surface crystal significantly

no matter the anode materials are in crystalline phase or in amorphous phase.

But when reduction reaction happened, there were two kinds of reduction

products found, Co and Co(OH)2. Combined XRD and EDX results show that

crystal Co3O4 turned into amorphous Co or small sized Co3O4. From the

experimental results, it is hypothesized that metal Co and small sized Co3O4

exist in a cage of the spinel lattice. With the process time increasing, more and

more black powders could be discovered in the electrolysis cell. A preliminary

electrode deactivation mechanism can be derived as the metal oxide film

gradual detachment from the substrate, consumption of Co3O4 structure and

generation of TiO2 inner layer. It is worth to note that alternating current direction

process can prolong anode service life, with this phenomenon clearly explained

by the redox reaction of Co4+/Co3+, Co3+/Co2+ and the trapping of reduced Co or

Cu inside the crystal structure.

89

CHAPTER 6 CONCLUSIONS

6.1 Conclusions of Present Work

In this dissertation, major efforts were devoted to the fabrication and

characterization of high-performance electrodes for both hydrogen and oxygen

evolutions. The academic contributions are summarized subsequently.

• 16 types of metal oxides were tested for water electrolysis in acid or

alkaline solution, with Ti/Cu1.5Co1.5O4 found to give the longest service

life.

• Ti/CuxCo3-xO4 was found to be able to act as one material anode and

cathode in an electrolysis cell with x varying from 0 to 1.5. The optimal

value of x = 0.3 was determined, with the ultimate service life reached

1080 h under accelerated life test in 1M NaOH.

• The surface morphology of the as-prepared Ti/CuxCo3-xO4 electrodes

become gradually smooth and compact until x reached to 0.3 then turn to

be porous and mesoscopic structure with further increasing in Cu content.

When the Ti/Cu0.3Co2.7O4 is applied in water electrolysis, surface

morphology shows a trend of no cracks → tiny cracks → big cracks → top

surface film detachment from substrate → porous structure → many holes

randomly distributed.

• XRD diffraction analysis shows that Ti/Cu0.3Co2.7O4 electrodes display a

similar diffraction pattern compared with pure Co3O4 which indicates that

Cu is replacing Co in the spinel structure of the cobalt oxide.

• By checking electrodes used after one day, two days……until used out in

1 M NaOH solution, a simple conclusion could be drawn that crystal

90

cobalt oxide was reduced to metal Co and Co(OH)2 and then changed

into amorphous phase. Reduction reaction also happened to copper

oxide to form metal Cu. Amorphous Co formation and its trap in the spinel

structure are the most important features of the electrode material which

allow Ti/Cu0.3Co2.7O4 to function as anode and also cathode for water

electrolysis with a prolonged service life.

6.2 Recommendations for Future Work

Due to time limitation, many challenges should have been but could not be

tackled. Thus, further study is necessary. Specifically:

• Continue to investigate the reason of amorphous cobalt formation and it

trap in the structure.

• Although Ti/CuxCo3-xO4 electrodes could be applied in water electrolysis

for more than 1,000 h under accelerated service life test, this kind of

electrode required the solution pH near 14. It is still necessary to search

for another electrode material that can perform well under various pH

conditions.

• Electrochemical treatment of anionic inorganic toxins in solution. Anodic

oxidation has also been used to destroy toxic inorganic species in solution,

with the most common and important example being cyanide oxidation

from electroplating, surface finishing, and gold mining operation. The

general reaction mechanism has been known for a very long time as

below:

CN-+2OH-→CNO-+H2O+2e- (6.1)

Since Ti/CuxCo3-xO4 electrodes could resist high pH, it can be considered

as an appropriate candidate for anodic oxidation of cyanide.

91

• Cathodic removal of toxic metal ions from solution. Cathodic

electrochemistry is one of the largest industrial areas for inorganic

electrochemistry (along with the production of chlorine/caustic for pulp

and paper industry). It is known now that Ti/CuxCo3-xO4 electrode is a fine

choice for cathode and has the ability to resist metal ion poisoning. Thus,

this electrode may have a good performance in cathodic electrochemistry

industry.

92

Reference

Adumbrava, A., Petrescu, N. and Simionescu, L. “Improved Technology for

Manufacture of Carbon Electrodes,” Proc. Indian Acad. Sci., 112, 19-26,

2000

Alexandrova, L., Nedialkova, T. and Nishkov, I. “Electroflotation of Metal Ions in

Wastewater,” Int. J. Miner. Process, 41, 285-294, 1994

Alves, V., Silva, AL. A. D., Oliveira, E. D. and Boodts, J. F. C. “Investigation under

Conditions of Accelerated Anodic Corrosion of the Effect of TiO2

Substitution by CeO2 on the Stability of Ir-based Ceramic Coatings,” Mater.

Sci. Forum, 289-292, 655-666, 1998

Anderson, C. and Bard, A. J. “ Improved Photocatalyst of TiO2/SiO2 Prepared by

a Sol-Gel Synthesis,” J. Phys. Chem., 99, 9882-9885,1995

Antonelli, D. M. and Ying, J. Y. “Synthesis of Hexagonally Packed Mesoporous

TiO2 by a Modified Sol-Gel Method,” Angew. Chem.-Int. Edit. Engl., 34,

2014-2017, 1995

Ardizzone, S. and Bassi, G. “Preparation of Crystalline Zirconia Powders.

Characterization of the Bulk Phase and Interfacial Behaviour,” J. Colloids

Surf., 51, 207-217, 1990

Balko, E. N. and Nguyen, P. H. “Iridium-tin Mixed Oxide Anode Coatings,” J.

Appl. Electrochem., 21, 678-682, 1991

Bard, A. J. “Encyclopedia of Electrochemistry, Volume 5, Electrochemical

Engineering,” Preface, 2006

93

Beer, H. B. “Electrode and Coating Therefor,” US Patent 3,632,498, 1972

Beer, H. B. “Electrode Having Platinum Metal Oxide Coating Thereon, and

Method of Use Thereof,” US Patent 3,771,385, 1973

Beer, H. B. “The Invention and Industrial Development of Metal Anodes,” J.

Electrochem. Soc., 127, 303C-307C, 1980

Beer, H. B. and Hinden, J. M. “Coated Metal Electrode with Improved Barrier

Layer,” US Patent 4,331,528, 1982

Bockris, J. O. M. and Khan, S. U. M. “Surface Electrochemistry: A Molecular

Level Approach,” New York, Plenum Press, 1993

Burke, L. D., Murphy, O. J., O’Neill, J. F. and Venkatesan, S. “The Oxygen

Electrode. Part B ⎯ Oxygen Evolution at Ruthenium Dioxide Anodes,” J.

Chem. Soc. Faraday Trans., 73, 1659-1671, 1977

Burke L. D. and McCarthy, M. “Oxygen Gas Evolution and Deterioration of

RuO2/ZrO2-coatd Titanium Anodes at Elevated Temperature in Strong

Base,” Eletrochim. Acta, 29, 211-216, 1984

Burke, L. D., Naser N. S. and Ahern, B. M. “Use of Iridium Oxide Films as

Hydrogen Gas Evolution Cathodes in Aqueous Media.” J. Solid State

Electrochem., 11, 655-666, 2006

Burns, S. E., Yiacoumi, S. and Tsouris, C. “Microbubble Generation for

Environmental and Industrial Separations,” Sep. Purif. Technol., 11,

221-232, 1997

94

Campari, M., Tavares, A. C. and Trasatti, S. “Thermally Prepared Ti/RhOx

Electrodes II; H2 Evolution in Acidic Solution,” Chem. Ind., 56(6), 231-237,

2002

Casueira, R. G. Torem, M. L. and Kohler, H. M. “The Removal of Zina from

Liquid Streams by Electroflotation,” Miner. Eng., 19, 1388-1392, 2006

Chen, G. H. “Electrochemical Technologies in Wastewater Treatment,” Sep.

Purif. Technol., 38, 11-41, 2004

Chen, G. H., Chen, X. M. and Yue, P. L. “Electrocoagulation and Electroflotation

of Restaurant Wastewater,” J. Environ. Eng.-ASCE, 126, 858-863, 2000

Chen G. H., Chen, X. M. and Yue, P. L. “Stable Ti/IrOx-Sb2O5-SnO2 Anode for O2

Evolution with Low Ir Content,” J. Phys. Chem. B, 105, 4623-4628, 2002

Chen, X. M. “High-performance Electrodes for Wastewater Treatment,” Ph.D.

Thesis, The Hong Kong University of Science and Technology, Hong Kong,

2002

Chen, L. L. and Guary, D. “Kinetics of the Hydrogen Evolution Reaction on RuO2

and IrO2 Oxide Electrodes in H2SO4 Solution: An AC Impedance Study.” J.

Electrochem. Soc., 143, 3576-3584, 1996

Chen, X. M., Chen, G. H. and Yue, P. L. “Novel Electrode System for

Electroflotation of Wastewater,” Environ. Sci. Technol., 36, 778-783, 2002

Chen, X. M. and Chen, G. H. “Stable Ti/RuO2–Sb2O5–SnO2 Electrodes for O2

Evolution,” Electrochim. Acta, 50 (20), 4155-4159, 2005

95

Chen, W. and Horan, N. J. “The Treatment of a High Strength Pulp and Paper Mill

Effluent for Wastewater Reuse (III) Tertiary Treatment Options for Pulp and

Paper Mill Wastewater to Achieve Effluent Recycle,” Environ. Technol., 19,

173-182, 1998

Colamn, J. E. “Electrochemical Cogeneration of Alkali Metal Halate and Alkaline

Peroxide Solutions,” US Patent 5,074,975, 1981

Comninellis, Ch. and Vercesi, G. P. “Characterization of DSA®-type Oxygen

Evolving Electrodes: Choice of a Coating,” J. Appl. Electrochem., 21,

335-345, 1991

Conway B.E. and Jerkiewicz, G. “Voltammetric Investigation of Platinum Oxides

II. Effect of Hydration on Their Reduction Behavior,” Electrochim. Acta, 45,

3063-3068, 2000

Cornell, A. and Sinomsson, D. “Ruthenium Dioxide as Cathode Material for

Hydrogen Evolution in Hydroxide and Chlorate Solutions.” J. Electrochem.

Soc., 140, 3123-3129, 1993

Da Silva, L. M., Boodts, J. F. C. and De Faria, L. A. “Oxygen Evolution at RuO2(x)

+ Co3O4(1-x) Electrodes from Acid Solution,” Electrochim. Acta, 46,

1369-1375, 2001

De Faria, L. A., Boodts, J. F. C. and Trasattic, S. “Electrocatalytic prosperities of

Ru+Ti+Ce Mixed Oxide Electrodes for the Cl2 Evolution Reaction,”

Electrochim. Acta, 42, 3525-3530, 1997

Elmore, F. E. “A Process for Separating Certain Constituents of Subdivided Ores

and Like Substances, and Apparatus therefore,” British Patent 13,578, 1905

96

Elumalai, P., Vasan, H. N., Munichandraiah, N. and Shivashankar, S. A.

“Kinetics of Hydrogen Evolution on Submicron Size Co, Ni, Pd and Co–Ni

Alloy Powder Electrodes by D.C. Polarization and A.C. Impedance Study,”

J. Appl. Electrochem., 32, 1005-1010, 2002

Eugene, J. M., O'Sullivan and Burke, L. D. “Kinetics of Oxygen Gas Evolution on

Hydrous Rhodium Oxide Films,” J. Electrochem. Soc., 137, 466-471, 1990

Fachinotti, E., Guerrini, E., Tavares, A. C. and Trasatti, S. “Electrocatalysis of H2

Evolution by Thermally Prepared Ruthenium Oxide Effect of Precursors:

Nitrate vs. Chloride,” J. Electroanal. Chem., 600, 103-112, 2007

Fan, C., Piron, D. L. and Paradis, P. “Hydrogen Evolution on Electrodeposited

Nickel-cobalt-molybdenum in Alkaline Water Electrolysis,” Electrochim.

Acta, 39, 2715-2722, 1994

Fang, Q., Shang, C. and Chen, G. H. “MS2 Inactivation by Chloride-Assisted

Electrochemical Disinfection,” J. Environ. Eng., 132, 13-22, 2006

Gang, W., Ning, L., Zhou, D. R., Mitsuo, K. and Xu, B. Q. “Anodically

Electrodeposited Co + Ni Mixed Oxide Electrode: Preparation and

Electrocatalytic Activity for Oxygen Evolution in Alkaline Media,” J. Solid

State Chem., 177, 3682-2692, 2004

Gaudet, J. Tavares, A. C. Trasatti, S. and Guay, D. “Physicochemical

Characterization of Mixed RuO2-SnO2 Solid Solution,” J. Chem. Mater., 17,

1570-1579, 2005

Gautier, J. L., Trollund, E., Nkeng, P., Poillerat, G. “Characterization of Thin

CuCo2O4 Films Prepared by Chemical Spray Pyrolysis. Study of Their

Electrochemical Stability by Ex-situ Spectroscopic Analysis,” J. Electroanal.

Chem., 428, 47−56, 1997

97

Grupioni, A. A. F. and Lassali, T. A. F. “Effect of the Co3O4 Introduction in the

Pseudocapacitive Behavior of IrO2-Based Electrode,” J. Electrochem. Soc.,

148, A1015-A1022, 2001

Guo, L. and Chen, G. H. “Long-term Stable Ti/BDD Electrode Fabricated with

HFCVD Method Using Two-Stage Substrate Temperature,” J. Electrochem.

Soc., 154, 657-661, 2007

Hamdami, M. Koenig, J. F. and Chartier, P. “Films minces de Co3O4 et NiCo2O4

obtenus par nébulisation réactive (spray) pour l'électrocatalyse. II. Etude par

Voltampérométrie Cyclique,” J. Appl. Electrochem., 18, 568-576, 2005

Ho, C. C. and Chan, C. Y. “The Application of Lead Dioxide-coated Titanium

Anode in the Electroflotation of Palm Oil Mill Effluent,” Wat. Res., 20,

1523-1527, 1986

Ho, C. C., Chan, C. Y. and Khoo, K. H. “Electrochemical Treatment of Effluents:

a Preliminary Study of Anodic Oxidation of Simple Sugars using Lead

Dioxide-coated Titanium Anodes,” J. Chem. Tech. Biotechnol., 36, 7-14,

1986

Horiuti, J. and Polanyi, M. “Horiuti’s Generalized Rate Expression and Hydrogen

Electrode Reaction,” Acta. Physicochim., USSR 2, 505, 1935

Hosny, A. Y. “Separating Oil from Oil-water Emulsions by Electroflotation

Technique,” Sep. Technol., 6, 9-17, 1996

Houda, H., Noaki Y. and Sako, K. “Electrode for Use in Hydrogen Generation,”

US Patent 7,229,536 B2, 2003

98

Huang C. J. and Liu, J. C. “Precipitate Flotation of Fluoride-containing

Wastewater from a Semiconductor Manufacturer,” Wat. Res., 33,

3403-3412, 1999

Hussanova, A., Guerrini, E. and Trasatti, S. “Thermally Prepared Ti/RhOx

Electrodes (IV): O2 Evolution in Acid Solution,” J. Electroanal. Chem., 564,

151-157, 2004

Iwakura C. and Sakamoto, K. “Effect of Active Layer Composition on the Service

Life of (SnO2 and RuO2)-coated Ti Electrodes in Sulfuric Acid Solution,” J.

Electrochem. Soc., 132, 2420-2423, 1985

Il’in, V. I. and Sedashova, O. N. “An Electroflotation Method and Plant for

Removing Oil Products from Effluents,” Chem. Petrol. Eng., 35, 480-481,

1999

Iwakura, C., Furukawa, N. and Tanaka, M. “Electrochemical Preparation and

Characterization of Ni/ (Ni + RuO2) Composite Coatings as an Active

Cathode for Hydrogen Evolution,” Electrochim. Acta, 37, 757-758, 2001

Janssen, L. J. J. and Koene, L. “The Role of Electrochemistry and

Electrochemical Technology in Environmental Protection,” Chem. Eng. J.,

85, 137-146, 2002

Jirkovsky, J., Makarova, M. and Krtil, P. “Influence of Oxygen on Reactivity of

RuFeO-doped Materials,” Electrochem. Solid-State Lett., 8, 1417, 2006

Kawashima, Y., Kobayashi, M. and Murakami, M. “Electrode for Electrolysis and

Process for Its Production,” US Patent 4443317, 1984

Ketkar, D. R., Mallikajunan R., and Venkatachalam S. “Electroflotation of Quartz

Fines,” Int. J. Miner. Process., 31, 127-138, 1991

99

King, W.J. and Tseung, A. C. C. “The Reduction of Oxygen on Nickel-Cobalt

Oxides,” Electrochim. Acta, 19, 493, 1974

Kinoshita, K. “Electrochemical Oxygen Evolution, Wiley-Interscience,” New York,

352, 1992

Krstajic, N. and Trasatti, S. “Cathodic Behavior of RuO2-doped Ni/Co3O4

Electrodes in Alkaline Soultions: Hydrogen Evolution,” J. Appl. Electrochem.

28, 1291-1297, 1998

Kotz, E. R. and Stucki, S. “Ruthenium Dioxide as a Hydrogen-evolving Cathode,”

J. Appl. Electrochem., 17, 1190-1197, 1987

Lafrance, P. and. Grasso, D. “Trajectory Modeling of Non-Brownian Particle

Flotation Using an Extended Derjaguin-Landau-Verwey-Overbeek

Approach,” Environ. Sci. Technol., 29, 1346-1352, 1995

La Rosa-Toro, A., Berenguer, R., Quijada, C., Montilla, F., Morallon, E. and

Vazquez, J. L. “Preparation and Characterization of Copper-doped Cobalt

Oxide Electrodes,” J. Phys. Chem. B, 110, 24021-24029, 2006

Lakshmi, B. B., Patrissi, C. J. and Martin, C. R. “Sol-Gel Template Synthesis of

Semiconductor Oxide Micro- and Nanostructures,” Chem. Mat., 9,

2544-2550, 1997

Lister, T. E., Tolmachev, Y. V., Chu, Y., Cullen, W. G., You, H., Yonco, R. and

Nagy, Z. “Cathodic Activation of RuO2 Single Crystal Surfaces for

Hydrogen-evolution Reaction.” J. Electroanal. Chem., 71-76, 2003

Liu, T. C., Pell W. G. and Conway, B. E. “Stages in the Development of Thick

Cobalt Oxide Films Exhibiting Reversible Redox Behavior and

Pseudocapacitance,” Electrochemica. Acta, 44, 2829-2842, 1999

100

Loucka, T. “The Reason for the Loss of Activity of Titanium Anodes Coated with a

Layer of RuO2 and TiO2,” J. Appl. Electrochem., 7, 211-216, 1977

Macdonald, D. D. and Schmuki, P. “Encyclopedia of Electrochemistry, Volume 5,

Electrochemical Engineering,” 2007

Makarova, M. V., Jirkovsky, J., Klementova, M. ,Jirka, I., Macounova, K. and Krtil,

P. “The Electrocatalytic Behavior of Ru0.8Co0.2O2−x—the Effect of Particle

Shape and Surface Composition,” Electrochim. Acta, 53, 2656-2664, 2008

Manjunath, N. Mehotra, T. I. and Mathur, R. P. “Treatment of Wastewater from

Slaughter-house by DAF-UASB System,” Wat. Res., 34, 1930-1936, 2000

Marco, J. F., Gancedo, J. R., Gracia, M., Guautier, J. L., Rios, E. I., Palmer, H.

M., Greaves, C. and Berry, F. J. “Cation Distribution and Magnetic Structure

of the Ferrimagnetic Spinel NiCo2O4,” J. Master. Chem., 11, 3087-3093,

2001

Melsheimer, J. and Ziegler, D. “The Oxygen Electrode Reaction in Acid Solutions

on RuO2 Electrodes Prepared by the Thermal Decomposition Method,” Thin

Solid Films, 163, 301-308, 1988

Miloš, H. “Reductions in Organic Chemistry,” American Chemical Society, 429,

ISBN 0-8412-3344-6, 1996

Miloš, H. “Oxidations in Organic Chemistry. Washington,” American Chemical

Society, 456, ISBN 0-8412-1780-7, 1990

Morimitsu, M., Otogawa, R. and Matsunaga, M. “Effects of Cathodizing on the

Morphology and Composition of IrO2-Ta2O5: Ti Anodes,” Electrochim. Acta,

46, 401–406, 2000

101

Mráz, R. and Krýsa, J. “Long Service Life IrO2/Ta2O5 Electrodes for

Electroflotation,” J. Appl. Electrochem., 24, 1262-1266, 1994

Nidola, A. and Schira, R. “Electrodes for Use in Electrochemical Process and

Method for Preparing the Same,” US Patent 4,975,161, 1990

Novak, D. M. Tilak, B. V. and Conway, B. E. “Fundamental and Applied Aspects

of Anodic Chlorine Production,” in Modern Aspects of Electrochemistry

No.14, Ed: J. O. Bockris, B. E. Conway, and R. E. White, New York,

Plenum Press, 195-318, 1982

Nunes, F., Mendonca, M. H., Da Silva Pereira, M. I. and Costa, F. M. “Preparation

and Characterisation of Spinel Type Cobalt and Rhodium Oxide Coatings

on Titanium,” Mater. Chem. Phys., 92, 526–533, 2005

Parsons, R. “The Rate of Electrolytic Hydrogen Evolution and the Heat of

Adsorption of Hydrogen,” Trans. Faraday Soc., 54, 1053, 1958

Parsons, R. “Advances in Electrochemistry and Electrochemical Engineering,”

Wiley-Interscience, New York, 1, 1961

Parsons, R. “The Kinetics of Electrode Reactions and the Electrode Material,”

Surf. Sci., 2, 418-435, 1964

Patrick, D., Jean-Francois, B., and Guy, M. “Review of Electrochemical

Technologies for Environmental Applications,” Recent Patents on

Engineering, 1, 257-272, 2007

Pereira, I., Nunes, F., Mendonca, H. and Costa, F. “Energy and Electrochemical

Processes for a Cleaner Environment,” J. Electrochem. Soc., 23, 414, 2001

Plambeck, J. A. “Session 11 Introductory University Chemistry II,” 1998

102

Poon, C. P. C. “Electroflotation for Groundwater Decontamination,” J. Hazard.

Mater., 55, 159-170, 1997

Pourbaix, M. “The Founding of the International Society of Electrochemistry,”

Electrochim. Acta, 16, 173, Abstract, 1971

Raj, I. A. and Vasu, K. I. “Transition Metal-based Cathodes for Hydrogen

Evolution in Alkaline Solution: Electrocatalysis on Nickel-based Ternary

Electrolytic Co-deposits,” J. Appl. Electrochem., 22, 471, 1992

Randon, J., Larbot, A., Guizard, C., Cot, L., Lindheimer, M. and Partyka, S.

“Interfacial Properties of Zirconium Dioxide Prepared by the Sol-Gel

Process,” Colloids Surf., 52, 241-255, 1991

Rashkova, V., Kitova, S. Konstantinov, I. and Vitanov, T. “Vacuum Evaporated

Thin Films of Mixed Cobalt-Nickel Oxides as Electrocatalyst for Oxygen

Evolution,” Electrochim. Acta, 47, 1555, 2002

Rios, E. Nguyuen-Cong, N, Marco, J. F., Gancedo, J. R., Chartier, P., Gauiter, J.

L. “Indirect Oxidation of Ethylene Glycol by Peroxide Ions at Ni0.3Co2.7O4

Spinel Oxide Thin Film Electrodes,” Electrochim. Acta, 45, 4431-4440,

2000

Roginskaya, Y. E., Galyamov, B. S., Belova, I. D., Shifrina, R. R., Kozhevnikov,

V. B. and Bystrov, V. I. “Metallic Clusters in Ti1-xO2 Solid Solutions, and the

Electrochamical Properties of Titanium-Ruthenium Oxide Anodes,” Soviet

Electrochemistry, Volume 18, 1179-1186, 1982

Santosa, M. R.G., Goularta, M. O. F., Tonholoa, J. and Zanta, C. L. P. S. “The

Application of Electrochemical Technology to the Remediation of Oily

Wastewater,” Chemosphere , 64, 393–399, 2006

103

Schmickler, W. and Stimming, U. “Redox Reactions at Titanium Electrodes

Covered with Doped Passive Films,” Thin Solid Films, 75, 331-340, 1981

Schumacher, L. C., Holzhueter, I., Hill, B. I. R. and Dignam, M. J.

“Semiconducting and Electrocatalytic Properties of Sputtered Cobalt Oxide

Films,” Electrochim. Acta, 35, 975-984, 1990

Sequeira, C. A. C. “Environmental Oriented Electrochemistry,” Amsterdam,

Elsevier, 1994

Shivastava, P. and Moats, M. S. “Wet Film Application Techniques and Their

Effects on the Stability of RuO2–TiO2 Coated Titanium Anodes,” J. Appl.

Electrochem. , 2008

Stucki, S. and Muller, R. in Hydrogen Energy Progress (Proceedings of the 3rd

World Hydrogen Energy Conference, Tokyo, 1980), Eds: T. N. Veziroglu, K.

Fueki, and T. Ohta, Oxford, Pergamon Press, 1799-1808, 1981

Singh, R. N. Hamdani, M. Koenig, J. Poillerat, F. G. Gautier, J. L. and Chartier, P.

“Thin Films of Co3O4 and NiCo2O4 Obtained by the Method of Chemical

Spray Pyrolysis for Electrocatalysis III. The Electrocatalysis of Oxygen

Evolution,” J. Appl. Electrochem. , 20, 442-446, 2005

Singh, R. N., Pandey, J. P., Singh, N. K., Lai, B., Chartier, P. and Koenig, J. F.

“Sol-Gel Derived Spinel MxCo3-xO4 (M=Ni, Cu; 0=<x=<1) Films and Oxygen

Evolution,” Electrochim. Acta, 45, 1911-1919, 2000

Subramanian, K. and Guruviah, S. “Electrochemical Behaviour of Titanium

Supported IrO2/RuO2 Electrodes in Acid and Alkaline Solutions,” Bul.

Electrochemistry, 2, 561-570, 1986

104

Tanabe, K. “Surface Properties of ZrO2 Dispersed on SiO2,” Mat. Chem. Phys.,

13, 347-364, 1985

Taniguchi, I., Landschoot, R. C. and Schoonman, J. “Fabrication of

La1-xSrxCo1-yFeyO3 Thin Films by Electrostatic Spray Deposition,” Solid

State Ionics, 156, 1-13, 2003

Tavares, A. C., Cartaxo, M. A. M., Da Silva Pereira, M. I. and Costa, F. M. “Effect

of the Partial Replacement of Ni or Co by Cu on the Electrocatalytic Activity

of the NiCo2O4 Spinel Oxide,” J. Electroanal. Chem., 464, 187-197, 1999

Tavares, A. C., Pereira, M. I. D., Mendoca, M. H., Nunes, M. R., Costa, F. M. and

Sa, C. M. “XPS and Voltammetric Studies on Ni1−xCuxCo2O4 Spinel Oxide

Electrodes,” J. Electroanal. Chem., 449, 91−100, 1998

Tavares, A. C. and Trasatti, S. “Ni+RuO2 Co-deposited Electrodes for Hydrogen

Evolution,” Electrochim. Acta, 45 (25-26), 4195-202, 2000

Trasatti, S. “Properties of Spinel-type Oxide Electrodes,” Electrodes of

Conductive Metallic Oxides, Part A, Elsevier Scientific,

Amsterdam-Oxford-New York, 221-222, 1980

Trasatti, S. “Electrocatalysis: Understanding the Success of DSA,” Electrochim.

Acta, 45 (15-16), 2377-2385, 2000

Trasatti, S. and Lodi, G. “Oxygen and Chlorine Evolution at Conductive Metallic

Oxide Anodes,” Electrode of Conductive Metallic Oxides, Part B, 627,

Elsevier Scientific, Amsterdam, 1981

Vaughan, R. L., Reed, B. E., Roark, G. W. and Masciola, D. A. “Pilot-scale

Investigation of Chemical Addition-dissolved Air Flotation for the Treatment

of an Oily Wastewater,” Environ. Eng. Sci., 17, 267-277, 2000

105

Ward, D. and KO, E. I. “Preparing Catalytic Materials by the Sol-Gel Method,”

Ind. Eng. Chem. Res., 34, 421-433, 1996

Wen, T. C. and Hu, C. C “Hydrogen and Oxygen Evlution on Ru-Ir Binary

Oxides,” Journal of Electrochem. Soc., 139, 2158-2162, 1992

Wen, T. C. and Kang, H. M. “Co+Ni+Cu Ternary Spinel Oxide-coated Electrodes

for Oxygen Evolution in Alkaline Solution.” Electrochim. Acta, 43,

1729-1745, 1998

Yeo, R. S., Orehotsky, J., Visscher, W. and Srinivasan, S. “Ruthenium-based

Mixed Oxides as Electrocatalysts for Oxygen Evolution in Acid

Electrolytes,” J. Electrochem. Soc., 9, 1900-1904, 1981

Zhang, H. J., Li, X. Y. and Chen, G. H. “Fabrication of Photoelectrode Materials,”

in Environmental Electrochemistry, EDS. Ch. Comninellis and G. Chen,

Springer, 2009

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