Oxygen Transport Measured by Isotope Tracing through Solid Oxides

by

Thomas Wood

A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science

Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Thomas Wood 2011

ii

Oxygen Transport Measured by

Isotope Tracing through Solid Oxides

Thomas Wood

Masters of Applied Science

Chemical Engineering and Applied Chemistry

University of Toronto

2011

Abstract

The following thesis demonstrates two isotope tracing experiments that measure oxygen

transport through electrochemically polarized solid oxides. Cathode-symmetric „button‟ cells

with yttria stabilized zirconia(YSZ) electrolytes and either strontium doped lanthanum

manganate(LSM) or composite LSM/YSZ cathodes were studied. The first experiment measured

the residence time distributions(RTD) of 34

O2. The measured RTDs were compared at different

temperatures(700-800°C) and applied potentials(-2 to -8V). Comparisons with simulated RTDs

revealed that oxygen transport was laterally heterogeneous. Delamination of the counter

electrode is likely the source of the heterogeneity. The second experiment measured a wave of

18O by exposing an interior cross section and applying ToF-SIMS analysis. A depth profile was

produced that spans the cathode and electrolyte interface. The depth profile was compared with a

variety of limiting oxygen activation scenarios predicted by a simple 1-D model. Comparisons

demonstrated that oxygen activation is likely not restricted to the cathode and electrolyte

interface.

iii

Acknowledgments

Thanks to Ben Kenney from the Fuel Cell Research Centre in Kingston, Ontario for providing

cells and helping me throughout this investigation.

Thanks to Peter Brodersen who helped me throughout the years, conducted the SIMS analysis on

the samples at SI-Ontario, and for being a friend.

A special thanks to Professor Charles Mims for being a great inspiration. Through his guidance

and insight I was able to learn a few valuable lessons that will stick with me the rest of my life.

iv

Table of Contents

Acknowledgments ........................................................................................................... iii

List of Figures ................................................................................................................. vi

Chapter 1 Introduction ..................................................................................................... 1

Chapter 2 Background ..................................................................................................... 4

2.0 Principles of Solid Oxide Fuel Cells ...................................................................... 4

2.1 Oxygen Activation ................................................................................................. 9

2.2 SOFC: Cathodes ................................................................................................. 13

2.3 SOFC: Electrolytes .............................................................................................. 15

2.4 Isotope tracing experiments ................................................................................ 17

2.5 Residence Time Distributions .............................................................................. 18

2.6 SIMS Depth Profiling ........................................................................................... 19

2.7 Simple 1-D Model ................................................................................................ 20

2.8 Objectives ........................................................................................................... 25

Chapter 3 Title of the First Chapter ................................................................................ 26

3.1 Materials .............................................................................................................. 30

3.2 Potentiostatic Current Measurements .................................................................. 31

3.3 Measuring Residence Time Distributions ............................................................ 31

3.5 Creating and Measuring Depth Profiles ............................................................... 33

Chapter 4 Results .......................................................................................................... 35

4.1 Residence Time Distributions .............................................................................. 35

4.1.1 A Typical RTD ............................................................................................ 36

4.1.2 General Considerations .............................................................................. 37

4.1.3 Best Fit RTD Simulations ........................................................................... 40

v

4.1.3 LSM Cathode – Cell y166 .......................................................................... 41

4.1.4 LSM Cathode – Cell y166: Constant Temperature ..................................... 42

4.1.5 LSM Cathode– Cell y166: Constant Current density .................................. 45

4.1.6 Composite Cathode- Cell y17 .................................................................... 47

4.2 Oxygen Depth Profiles in Cell Y17 ....................................................................... 49

Chapter 5 Discussion ..................................................................................................... 54

5.1 Depth Profiles in Cell Y17 .................................................................................... 54

Chapter 6 Conclusions ................................................................................................... 63

Chapter 7 Future Work .................................................................................................. 64

7.1 Motivation ............................................................................................................ 64

7.2 Current Work Continued ...................................................................................... 65

7.3 Proposed Experiments ........................................................................................ 66

7.3.1 ‘Macroscopic’ Effects (>10µm) ................................................................... 66

7.3.2 ‘Mesoscopic’ Effects (1-10µm) ................................................................... 68

7.3.3 ‘Microscopic’ Effects (0.1-1µm) .................................................................. 69

References:.................................................................................................................... 73

Parameters: ................................................................................................................... 75

Appendix ........................................................................................................................ 77

A.1 EZ Solve Code .................................................................................................... 77

A.2 MATLAB Code: ................................................................................................... 79

A.3 Experimental Equipment ..................................................................................... 82

vi

List of Figures

Figure 2.0.1 Schematic of a solid oxide fuel cell ............................................................................ 4

Figure 2.0.2: Example potential vs. current graph for a single cell planar SOFC at 800oC [14] ... 8

Figure 2.1.1: Overview of possible oxygen incorporation pathways at the triple phase boundary

region. [5] ...................................................................................................................................... 10

Figure 2.1.2: A simplified schematic of a proposed oxygen reduction mechanism [20,21]. ....... 12

Figure 2.2.1: Ideal structure of an ABO3 perovskite .................................................................... 14

2.3.1: Structure of yttria stabilized zirconia oxide with oxygen ions and aliovalent cations

labeled. .......................................................................................................................................... 15

Figure 2.4.1: Schematic for an experimental cell exposed to 18

O2 ............................................... 18

Figure 2.6.1: Representation of an exposed cross-section ............................................................ 19

Figure 2.7.1: Control volume ........................................................................................................ 21

Figure 3.0.1: Schematic for a symmetric „button‟ cell ................................................................. 26

Figure 3.0.2: Schematic of Equipment used in Experimental Set-Up .......................................... 27

Figure 3.0.3: Left: A schematic showing the interconnection of the quartz tubing and gas flow

streams on both sides of the cell. Right: Schematic for the cell held between two Macor discs

within the furnace. ........................................................................................................................ 28

Figure 3.0.4: A hypothetical example of an isotope pulse on the working side of the cell. ......... 29

Figure 3.0.5: Overview of data interpretation from a hypothetical static depth profiling using a

18O tracer and ToF-SIMS analysis on a polished cross section of a symmetrical SOFC „button‟

cell. ................................................................................................................................................ 30

Figure 4.1.1: A RTD measurement at 700oC with -3.4V and 205mA/cm

2 .................................. 32

vii

Figure 4.1.1.1: A RTD at 700oC, -3.4V and 205mA/cm

2 ............................................................. 36

Figure 4.1.2.1: Series of simulated RTDs using various current densities ................................... 37

Figure 4.1.2.2: I-V curve measurements for Y166 at 700 oC ........................................................ 38

Figure 4.1.2.3: Degradation of cell Y166 with time. .................................................................... 39

Figure 4.1.3.1 Comparison of measured and simulated RTDs at 700oC. ..................................... 40

Figure 4.1.3.2: Comparison of a simulated and measured RTD at 700oC. The simulated RTD

uses a current density of 288 mA/cm2. ......................................................................................... 41

Table 4.1.3.1: Matrix of RTD measurements. .............................................................................. 42

Figure 4.1.4.1: RTDs measured at 700oC at -3V and 178mA/cm

2, -3.5V and 205 mA/cm

2, and -

4V and 260mA/cm2. ...................................................................................................................... 43

Figure 4.1.4.2: A comparison of RTDs measured at 750oC and -2.76V and 178mA/cm

2, and -

3.4V and 205 mA/cm2. .................................................................................................................. 44

Figure 4.1.5.1: Simulated RTDs at constant current density with change in temperature. .......... 45

Figure 4.1.5.1: A set of RTDs measured at 205 mA/cm2 at 700

oC and 750

oC with applied

potentials of -3.5V and -3.4V respectively. .................................................................................. 46

Figure 4.1.5.2: A set of RTDs at 178 mA/cm2 and 700

oC, 750

oC and 800

oC with applied

potentials of -3V, -2.76V and -4.6V, respectively. ....................................................................... 47

Figure 4.1.6.1: A RTD measured at 700oC with an Eapp of -6V and 102mA/cm

2. ....................... 48

Figure 4.2.1: Images of ions from an exposed cross section collected using ToF-SIMS. Each

image is 500μm x 500μm. ............................................................................................................. 50

Figure 4.2.2: 1-D profiles for a select set of secondary ions taken from the 500μm x 500μm ToF-

SIMS images. ................................................................................................................................ 51

viii

Figure 4.2.3: Images of ions in exposed cross section collected using ToF-SIMS. Each image is

60.5 μm x60.5 μm. ........................................................................................................................ 52

Figure 4.2.4: 1-D profiles for a select set of secondary ions taken from the 60.5μm x 60.5μm

ToF-SIMS images. ........................................................................................................................ 53

Figure 5.1.1: An 18

O fraction depth profile produced from 500µm x 500µm ToF-SIMS images 55

Figure 5.1.2: An 18

O fraction depth profile produced from 60.5µm x 60.5µm ToF-SIMS images

....................................................................................................................................................... 55

Figure 5.1.3: Oxygen activation scenarios. ................................................................................... 56

Figure 5.1.4: Simulated depth profiles for three oxygen activation scenarios compared to the

depth profile produced from the 500µm x 500µm ToF-SIMS images ......................................... 57

Figure 5.1.5: Scenario A simulations with changes in E compared to the depth profile produced

from the 500µm x 500µm ToF-SIMS images .............................................................................. 58

Figure 5.1.6: Best fit Scenario C simulation compared to the depth profile produced from the

500µm x 500µm ToF-SIMS images ............................................................................................. 59

Figure 5.1.7: Scenario C simulation with 45s of diffusion after quenching. ................................ 60

Figure 5.1.8: Best fit Scenario B simulation compared to the depth profile produced from the

500µm x 500µm ToF-SIMS images ............................................................................................. 61

Figure 7.3.1.1: Schematic of a cell with a uniform isotope incorportation flux gradient across a

porous cathode. ............................................................................................................................. 67

Figure 7.3.1.2: Overview of isotope depth profiles in cathodes produced from corresponding

isotope incorportation flux gradients.. .......................................................................................... 67

Figure 7.3.2.1: Under low polarization (-0.1V) the oxygen transport is limited by oxygen

migration through cathode. ........................................................................................................... 69

ix

Figure 7.3.2.2: Under high polarization (-0.3V) the oxygen transport is limited by oxygen

reduction on the surface of the cathode. ....................................................................................... 69

Figure 7.3.3.1: Schematic for an experimental cell. ..................................................................... 70

Figure 7.3.3.2: Overview of XPS analysis.. .................................................................................. 71

Figure 7.3.3.3: Schematic of 1-D surface profiles produced from different polarization regimes.

....................................................................................................................................................... 72

1

Chapter 1 Introduction

The focus of this thesis is to apply and evaluate two isotope tracing techniques which measure

oxygen transport in polarized solid oxide materials. In an isotope tracing experiment, excess 18

O

is introduced into a system and mass spectrometry differentiates between the oxygen isotopes to

evaluate the movement of oxygen. Despite proving quite valuable in identifying preferred

oxygen transport pathways under electrochemical polarization in solid oxide fuel cell (SOFC)

cathode materials[1,2,3,4], the use of isotope tracing remains limited to isotope exchange depth

profiling (IEDP). Understanding oxygen transport and oxygen activation in the cathode is of

critical important to SOFC development[5]. With advances in both analytical tools and

knowledge of oxygen activation pathways it may be possible to design cathodes using materials

tailored for optimal SOFC performance [6].

SOFCs are electrochemical power conversion devices that convert chemical energy directly into

electricity with the benefit of high efficiencies. The operating temperature of a SOFC can range

from 600 to 1000oC. The high temperature is required in order to facilitate the movement of

oxygen ions through the solid oxide electrolyte and activate the sluggish reaction kinetics of

oxygen reduction in the cathode.

The high operating temperatures offer SOFCs distinct advantages. At these temperatures internal

reformation of hydrocarbon gases is possible. SOFCs are also resistant to poisoning from CO

which is instead oxidized at the anode. The ability to effectively utilize a variety of fuels is a

significant advantage over lower temperature fuel cells[7].

Additionally, the high operating temperature allows SOFCs to produce high quality steam. In

large installations SOFC-steam hybrid systems are able to recover heat from the steam resulting

in an overall system efficiency of 70% [8]. In smaller installations the steam can be used for

ancillary heating.

Despite the many advantages, there is high cost associated with the strict material requirements

caused by the high operating temperatures. SOFCs are required to be made of ceramics which

2

are brittle and expensive to manufacture. Further discussion of the material properties of the

cathode and electrolyte is in Sections 2.2 and 2.3, respectively. This high cost is a significant

disadvantage which has limited widespread adoption of SOFCs.

In order to alleviate this cost, intermediate temperature solid oxide fuel cells (IT-SOFC) are

currently being developed to operate at lower operating temperatures to allow metal supports,

interconnects and seals. While there are many factors that contribute to performance losses at the

cathode one long standing problem is sluggish oxygen activation, the reduction of oxygen. As

the operating temperature decreases, oxygen activation becomes a significant source of

polarization [9]. Oxygen activation is discussed in further detail in Section 2.1.

While the oxygen activation mechanism is not completely understood, the charge-transfer step

for oxygen reduction is believed to occur at the boundary of the three reaction constituents. This

triple phase boundary (TPB) is the intersection between a gas phase, an electronic conducting

phase and an ionic conducting phase in the cathode. Composite cathodes, which combine both an

ionic conducting phase and electronic conducting phase in the cathode, increase the TPB length

which increases the amount of oxygen incorporation and thus electrochemical performance.

In 1998 isotope tracing was applied by Horita to measure preferred oxygen pathways in

electrochemically polarized materials under different potentials[1,2,3,4]. Under low polarizations

it was found that the preferred oxygen pathway was conduction through the cathode material.

This reinforced the application of mixed ionic and electronic conducting (MIEC) materials as

cathodes. MIEC cathodes, such as strontium doped lanthanum iron cobaltite (LSCF), achieve

good power densities at lower operating temperatures [16].

However, the majority of isotope tracing experiments are equilibrium experiments such as

isotope exchange depth profiling (IEDP). While capable of providing key transport properties,

the measurements are conducted on unpolarized samples. Isotope tracing experiments are

described further in Section 2.6. Despite the prevalence of equilibrium isotope tracing

experiments, there is a dearth with respect to polarized materials.

This thesis applies two different isotope tracing experiments to electrochemically polarized cells

in order to evaluate oxygen transport. In the first experiment a mass spectrometer measures 34

O2

and 36

O2 gas in the exhaust stream in order to obtain a residence time distribution (RTD).

3

Comparisons of the measured and simulated RTDs provide insight into the oxygen transport

when evaluating differences in the mean residence time and the shape of the distribution. This

experiment is nondestructive which allows multiple measurements from a single sample. In the

second experiment a wave of 18

O is infused into a polarized sample. Secondary ion mass

spectroscopy (SIMS) is applied to measure depth profiles of infused 18

O. Each sample is only

capable of a single infusion as an interior cross section must be revealed for SIMS analysis.

Although destructive, this technique provides more detailed information on the oxygen activation

processes. The depth profiles are compared to 1-D models in order to distinguish between

limiting oxygen activation profiles discussed in Section 3.1.

The materials studied are strontium doped lanthanum manganate (LSM) and yttria stabilized

zirconia (YSZ), which are typically used as cathodes and electrolytes, respectively, in SOFCs.

The sample cells are cathode-symmetric „button‟ cells, which are cells that use the cathode

material for both electrodes.

In Section 2.0 SOFCs are described in further detail. In Section 3.0 the equipment and

experimental protocol are outlined in detail. Section 4.0 presents results from both sets of

experiments along with the rationale behind experimental conditions. In Section 5.0, the 1-D

profiles produced from the SIMS images are analyzed. In Section 6.0 conclusions are presented.

The thesis concludes with Section 7.0 which includes a set of future experiments that expand on

the current work.

4

Chapter 2 Background

2.0 Principles of Solid Oxide Fuel Cells

In this section the theory behind SOFC operation is introduced and described. Additional

information may be sought in a variety of texts such as [10] and [11].

Solid oxide fuel cells (SOFC) are galvanic electrochemical cells that convert a continuous supply

of fuel and oxidant into heat and power. Typically hydrogen gas or a hydrocarbon gas such as

methane is the fuel and oxygen or air is the oxidant. SOFCs are named after its solid oxide

electrolyte which requires a high operating temperature in order to conduct oxygen ions at

sufficient rates. The high operating temperature, ranging from 600-1000oC, provides advantages

over other fuel cells, and being a fuel cell has advantages over traditional power sources. Figure

2.0.1 shows a schematic of a SOFC and its components; the anode, cathode and electrolyte.

Figure 2.0.1 Schematic of a solid oxide fuel cell

A single cell has a limited power capacity. For example a single thin-film SOFC planar cell

made using standard materials can provide 1 W/cm2 at 0.6 V and 1.8 A/cm

2 at 800

oC [12].

Depending on the intended use, fuel cells are arranged in parallel and/or series in order to obtain

5

a desired voltage or current output. Interconnects connect the electrical current from multiple

cells in a fuel cell stack.

SOFCs convert the free energy change, ∆Grxn , of a combustion reaction directly into electricity.

This is accomplished by creating an electrochemical cell, which separates the electron transfer

steps of the overall reaction into two half-cell reactions. A half-cell reaction is either the

reduction or oxidation part of the overall reaction that occurs at one of the two electrodes.

The oxidation reaction ( eOHOH 22

2

2) occurs at the anode and the reduction reaction

( 2

22

12 OOe ) occurs at the cathode. The reactions are separated by an electrolyte, which

only allows the passage of O2-

ions. The ionic transport is driven by the electrochemical

potential gradient that forms across the cell due to the reaction. The supply of electrons generated

at the anode passes through an external load as it travels toward the cathode to complete the

circuit.

Although SOFCs are capable of using a variety of fuels, for simplicity the reaction between

hydrogen and oxygen,

OHOH 2222

1 , is used to describe the overall reaction. Table 2.0.1

shows half-cell reactions that occur at the cathode and anode for various fuels.

Table 2.0.1: Series of reactions in SOFC for a variety of fuels (13)

Fuel Overall Cathode Anode

H2 OHOH 222

2

1

2

2 24 OeO

eOHOH 22

2

2

CO 22

2

1COOCO

eCOOCO 22

2

CH4 OHCOOCH 2224 22 eOHCOOCH 824 22

2

4

6

The fuel is oxidized at the anode surface consuming O2-

from the lattice producing water or CO2

and liberating electrons. Driven by the difference in potential across the cell, electrons travel to

the cathode where oxygen is reduced at the cathode surface. A high operating temperature (800-

1000oC) is required for current commercially viable solid oxide electrolytes to conduct oxygen

ions at sufficient rates.

The reaction is driven by the free energy change of the fuel oxidation reaction, ∆Grxn, the free

energy difference between the reactants and products. The first law of thermodynamics states

that energy must be conserved within a system. An energy balance across an ideal fuel cell (no

mixing of reactants and products while at a constant temperature and pressure, and reversible

heat transfer to and from the system) reveals, by definition, that the enthalpy change of reaction

is equal to reversible heat loss, Qrev, and the reversible work, Wrev.

The energy available for work is defined as the reversible work and is also the free

energy change of the reaction, ∆Grxn. The reversible heat loss to the environment is proportional

to the change in entropy, ∆Srxn for the reaction and the temperature, T. Thus the theoretical

available energy under ideal and equilibrium conditions can be expressed by the following

equation:

Any resistances that arise limit the available energy output reducing the theoretical cell voltage,

Ecell. At standard state, the maximum theoretical cell voltage is the standard cell voltage, Eo,

which is directly proportional to the free energy driving force for the overall reaction, o

rxnG ,

when each electrode reaction is at equilibrium and at standard state:

7

F is Faraday‟s constant equal to 96485 C/mol and z is the charge transfer coefficient

equal to 2 for O2-

. The standard cell voltage, Eo, for hydrogen oxidation is 1.229 V at 25

oC and 1

atm. Ecell changes with reactant and product compositions as well as temperature when the

operation conditions deviate from standard state. The Nernst equation correlates changes

operating conditions with the theoretical cell voltage, Ecell.

R is the standard gas constant equal to 8.314 J/ mol K and Pi is the partial pressure in bar of

species i. Ecell changes with changes in gas composition at the electrodes. As fuel is depleted at

the anode, the voltage decreases. The open circuit voltage is the cell voltage when no current is

drawn and is the maximum output voltage from the cell. When current is drawn from the cell, the

cell voltage begins to decrease as the electrode reactions deviate from equilibrium.

Overpotentials, also called overvoltages or polarizations, cause the output voltage, Eout, to

deviate from Ecell related by the following expression. a

outE is the output cell voltage, cellE is the theoretical cell voltage, I is the total current moving

through the cell, R is the ohmic resistance through the cell, and ηc and ηa represents the

polarization resistance at the cathode and anode respectively. There are three major sources for

overpotentials in fuel cells; activation polarization, ohmic losses and concentration polarization.

A simulated performance curve for a single planar SOFC in Figure 2.0.2 shows the contribution

of each polarization.

8

Figure 2.0.2: Example potential vs. current graph for a single cell planar SOFC at 800oC [14]

Figure 2.0.2 shows a simulated operating voltage for a planar SOFC operating at 800oC along

with the overpotential contributions of ohmic, concentration and activation losses. Ohmic losses

are the losses in cell voltage associated with the transport of electrons and ions through a

material. The relationship between the driving force, V, total flow, I, and resistance to flow, R, is

expressed by Ohm‟s Law.

V is the potential in volts, I is the current in Amps, and R is the resistance in ohms. The ohmic

losses in SOFC operation are attributed to resistance to O2-

transport within the electrolyte.

Ohmic losses can also be generated in electrical contacts and interconnects. However, in an

electrolyte supported SOFC ohmic losses are dominant.

Concentration polarization refers to losses associated with mass transfer limitations, i.e. supply

of reactants at either of the two cathodes. When the operating current becomes very great,

reactants cannot physically move fast enough for the reaction to proceed. At this point the

maximum operating current is reached and is referred to as the limiting current density.

9

Activation polarization refers to losses associated with sluggish reaction kinetics. Activation

losses occur at both the anode (fuel oxidation) and cathode (oxygen reduction) reactions. This

loss generally makes up the initial drop in potential when current is drawn from open circuit.

This loss is usually described for a single electrode by the Butler-Volmer equation shown below.

Where io is the exchange current density, and are the transfer coefficient, F is Faraday‟s

constant, R is the gas constant, T is temperature and ∆Vact is activation polarization.

There are a variety of approaches to minimize polarization losses. Modifying the cell

architectures by using thin electrolytes and electrodes can reduce ohmic polarization losses for

the thin components. Generally two components are made as thin as possible and the third

component is thick in order to provide structural support. The thick component is said to support

the cell. When a cell has thin film electrodes and a relatively thick electrolyte, the cell is said to

be electrolyte supported.

Activation polarization associated with the sluggish reaction kinetics of the reduction of oxygen

at the cathode is one of the most significant problems holding back SOFCs. In order to reduce

the polarization loses associated with oxygen activation, or the reduction of oxygen, in the

cathode, it is necessary to understand the process. The following section discusses oxygen

activation in more detail.

2.1 Oxygen Activation

Oxygen activation, the reduction of oxygen, is considered to be a limiting process and

contributes greatly to activation polarization losses, especially at lower temperatures [9]. It is

possible to increase oxygen activation by maximizing the effectiveness of the cathode

10

architecture and material properties. This is done by understanding the process by which an

oxygen gas molecule is converted into two O2-

ions at the cathode.

As mentioned briefly in Section 1.0, the triple phase boundary (TPB) is a region which contains

the three phases required for oxygen activation; (1) the gas phase which provides the O2 , (2) the

cathode which conducts electrons and (3) the electrolyte that provides the O2-

pathway through

the cell. The TPB area in the cathode directly relates to polarization resistance [15,16].

Oxygen incorporation, the way by which an oxygen molecule is reduced and conducted to the

electrolyte, has two overall limiting processes as was originally reported by Bouwmeester et al

[17]. The first is the adsorption and reduction chemical processes for an oxygen molecule at the

surface of the cathode. The second step is oxygen ion migration through the cathode. Depending

on the material properties of the cathode either of the limiting processes can dominate.

A summary of the possible pathways for oxygen incorporation have been summarized by Adler

in a recent review paper [5] and are shown diagrammatically in Figure 2.1.1.

Figure 2.1.1: Overview of possible oxygen incorporation pathways at the triple phase boundary region.

Alpha ( ) is the mixed electron and ionic conducting phase (cathode). Beta ( ) is the oxygen gas

containing phase. Gamma (

) is the oxygen ion conducting phase (electrolyte). [3]. The triple phase boundary (TPB) is the region that constitutes all three phases(3). [5]

Figure 2.1.1 shows several possible pathways for an oxygen molecule to be reduced and transfer

to the electrolyte. The pathways represent scenarios with various restrictions to oxygen

incorporation. The pathway in panel a) begins with O2 adsorbing onto the surface on the cathode

followed by dissociative reduction and finished with O2-

migration only through the cathode. The

pathway in panel b) again begins with O2 adsorption and dissociative reduction on the cathode

11

surface. The O2-

then migrates toward the electrolyte only across the surface of the cathode. The

pathways in panel c,d) show when a dissociated oxygen species migrates both across the

electrode surface (d) and through the electrode(c). In panel e,f) O2-

migration through the

electrode is limited but migration occurs across the cathode surface (e) and through the

cathode(f). The pathway in panel g) begins with O2 dissociation at the gas/cathode/electrolyte

interface with electron conduction through the electrolyte toward the reaction site[5].

When a cathode is a poor ion conductor, such as LSM, oxygen incorporation is limited to the

TPB, as in g) in Figure 2.1.1. In order to increase electrochemical performance the TPB length is

increase by using composite cathodes. A composite cathode consists of an electronic conduction

phase and an ionic conducting phase. It was demonstrated that the oxygen conductivity can be

increased in the cathode using a composite cathode from LSM and YSZ [18].

When SOFC cathodes are made using MIEC materials, such as LSCF, the oxygen ion

migration pathway becomes significant, as in a) in Figure 2.1.1. Fleig et al demonstrated that the

resistance was proportional to the area of a circular microelectrode and not the circumference,

indicating that the oxygen is limited by oxygen migration through the LSM [19]. However, it has

been demonstrated that at different polarizations the rate limiting process can be either

dominated by oxygen ion migration or surface adsorption/reduction in terms of the location of

the active region [2,3]. The difficulty resolving the details of the limiting process of oxygen

reduction make it difficult to optimize cathode design. Table 2.1.1 summarizes three limiting

scenarios which depend on the cathode architecture.

Architecture Cathode Material Activation Location

Poor Ionic Conducting

Cathode

Good e- conductor and poor O

2-

conductor

LSM TPB at the electrolyte / cathode

interface

Composite Cathode Good e- conduction from

cathode material and good O2-

conduction from electrolyte

material

LSM / YSZ TPB throughout cathode

Mixed Ionic and Electronic

Conducting Cathode

Good e- and O

2- conduction

from cathode material

LSCF Cathode surface throughout

cathode

Table 2.1.1: Various cathode architectures and their limiting oxygen transport pathway

12

Although three cases are presented they may not be the only possible cases. While oxygen

activation is likely to occur at the TPB, an oxygen molecule may adsorb and be reduce near the

TPB and migrate across the surface of the cathode, as in case b) in Figure 2.1.1. The distance is

difficult to measure.

One point of contention is the oxygen partial pressure dependence for the rate limiting step in the

oxygen reduction mechanism, which is a series of simple reactions that take O2(gas) to a O2-

in the

cathode or electrolyte. Recently Adler, using differential impedance spectroscopy techniques,

found that oxygen is limited by the oxygen reduction step [19], which can be described by first

order kinetics. Figure 2.1.2 shows a simplified version of possible oxygen reduction mechanistic

pathways [20,21].

Figure 2.1.2: A simplified schematic of a proposed oxygen reduction mechanism [20,21].

In order for first order kinetics to be observed, rates (a), (b), or (c) must be rate limiting. A first

order rate, in this case, is proportional to the partial pressure of O2, pO2. Regardless of which step

( a, b or c) is rate limiting, there will exist a adsorbed diatomic oxygen species. Therefore, there

should be some diatomic oxygen intermediates on the surface at steady state. It is possible to

gather some insight into the reduction mechanism through experiments proposed in future work.

In order to take advantage of this information it is important to understand the materials used in

each of these components. Oxygen incorporation describes how an oxygen molecule becomes an

22)(2

)(

)(

,2

2

2

2

2

2

2

,2

,2

OO

ee

OO

ee

OO

O

ec

eb

ad

a

ad

ad

ad

13

oxygen ion. It is also important to understand the materials on and through which this process

takes place. SOFC components are made from solid oxide materials. The most commonly

studied SOFC materials are yttria stabilized zirconia(YSZ) electrolytes, strontium doped

lanthanum manganite(LSM) cathodes and nickel yttria-stabilized zirconia(Ni-YSZ) anodes[5].

Sections 2.2 and 2.3 discuss the SOFC cathode and electrolyte in greater detail.

2.2 SOFC: Cathodes

The primary function of the cathode is to reduce oxygen gas and transport O2-

to the electrolyte.

The cathode is generally porous to increase active surface area and to allow gas phase transport

and is made using solid oxide materials since they meet the strict material requirements. Ideal

cathode materials require the following properties (10,22):

High electronic conductivity

High activity of the oxygen reduction reaction

The solid oxide material typically used in SOFCs as cathodes are a class of complex ABO3-x type

perovskites. The perovskite is a class of solid oxides structures named after the mineral CaTiO3

which is called perovskite, but which also has the simplest form of the perovskite structure.

Figure 2.2.1 shows the ideal unit cell structure of an ABO3-x type perovskite. represents a

larger ion, usually a rare earth or transition metal. represents a smaller ion, typically a

transition metal. The properties of a perovskites material can be tailored based on the and

type ions. It is also possible to create complex perovskites using multiple site and site

substitutions. and site substitutions are usually completed with aliovalent transition

metals creating a 1A1-x

2Ax

1B1-y

2ByOz type material perovskite, where

1A,

2A,

1B and

2B are

different species. (23,24)

14

Figure 2.2.1: Ideal structure of an ABO3 perovskite (Adapted from 23) A – metal ions in the crystal lattice (typical oxidation state of +2) B – metal ions in the crystal lattice (with oxidation state of +3) O- Oxygen ion in the crystal lattice (oxidation state -2)

The most common cathode material is strontium doped lanthanum manganite LaxSrx-1MnO3+δ

(LSM). Sr+2

is a partial site aliovalent substitution for La+3

. Mn+2

is the site ion. LSM is

used because of its good thermal and chemical compatibility with yttria stabilized zirconia (one

of the most commonly used SOFC electrolytes), while demonstrating suitable performance

characteristics at operating temperatures ranging from 800-1000oC. Sr

+2 is used as a dopant in

the site due to its similar size to La+3

but with a different oxidation state.

SOFC cathodes can exist in a variety of configurations. In order to increase the active regions

within the cathode, composites of a cathode and electrolyte material are often created. Composite

cathodes increase the TPB and O2-

conductivity within the cathode.

Current research to improve SOFCs uses mixed ionic and electronic conducting (MIEC)

cathodes such as strontium doped lanthanum cobalt ferrite (LSCF). Another MIEC cathode

material uses double perovskite materials such as lanthanum barium doped cobalt oxide (LBCO)

[25] or praseodymium barium doped cobalt oxide (PBCO)[26]. Double perovskites form when

the difference between the site ions size is great enough so that the crystal lattice forms

alternating layers of each site ion.

15

2.3 SOFC: Electrolytes

The solid oxide electrolyte is the barrier between the two half-cell reactions, allowing only the

passage of O2-

from the cathode to the anode. Therefore, an electrolyte must be dense. The

following is a set of criteria for material selection for solid oxide electrolytes (10, 27).

High ionic conduction of O2-

No electronic conduction

Many solid oxides have been studied, with various cations substituted. Nernst identified what is

known as the Nernst mass, which is 15% mol yttria in Y2O3-ZrO2 and which is remarkably close

to the most commonly used electrolyte for SOFCs today: 8 mol% yttria stabilized zirconia,

Y2O3-ZrO2 (YSZ). The back bone of the material is zirconia (ZrO2) which has the fluorite

structure and which is named after the structure of the mineral calcium fluoride (CaF2). Figure

2.3.1 shows the ideal fluorite structure of YSZ.

2.3.1: Structure of yttria stabilized zirconia oxide with oxygen ions and aliovalent cations labeled. (37) +4 – Zirconium metal ion in the crystal lattice (typical oxidation state of +4) +2 or +3 – Yttrium metal ion in the crystal lattice (with oxidation state of +3) O- Oxygen ion in the crystal lattice (oxidation state -2) Empty – Vacancy within the lattice structure

16

Yttria oxide (Y2O3) is combined with zirconia in order to stabilize the cubic zirconia structure

and increase the number of oxygen vacancies within the lattice. At SOFC operating temperatures

zirconia has a monoclinic crystal structure with poor oxygen ion conduction. The addition of Y3+

to the structure, which is an aliovalent substitution of Z4+

, stabilizes the structure to a cubic

fluorite zirconia. Y3+

has a lower valence then zirconia which creates oxygen vacancies within

the fluorite structure (28). Yttria influences the ionic conductivity of the oxide by stabilizing the

zirconia structure at regular SOFC operating temperatures. The yttria substitution also maintains

the cubic structure that would otherwise be tetragonal avoiding destructive phase changes (29).

The charge difference between the two cations effects the charge balance within the structure,

which is rectified by the creation of vacancies at the oxygen sites. These vacancies increase the

mobility of O2-

within the lattice. As the number of vacancies increase so does the mobility of the

oxygen ion. During SOFC operation, vacancies form on the anode side of the electrolyte when

oxygen combines with hydrogen to create water. Oxygen within the lattice swap positions

allowing the vacancies to migrate toward the cathode side.

Ceria based electrolytes like gadolinia doped ceria (CGO) are used as electrolytes in intermediate

temperature SOFCs due to good performance. A summary of SOFC materials and their

performance properties has been produced by Fergus et al [30].

Knowing both the oxygen incorporation pathways and materials is one part of the puzzle. In the

next section isotope tracing experiments will be introduced and described in detail. Isotope

tracing is an experimental technique that when combined with SIMS has been able to produce

information that can enhance material selection criteria or cathode design in order to improve

performance.

17

2.4 Isotope tracing experiments

In order to study oxygen transport this thesis applies dynamic isotope tracing and isotope

depth profiling. In these experiments 18

O, an isotope of oxygen, is used. The natural atomic

abundance of 18

O is 0.2%. Carefully designed experiments provide measurements of the isotope

which show symptoms of problems with the oxygen transport.

Isotope tracing experiments are used to study material properties. As the sophistication of

analysis tools increases so does the value of isotope tracing experiments. The basic isotope

tracing experiment is the isotope exchange depth profiling experiment (IEDP) which has been

used to determine properties such as surface exchange coefficients, k, and oxygen diffusion

coefficients, Do, of solid oxides. This technique measures the exchange of isotope into a material

under equilibrium conditions and no polarization. The use of this technique has allowed

materials to be compared for use in oxygen permeable membranes and SOFC electrolytes.

Secondary ion mass spectroscopy (SIMS) measures the depth profiles produced. Using Crank‟s

solution for 1-D diffusion in an infinite plane, the values for k and D can be calculated [31].

Non-equilibrium based isotope tracing techniques measure the movement of a species while the

cell is at steady-state. Usually a potential is applied to a sample to facilitate species transport.

The most common tracer is 18

O2, however 2H2O has been used as well. Horita et al. applied

isotope tracing using isotopes of oxygen and hydrogen in order to locate the active region in the

cathode and anode under polarization. Oxygen isotope tracing was applied by Horita et al to

determine the location of active sites and the effect of polarization on oxygen transport using

LSM cathodes and YSZ electrolytes [1,2,3,4]. In these experiments oxygen isotope exchange

was enhanced by an applied potential. SIMS measurements produced oxygen distributions at

various applied voltages, revealing the TPB as the active region at high polarizations. Using

oxygen isotope it became clear that the triple phase boundary and oxygen conduction through the

electrode played important roles in the oxygen reduction and incorporation process [1,2,3,4]. Our

work aims to expand this type of investigation with a variety of techniques and materials.

As stated previously, this thesis uses two techniques to examine a cathode at the 10 micron scale

in the porous cathode. Both experiments in this thesis use a cathode-symmetric cell with Pt mesh

18

pressed against each electrode as electrical contacts. Figure 2.4.1 shows an experimental cell

schematic. Isotope pulses are delivered across the working electrode, which is the negative

electrode. During the RTD experiments the isotope will travel from the working electrode to the

counter electrode, where the isotope mostly evolves as 34

O2 gas. At this point the exhaust stream

at the counter electrode is sampled and analyzed by a mass spectrometer.

Figure 2.4.1: Schematic for an experimental cell exposed to 18

O2

2.5 Residence Time Distributions

The first experiment in this thesis measures residence time distributions of oxygen

isotope moving through a cell. A residence time distribution (RTD) represents the range of times

a molecule resides within a continuous flow system. Traditionally, RTDs are used to provide

insight into the mixing processes within reactor vessels. This is important for optimizing

reactors. In a continuously stirred tank reactor (CSTR) it is assumed that the tank is mixed well

enough that the properties within the system are uniform. In a plug flow (PF) reactor it is

assumed that mixing is limited to the radial dimension. Each flow regime has a distinct RTD

associated with it. Comparing RTDs to ideally mixed flow can provide insight into the physical

processes occurring within the system [32]. Such experiments are also used in reaction chemistry

to test a variety of mechanistic models.

19

In SOFCs, oxygen transport through the electrolyte can be viewed as a continuous flow

system. Measuring an isotope signal can provide insight into oxygen transport through the cell.

To the author‟s knowledge isotope tracing to measure the RTD of oxygen through a SOFC have

not been conducted upon reviewing. The methods used to measure a RTD in this thesis are

described in Section 3.4.

2.6 SIMS Depth Profiling

The second experiment conducted in this thesis is the isotope depth profiling

measurement. Secondary ion mass spectroscopy (SIMS) measures the composition of an isotope

infused surface from within a sample cell. The images produced from the SIMS analysis are the

isotope depth profiles. These experiments freeze the processes in the previous experiment so that

details of the activation processes can be revealed.

A polarized cell, held at constant temperature and applied potential, is exposed to a dose

of 18

O2. After a set time limit, the cell is quenched which freezes the isotope wave within the

cell. The cell is then cut to expose an interior surface. SIMS imaging measures the composition

of the exposed cross section. Figure 2.6.1 shows the exposed cross-section referred to within this

thesis, as well as the defined dimensions of the cross section.

Figure 2.6.1: Representation of an exposed cross-section

20

The exposed surface is sampled by SIMS, using primary ions which bombard the surface of a

sample. The primary ions disrupt the surface enough to knock off secondary ion fragments. The

secondary ion fragments are analyzed yielding a mass spectrum. Different primary ion beam and

energies allow variations in the ion fragments providing different molecular information.

Additional information on the SIMS measurements is available in Section 3.5.

Imaging the isotope information involves the use of a small spot primary ion beam which is

rastered over the surface. The presence of 18

O is revealed by 18

O- or M

18O

- secondary ions, where

M represents another ion. The depth profile of the isotope is compared to 16

O which is revealed

by 16

O- or M

16O

- secondary ions. Further information can be found in Section 3.5.

2.7 Simple 1-D Model

This thesis employs a simple 1-D mathematical model in an attempt to simulate the results of

both isotope tracing experiments. There are two objectives for the modeling work: (1) To

compare a theoretical residence time distribution to an observed residence time distribution (2)

To compare frozen transients to theoretical depth profiles.

A simple 1-D model is used to simulate a residence time distribution (RTD) of 18

O

travelling through a model cell. Each RTD represents the range of times an O2-

can take to

migrate from the point of O2 reduction to the point of O2 evolution from opposite end of the cell.

The model considers both conductive and diffusive transport. Depending on the cathode

architecture an O2-

can migrate by ionic conduction through the cathode or by-pass the cathode

entirely if reduction occurs only at the TPB at the cathode and electrolyte interface.

Under electrochemical polarization, the conductive flux is a dominant mode of transport. The

simple model assumes that each O2-

moving through the cell replaces every O2-

within the oxide

lattices. It also assumes that O2-

transport occurs vertically through the cell, implying that the

lateral transport is homogenous across the geometric area of the cell. This model also assumes

that the material properties are the bulk material properties and are homogeneous throughout.

This assumption extends to uniform molar density within the electrolyte and cathode. Finally, it

21

assumes that the isotope only enters through the working cathode and exits from the counter

electrode.

A mass balance across a control volume within the cell on a mole 18

O basis is as follows:

n is the number of moles of 18

O per cm2 and t is time in seconds.

Figure 2.7.1: Control volume within a cell

Figure 2.7.1 shows a control volume with varying thicknesses of each compartment. fi is the

fraction of 18

O within compartment i, Do is the diffusion coefficient in cm2/s, Jo is the conductive

flux in moles of O2-

per cm2 s, ∆x is the difference of each control volume in cm, and i is the

index that indicates a particular volume.

The conductive flux is the result of a charge balance across the cell under electrochemical

polarization. Since the ionic conductors conduct only O2-

and not e- , yet the cell is part of a

circuit, a charge balance is required(33). The charge balance is as follows:

2 x (charge carried by e-) = (charge carried by O

2-)

22

From the charge balance, the conductive flux of oxygen ions through the electrolyte, , can be

derived from the total current, , flowing through the external circuit, by the following

relationship:

Where Itot is the total current in A, 2 is the moles of electrons per moles of O2-

, F is

Faraday‟s constant equal to 96485 C per mole of electrons and Atot is the geometric square area

of the cell in cm2. Based on the assumption that the oxygen replaces every oxygen atom within

the YSZ lattice as it migrates through the cell, a relationship was developed to approximate the

mean residence time, t . An estimate of the time for the average O2-

to move through the cell can

be approximated using the molar concentration of oxygen in YSZ, [OYSZ], the cell thickness in

cm, d, and the conductive flux.

Based on the assumption that the molar oxygen density is uniform throughout, the conductive

flux term within the mass balance is derived. The change of 18

O fraction within the control

volume is proportional to the 18

O fraction gradient. When examining a control volume

sufficiently small such that the properties of the volume are uniform, the moles of 18

O leaving

the volume is equal to the moles within the volume. Depending on the location of the control

volume, the conduction term includes the reduction of 18

O from gas phase.

23

The diffusive flux is modeled using Fick‟s First Law which describes the diffusive flux

proportional to the change in the 18

O gradient across. The diffusive flux through the cell can be

described using Fick‟s First Law for diffusion through a plane.

C is concentration in moles per cm3. The diffusion coefficient, Do, for YSZ was calculated using

the ionic conductivity of oxygen in YSZ, ζYSZ, provided by the supplier of the YSZ,

NexTech[34]. The diffusion coefficient is related to the ionic conductivity via the Nernst-

Einstein equation:

35

ρ is density in moles per cm3, ζYSZ is the ionic conductivity in S per cm, R is the gas constant in

8.314 moles per cm3

Kelvin, T is the temperature in Kelvin and z is the charge transfer

coefficient in moles of electrons per mole of O2-

.The conductivity of YSZ is related to

temperature via the following equation:

34

The mass balance within a control volume can be described by the following equation:

The partial differential equation may be solved using discrete numerical methods. Matlab is used

to solve the model using a series of difference equations with sufficiently small ∆t and ∆x. By

24

creating a series of compartments the solution from the differenced equations are equivalent to

the solution of the partial differential equations. An example of a difference equation can be seen

below.

Depending on the location within the cell and the activation scenario, the difference equation is

different. In the cathode an additional conduction in term describes the incorporation of 18

O2 into

the cell from the gas phase. Depending on the oxygen activation scenario this term may occur at

the cathode platinum compartment, or the cathode electrolyte interface compartment.

An effective diffusion coefficient, Deff, is used to describe the oxygen self-diffusion through the

cathode region. The effective diffusion coefficient for oxygen moving through the cathode is the

following:

ε is an effectiveness factor. Oxygen diffusion is effected by tortuosity increasing the

diffusion time. Also, since the 1-D model uses average properties, the model may over

inaccurately predict oxygen diffusion due to transport between multiple phases and porosity

within the cathode. A more detailed cathode model is recommended for further studies.

The Matlab code for such a series can be found in A.2. EZ-Solve solves a set of partial

differential equations as ordinary differential equations using a Runge-Kutta method. The series

of differential equations can be solved as a series of ordinary differential equations when placed

into a series of 12 or more with a specified ∆x. The EZ Solve code can be found in A.1.

25

2.8 Objectives

The objective of the current work is to apply isotope tracing experiments in order to study

oxygen transport in electrochemically polarized solid oxides. The materials studied are LSM and

YSZ, which are commonly used in SOFCs. The experiments in this thesis examine transient

isotope transport signals in the form of residence time distributions (RTD) and isotope depth

profiles.

The first isotope tracing experiment measures RTDs of 34

O2 through an electrochemically

polarized cell. The key objective of this experiment is to compare a set of measured RTDs from

various temperatures and current densities. The measured RTDs are compared with simulated

RTDs produced using a simple 1-D model. It is shown through comparison that the active area is

less than expected. This difference is then attributed to delamination of the counter electrode

which is confirmed by post-mortem inspection of the cell.

The second isotope tracing experiment measures isotope depth profiles. The key objective of this

experiment is to measure an isotope depth profile of 18

O that spans the cathode and electrolyte

boundary at the delivery side of the cell. The depth profiles reveal information on oxygen

transport in greater detail than the RTD measurements. A variety of RTDs using different

limiting oxygen activation scenarios in the 1-D model are compared to the measured depth

profile. It is demonstrated that the depth profile is capable of discriminating between specific

activation scenarios under a given set of conditions.

The experiments are described in further detail in the following Section 3.0 Experimental

Methods.

26

Chapter 3 Title of the First Chapter

As previously introduced, two exploratory isotope tracing experiments are conducted in this

thesis. The first experiment measures an 34

O2 signal to evaluate a residence time

distribution(RTD). The second experiment infuses a cell with an 18

O tracer that is measured

using ToF-SIMS analysis in order to evaluate 1-D depth profiles .

Both isotope tracing experiments make use of an application of 36

O2 tracer gas to supply 18

O for

measurement. The pulse of 36

O2 is applied to the working side of a electrochemically polarized

cell. Depending on the experiment, the isotope moves completely through the cell or remains

frozen within. RTD measurements sample the exhaust gas that includes gas evolved from the

counter electrode using a mass spectrometer. Figure 3.0.1 shows a schematic for the isotope

exchange tracing combined with the SOFC cell.

Figure 3.0.1: Schematic for a symmetric ‘button’ cell

Figure 3.0.2 shows an overview of the equipment. This set-up is used to conduct both isotope

tracing experiments. The overview shows a clear picture of the various gas flows and the

pathway for a pulse of 36

O2 delivered to the cell.

27

Figure 3.0.2: Schematic of Equipment used in Experimental Set-Up

In Figure 3.0.2 the 18

O2 pathway, for both tracing experiments, begins at the point labeled A

following each point alphabetically to point D. Point A indicates the pneumatic 6-port valve used

to control the delivery of the 18

O2 pulses. The isotope is introduced into the 6-port valve at port 5

and enters a dosing tube at port 6. The dosing tube connects port 6 to port 3 on the valve. The

volume of 18

O2 is controlled by adjusting the length of the tube. A 273cm length of plastic tubing

was used to maximize the pulse time while limiting the amount of 18

O2 used during tracing

experiments.

During steady state operation, air flows from a compressed air cylinder to point A. The air flows

through the 6-port valve, entering at port 1 and exiting at port 2. When the 18

O2 pulse is required,

the 6-port valve is pneumatically triggered forcing the air to flow through the isotope dosing tube

(1 to 6 and 3 to 2) pushing the 18

O2 through the remainder of the setup.

In Figure 3.0.2, Point B indicates the furnace. A cell sits in the furnace at a specific temperature

and applied potential in the holder described in 3.0.3. During dynamic isotope tracing

experiments, some of the isotope that is exposed to the working side of the cell, travels through

the cell. The isotope evolves from the cell at the counter electrode and is sampled at Point C by a

sniffer. The sniffer stream connected to the mass spectrometer (MS) is under vacuum at 10-3

bar.

28

This allows a quick response, 11-12s, between the time the 6-port valve is switched and

detection by the mass spectrometer. Point D indicates the stream that feeds the MS. The

roughing pump creates a vacuum in the sniffer steam and evacuates the dosing tube.

The left side of Figure 3.0.3 shows gas flow streams in the furnace. At the centre of the chamber

are two pieces of macor that are fabricated to hold „button‟ cells. A set of five quartz tubes

separate flows and provide support for the macor discs. The exterior tube is supported at the top

and bottom by stainless steel clamps. The middle quartz supports the lower macor piece. The

interior quartz tube forces air across the counter electrode to carry the exhaust gas to the quartz

sniffer. The quartz tube that enters from above delivers the gas flow to the working side of the

cell. A metal disc is attached to this quartz tube to apply pressure to the top macor disc, creating

a pressure seal. The fifth quartz tube is the sniffer which samples the gas from the exhaust stream

of the counter electrode.

Figure 3.0.3: Left: A schematic showing the interconnection of the quartz tubing and gas flow streams on both sides of the cell. Right: Schematic for the cell held between two Macor discs within the furnace.

29

The right side of Figure 3.0.3 shows the symmetric „button‟ cell housed within the

furnace. The cell sits within a depression in the macor disc and platinum mesh contacts both

cathodes. In the schematic, 18

O2 flows from point A to D. Point A delivers the isotope pulse to

the working side of the cell when the 6-port valve is switched. When the cell is polarized, a

fraction of the isotope will travel through the electrolyte. The remainder of the gas delivered

from point A travels around the macor disc through the exterior and middle quartz tubes. At

point C, the isotope tracer evolves from the counter electrode. Air flows across the bottom of the

cell down across the sniffer. A fraction of the flow at point D is sampled by the sniffer to

represent the exhaust.

Using the equipment described above, the two isotope tracing experiments are conducted. The

RTD is the 34

O2 signal measured using quadrapole mass spectrometer combined with an analog

controller and a PC that analyzes the composition of the exhaust stream. For this set of

experiments the cell was held at a constant temperature ranging from 700oC-800

oC and polarized

with a potential ranging from 0 to -8V. Adequate time was allowed for the cell to reach steady

state conditions. An example isotope pulse and response are shown in Figure 3.0.4. Further

information can be found in Section 3.3.

Figure 3.0.4: A hypothetical example of an isotope pulse on the working side of the cell (left) and the measured response in the exhaust from the counter electrode (right).

The depth profiling experiment freezes an 18

O wave during its passage across the cathode and

electrolyte boundary. The profile is frozen by quenching (rapid cooling and setting the applied

voltage to 0V) the cell after a certain set time. An appropriate time is chosen from 1-D modeling

30

and RTD measurements. The cross section of a cell is analyzed by SIMS. Vertical 1-D profiles

18O can be produced from the 2-D SIMS images. Figure 3.0.5 shows a summary of events that

produce an isotope depth profile. Further information can be found in Section 3.4.

Figure 3.0.5: Overview of data interpretation from a hypothetical static depth profiling using a 18

O tracer and ToF-SIMS analysis on a polished cross section of a symmetrical SOFC ‘button’ cell. (Left) Exposed interior cross section of a cell. (Centre) Example SIMS images showing an

18O infusion within the

exposed cross section. (Right) A depth profile shows the fraction of 18

O

3.1 Materials

Gases:

Isotope: 99% 18

O2 : Ar in the ratio 21:79 ISOTEC

Air: 20% O2 Balance N2 (compressed air, extra dry) BOC GASES

Cathode-symmetric „button‟ cells: A limited supply of symmetric button cells was provided by

the Fuel Cell Research Centre (FCRC), Kingston, Ontario. Porous composite thin film

YSZ/LSM cathodes were solution spray-deposited on a 0.9 mm thick dense YSZ electrolyte.

These composite cells were 1mm thick after processing with 12.7 mm diameters. Cells using

only LSM as a cathode were fabricated on 0.65mm YSZ electrolytes. These cells were 0.9 mm

thick with 12.7 mm diameters. A detailed report of the results can be found in Section 5.0. Data

from cells Y166 (LSM cathode with 0.65 mm YSZ electrolyte) and Y17 (composite cathode

with 0.9 mm electrolyte) are described within this report.

31

3.2 Potentiostatic Current Measurements

The resistance across the cell is measured for various applied potentials and at various

temperatures. A negative potential is applied to the working cathode and the current is recorded.

Measurements started at 0V and are decreased at set intervals to -1V before returning to 0V

repeating measurements at the same intervals. The potentiostat ran in potentiostatic operation,

which is constant applied voltage.

3.3 Measuring Residence Time Distributions

The residence time distribution (RTD) is a measurement of the 34

O2 in the exhaust stream at the

counter electrode with time. A sample of the exhaust stream is passed to a mass

spectrometer(MS) using a sniffer. The pressure in the MS side of the sniffer stream is held at 10-3

bar in order reduce the sampling time. The MS cycles between several masses during analysis.

Each point is measured at specified intervals in time, generally once every 10s, averaged over a

specified length of time, generally 5s before the measurement is taken.

Each data point of a RTD measurement is a magnified count of 16-18

O-2

, an ionized O2 that

weighs 34 AMU. The presented values of the 34 AMU signal, [34

O2], are normalized by

subtracting the mean of the measured signal, [34

O2]ave, and then divided by the mean. The

normalized mass 34 signal, [34

O2]norm, was calculated by the following equation.

An 16-18

O2 molecule is used to indicate the isotope RTD instead of the 36 AMU oxygen molecule

due to greater abundance. The isotope 18

O travels through the cell will eventually combine with

either a 16

O or an 18

O when it leaves the counter electrode. Statistically it is more likely for the

32

18O to combine with a

16O. Also, there is a

36Ar isotope and Ar is present in the isotope tracing

gas supply.

Before isotope dosing, each of the masses of interest was measured over extended periods of

time to establish steady state values and to determine whether the signals remain constant over

time. Dosing begins when the 6-port valve, shown in Figure 3.0.2, is switched to allow an 18

O2

stream to contact the working side for a set duration depending on the flow and isotope volume.

Measuring the 16-18

O2 signal determined the transient flow characteristics of the isotope through

the cell. Transient analysis provided the time for oxygen to travel through the cell for various

temperatures and polarizations. This information was used to determine the time until quenching

for static isotope profile infusion experiments. The length of each RTD experiment was 3 hours

to 12 hours depending on the applied potential, temperature and cell thickness.

A typical measured RTD is presented in Figure 4.1.1. This RTD is measured from cell Y166 at

700oC with an applied potential of -3.4V and 205 mA/cm

2.

Figure 4.1.1: A RTD measurement at 700oC with -3.4V and 205mA/cm

2

The first peak in the measured data set begins at 0 minutes and continues to roughly 10 minutes.

This peak represents isotope from the dosing period that leaked into the exhaust stream during

dosing. Since the 18

O signal for this leak returns to 0 quickly it is not considered to have any

effect on the measured RTDs. The second peak of the measured data set is a wave of 16-18

O2

which evolved from the counter electrode, otherwise known as the RTD.

33

3.5 Creating and Measuring Depth Profiles

The depth profile is a measurement of the frozen wave of 18

O infused into the cell. The location

of the frozen wave can be controlled by setting the current density and the length of the applied

potential. In this experiment the wave is frozen close to the surface of the electrolyte and cathode

interface, by exposing the cell to a pulse of 18

O2 with an applied potential of -8V and quenched

after 10 minutes. These conditions are estimates from analysis of the RTD data combined with 1-

D simulations of depth profiles.

The average time an 18

O2-

takes time to travel through the cell is the time between the onset of

the 18

O2 contact and peak mass 34 signal measured from the exhaust is the mean residence time.

This time is inversely proportional to the current density. Thus the average location of the wave

can be approximated.

After 18

O2 dosing and quenching, a distribution of 18

O is captured near the cathode and

electrolyte interface at the working cathode side of the cell. The sample in this study was held at

700oC and a potential of -8V was applied. Following an exposure time of 10 minutes to

18O2 the

applied potential is returned to 0V and the furnace chamber is powered off and opened. Once

cooled, the sample is cut to reveal a cross-section of the cell. The sample is placed within an

electrical conducting resin and polished before ToF-SIMS analysis.

Prior to ToF-SIMS analysis, 3 keV Cs+ ions were used to sputter a 700 μm by 700 μm area to

remove surface contaminants. ToF-SIMS is conducted using an ION TOF IV. Within the cleaned

region a 500 μm by 500 μm area was analyzed by Bi1+

ions at 25 keV using high compression

burst to obtain a mass spectrum. Images are obtained within a 60.5 μm by 60.5 μm area to obtain

higher resolution data of the electrode and electrolyte interface. Each SIMS image is a matrix of

analysis with 256 x 256 data points. The resolution relates to the length divided between 256

data points. Higher resolution refers to a SIMS image with 256 samples within 60.5 μm.

The constraints for the experiment presented in Section 4.2 and Section 5.0 are that a profile of

10 μm to 100 μm in length is frozen near the working electrode and electrolyte interface while

limiting the amount of 18

O2 used. The cell in this case is Cell Y17 and is a thick electrolyte

34

supported composite (YSZ-LSM) cathode symmetric cell. A high applied potential, one that

exceeds typical operating cell voltages, is used to create a profile with the desired length in the

desired time and also matches conditions for the RTD measured in Section 4.1.4. A question

arose whether the cell has homogenous transport which can be answered by examining the 2-D

SIMS images. It is believed that this data might help resolve that issue.

ToF-SIMS measurements are done on a polished cross-section of the cell after infusion. In order

to determine the components of the cell secondary ions peaks are identified. Intensity versus time

plots are obtained from the ToF-SIMS analysis. A calibration is performed by manually

identifying known peaks in the mass spectrum. The ions used for calibration are H-,

16O

-,

18O

-

and ZrO- LaO

-.

Select secondary ions in the negative spectrum were used to indicate various parts of the cell.

The ions of interest include the 16

O-,

18O

-, YO

-, LaO

- and MnO

- peaks. In order to enhance the Zr

signal the ZrO- peak is the sum of the various Zr containing fragments which is the sum of all

isotope variants and various other Zr containing species. The ZrO- peak includes

90ZrO

-,

91ZrO

-,

92ZrO

-,

94ZrO

-,

90ZrO2

- 90

ZrO2-.

35

Chapter 4 Results

4.1 Residence Time Distributions

In this section measured RTDs produced from the first isotope tracing experiment are presented,

compared and discussed. By comparing measured RTDs to simulations, an attempt is made to

discriminate the source of the heterogeneity of the oxygen transport through the cell.

Since this thesis is primarily an exploratory study, the results presented are from both

preliminary experiments from when the apparatus was not finalized as well as the finalized

apparatus under a variety of conditions. These experiments attempt the following:

1. Measure a RTD

2. Examine the changes of RTDs with changes in applied potential and temperature

3. Evaluate the RTDs against a simple 1-D model

Before the results are presented, example RTDs are discussed which is followed by the

development of experimental conditions for the measurements. This is followed by the

development of the best fit simulated RTDs produced using the simple 1-D model. After this, the

results are presented along with their best fit simulated RTDs.

RTDs from two cells are presented and discussed. Cell Y166 uses LSM cathodes and a YSZ

electrolyte with a total cell thickness of 6.5 mm. Cell Y17 uses a composite cathode of LSM and

YSZ, and a YSZ electrolyte with a total cell thickness of 9 mm. Each data set is presented as dots

and simulated RTDs are presented as lines.

36

4.1.1 A Typical RTD

A typical measured RTD is presented in Figure 4.1.1.1. This RTD is measured from cell Y166 at

700oC with an applied potential of -3.4V and a current density of 205 mA/cm

2.

This RTD is taken from a set of experiments that examines the effects caused by changes in

applied potential, Eapp , and temperature. The methodology used to determine the experimental

conditions is described in the following section.

Figure 4.1.1.1: A RTD at 700oC, -3.4V and 205mA/cm

2

As described in Section 3.3, a measured RTD is an 34

O2 signal measured with time. The source

of the 18

O tracer is a pulse of 36

O2 applied to the working side of the cell. The tracer is pulled

through the cell by an Eapp. In this case, the RTD presented in Figure 4.1.1.1 begins at 40 minutes

and extends to 175 minutes. The mean residence time, , is 60 minutes. The tail end of the RTD

is the end of the RTD, in this case extending to 200 minutes.

37

4.1.2 General Considerations

The basis for selecting the experimental conditions for each RTD are that they must be

measureable and distinct using the equipment available. A sufficient current density is required

to move a wave of 18

O quickly through the cell in order to limit the spread by diffusion. A visual

representation of the effect of current density is presented using simulated RTDs produced using

the simple 1-D model described in Section 2.7. These simulations are produced using EZ-Solve

software and the code in Appendix A.1. Once an appropriate current density is established, I-V

curves are examined to determine the required Eapp at a given temperature. A series of simulated

RTDs, presented in Figure 4.1.2.1, show that as current density increases the mean residence

time, , decreases.

Figure 4.1.2.1: Series of simulated RTDs using various current densities

Referring back to Section 2.6, the current density is inversely proportional to . As the current

density decreases, the oxygen resides within the cell for longer, allowing the isotope to continue

to diffuse vertically. A long produces a RTD with a low intensity and a wide breadth. For

example, the simulated RTD at 60mA/cm2 spans 16 hours.

38

In order to measure a clear RTD in a reasonable amount of time a current density is chosen such

that is less than 2 hours. A current density of 150 mA/cm2 corresponds to a of 2 hours.

The Eapp necessary to attain the required current densities are much higher than what a typical

SOFC may experience. A typical single SOFC may operate at 0.5V while the applied potentials

in these experiments run the range of 2 to 8V. Since these studies are exploratory in nature the

high potentials are acceptable. However, it is expected that the cell will degrade under these

conditions as described later.

The high Eapp is necessary since the cells have high resistance across the cell. Figure 4.1.2.2

shows an I-V curve for cell Y166 at 700 o

C. The electrolyte support structure and a relatively

thick electrolyte contribute significantly to the cell resistance. A comparison with an anode

supported cell, demonstrates the difference in performance. Cell Y166 produces 8 mA/cm2 at

500mV compared with a cell that produced 200 mA/cm2 at 500 mV, both at 700

oC (48).

Figure 4.1.2.2: I-V curve measurements for Y166 at 700 oC

At these high potentials it is expected that the cell with degrade with time. After all RTD

measurements were completed, visual inspection of cell Y166 revealed significant delamination

of the counter electrode. Figure 4.1.2.3 shows the resistance with time for Cell Y166. During the

set of measurements, the operating temperature increased twice, once from 700oC to 750

oC at 60

hours and then from 750oC to 800

oC at 75 hours. At 800

oC the total cell resistance is 17Ω which

is greater than the resistance at 700oC and is not expected from a normal operating cell.

39

Figure 4.1.2.3: Degradation of cell Y166 with time.

While conducting the isotope transient experiments the observed resistance gradually increased

with time. At 700oC the resistance across the cell is 13.8Ω compared with 13Ω at 750

oC. The

change in resistance is small compared to the expected change with temperature. For example,

based on the change in the conductivity alone, the ohmic resistance should decrease by a factor

of 2. After 96 hours of continued polarization, the cell the resistance increased by 50%.

The rise in resistance can be attributed to the destruction of the counter electrode due to

delamination. As the electrode becomes inactive, a larger potential is required to drive O2-

through the cell. The delaminaion may occur in different configurations. Large portions of the

counter electrode may become inactive at one time. Alternatively, the deactivation of the

electrode may occur at a smaller scale but in more locations spread across the electrode. In either

case, the current density increases as a result of the reduction of active area. However, post-

mortem inspection of the cell revealed that the counter electrode has indeed delaminated. Larger

portions of the counter electrode, roughly 0.3 mm in diameter, had detached from the cell. This

supports the hypothesis that the counter electrode delaminated during RTD analysis.

Despite the destruction of the cell due to the high Eapp, this set of RTD measurements and the

comparison to simulations demonstrate an ability to detect the effectiveness of the cell. The

measurements show that they can discriminate between the reduction in active area and poor

activation at the cathode. Had oxygen activation limited transport through the cell, increasing the

temperature would have increased the .

700o

C

800o

C

750o

C

40

4.1.3 Best Fit RTD Simulations

The measured RTDs in the following sections are presented along with best fit simulated RTDs.

These best fit RTDs are produced using the simple 1-D model described in Section 2.7. In order

to produce the best fit, the current density of the simulated RTDs are adjusted by decreasing the

cell area and holding the total current constant. Figure 4.1.3.1 presents a measured RTD with an

unadjusted simulated RTD.

Figure 4.1.3.1 Comparison of measured and simulated RTDs at 700oC.

It is observed that the simulated RTD using the experimental conditions is slower than the

measured RTD. In this example case the measured is 67 minutes corresponding to a current

density of 288 mA/cm2 , which is 40% greater than the measured nominal current density of 205

mA/cm2.

In order to correct the difference in current densities, the cell area is decreased in the model,

simulating partial delamination of the counter electrode, which effectively increases the current

density. In this case the area is decreased to 0.9 cm2

from 1.27cm2, while the total current

remains 260mA. Figure 4.1.3.2 shows a simulated RTD using the adjusted area.

41

Figure 4.1.3.2: Comparison of a simulated and measured RTD at 700oC. The simulated RTD uses a

current density of 288 mA/cm2.

The simulated RTD in Figure 4.1.3.2 is able to match the measured RTD fairly well but for two

minor differences in shape. The first is that the simulated RTD predicts an isotope signal earlier

than measured. In other words, the fastest oxygen within the RTD travels slightly slower. The

second is that the decay of the measured isotope signal at the tail end of the RTD is less than

predicted. At 100 minutes, the tail end of the measured RTD is appears to decrease linearly with

time, while the predicted RTD returns to baseline much faster. Similar observations are made

with other measured RTDs.

Each of the RTDs presented in the following section include predicted RTDs using adjusted

current densities using the same methodologies described above.

4.1.3 LSM Cathode – Cell y166

The RTDs are measured from cell Y166 are from a variety of experimental conditions to

determine the effect of changes in temperature and potential on the RTDs. Table 4.1.1.1 outlines

the set of experimental conditions. The temperatures range between 700oC and 800

oC. As

42

described above, Eapp are required in the range of -3 to -4 volts from reference, to attain the

desired total current.

Table 4.1.3.1: Matrix of RTD measurements.

A RTD for each condition detailed in Table 4.1.3.1 was measured. The resulting RTDs are

normalized based on the methods described in Section 3.3 and compared to a simple 1-D model

described in Section 2.7. The following section presents the various RTDs compared based on

constant current density or constant temperature.

4.1.4 LSM Cathode – Cell y166: Constant Temperature

In this section RTDs are presented from constant temperature measurements at both

700oC and 750

oC. Refer to Figure 4.1.4.1 for the temperatures and total currents. At these

temperatures the Eapp ranged from -3V to -4 V, depending on the desired total current. The

measured RTDs along with simulations are shown in Figure 4.1.4.1 and Figure 4.1.4.2.

43

Figure 4.1.4.1: RTDs measured at 700oC at -3V and 178mA/cm

2, -3.5V and 205 mA/cm

2, and -4V and

260mA/cm2.

Figure 4.1.4.1 shows three RTDs measured at 700oC. The current densities necessary for the

simulated RTDs to fit the measured RTDs are greater than those measured. In one case a

simulated RTDs uses a current density that is greater by 100 mA/cm2. However, in all cases the

cell area required to increase the current density is 0.9 cm2.

At different Eapp it is observed that the RTDs demonstrate an increased intensity and decreased .

The model handles these changes well. As mean residence times increase the peak signal

decreases and the spread by diffusion is wider. The simulated fit matches the shape of each of the

measured RTDs adequately. As stated previously there are two observable differences. The first

is that the model predicts a quicker appearance of the isotope signal. The second is that the tail

end of the measured RTD decays significantly slower.

44

Figure 4.1.4.2: A comparison of RTDs measured at 750oC and -2.76V and 178mA/cm

2, and -3.4V and

205 mA/cm2.

Figure 4.1.4.2 shows two RTDs measured at 750oC. The trends observed in the 700

oC RTDs are

also seen in the 750oC RTDs. The simulated models again require higher current densities of 348

mA/cm2 and 486 mA/cm

2 with modified cell areas of 0.51cm

2 and 0.53 cm

2, respectively.

Again it is observed that the simulated RTDs somewhat match the shape of the measured RTDs

with the same deviances. The simulated fit of the 205 mA/cm2 RTD does not fit the tail end.

However, the simulated RTD at 348 mA/cm2 match the tail fairly well. Also, both measured

RTDs appear to have a similar decay rate and magnitude. The simple 1-D model shows that it

can adequately predict the change in RTDs with changes in applied potentials at constant

temperature by adjusting the current density.

45

4.1.5 LSM Cathode– Cell y166: Constant Current density

In this section RTDs are presented from measurements at constant current densities of

178mA/cm2 and 205 mA/cm

2 conducted at 700

oC, 750

oC and 800

oC. Before the results are

presented, a simulated set of RTDs is presented in Figure 4.1.5.1 which shows the expected

result from a change in temperature.

Figure 4.1.5.1: Simulated RTDs at constant current density with change in temperature.

The predicted effect of a change in temperature is slight. The intensity of the signal decreases

since the diffusion coefficient increases with temperature. A greater amount of vertical diffusion

occurs, allowing a portion of the isotope to leave the cell quicker and a portion to advance more

slowly. However, since the model uses properties averaged over the scale of the cell, it does not

take into account any 2-D heterogeneities that may be affected by an increase in temperature. For

example, lateral diffusion is enhanced as temperature increases.

Measured RTDs at constant current densities of 178mA/cm2 and 205 mA/cm

2 conducted at

700oC, 750

oC and 800

oC are presented below. Each of the measurements applied a different

voltage ranging from -2V to -4V to obtain the desired current density. The resulting RTDs along

with best fit simulated RTDs are shown in Figure 4.1.5.1 and Figure 4.1.5.2. The current density

of each of simulation is adjusted to produce fits of the measured RTDs.

46

Figure 4.1.5.1: A set of RTDs measured at 205 mA/cm2 at 700

oC and 750

oC with applied potentials of -

3.5V and -3.4V respectively.

Figure 4.1.5.1 shows two RTDs produced with a current density of 205 mA/cm2. The simulated

RTD at 700oC manages to fit the tail end of the measured RTD well. It is observed that the RTDs

produced at 750oC have a shorter and decreased rate of decay. This is true for the RTDs

measured at both current densities.

It is observed that the RTDs at higher temperatures have a shorter shorter despite having the

same measured current density. This is unexpected assuming the conductive flux is spread

evenly across the horizontal area of the cell. However, if a shorter in this case corresponds with

a reduced area then the average cell area that is active decreases to 0.5 cm2 at 750

oC compared to

0.9 cm2 at 700

oC. Thus oxygen transport is heterogeneous through the cell with respect to the

horizontal area. It is likely that this heterogeneity arose from delamination of the counter

electrode and corresponds with an increase in resistance with time.

Figure 4.1.5.1 shows three RTDs produced at different temperatures with a constant current

density of 178mA/cm2. Similar to the comparisons between measured and simulated RTDs at

constant temperature, it is observed that the simulated RTDs fit the shape of the measured RTDs.

47

Figure 4.1.5.2: A set of RTDs at 178 mA/cm2 and 700

oC, 750

oC and 800

oC with applied potentials of -3V,

-2.76V and -4.6V, respectively.

It should be pointed out that the measured RTD at 800oC in Figure 4.1.5.2 is unique compared to

the other RTDs. This RTD has two peaks. In addition, the required potential to reach 178

mA/cm2 is -4.6V. Increasing temperature generally decreases the required potential, but in this

case it was much higher. This change is due to delamination of the counter electrode.

An attempt to produce depth profiles using this sample did not yield any depth profiles from

static ToF-SIMS analysis. This was the result of the destruction of the cathode material.

However, the 1-D simulations show that the model is able to handle both changes in time as well

as changes in applied potential for working cells.

4.1.6 Composite Cathode- Cell y17

In this section a RTD measurement is presented that was measured using cell Y17, a composite

cathode cell at 700oC. This measurement determines the quenching time for depth profiling

experiment. While the composite cathode benefits from a larger TPB region compared to a LSM

only cathode, the total resistance of the cell is much greater due to the thick electrolyte. In order

48

to obtain similar current densities to the RTDs measured from cell Y166, Eapp from 6-10V are

required. A benefit of a composite cathode cell is that is more resilient to delamination. Figure

4.1.6.1 shows a pulse that was produced with 102mA/cm2 from an applied potential of -6V.

Figure 4.1.6.1: A RTD measured at 700oC with an Eapp of -6V and 102mA/cm

2. The measured RTDs are

indicated by dots. The simulated RTDs are indicated by solid lines.

Figure 4.1.6.1 shows a measured RTD at 700oC and 102mA/cm

2. The simulated RTD required

an adjusted area of 0.45 cm2 with a current density of 235 mA/cm

2. It is observed that at this

current density the RTD is fairly broad. A moving average is presented in Figure 4.1.6.1 to help

distinguish the measured RTD. The noise in the signal is the result of a lose wire connection

between the mass spectrometer and the controller.

Since it is likely that there is horizontal heterogeneity across the cell with respect to

oxygen activation or conduction, the measured current density cannot be assumed to be an

accurate measure for the flow of oxygen. In order to determine a quenching time and current

density, the adjusted area of 0.45 cm2 is considered when designing the depth profiling

experiment.

49

4.2 Oxygen Depth Profiles in Cell Y17

In this section the results are presented from ToF-SIMS analysis of an exposed cross-section of

Cell Y17. These experiments provide the position of each of the cell components, as well as

profiles for the oxygen isotopes that are used to create the 18

O fraction depth profiles.

2-D images of secondary ions are presented from two resolutions, 500μm x 500μm and 60.5μm x

60.5μm. 1-D profiles are presented from the various secondary ions to reveal the location of the

cell components and the frozen 18

O wave. This experiment attempts the following:

1. Measure a depth profile for 18

O on the scale of the entire wave

2. Measure a depth profile for 18

O on the scale of the cathode and electrolyte interface

The data collected from the ToF-SIMS analysis of an exposed cross section is presented as a set

of four 2-D images, one for ZrO ions, one with both LaO- and MnO

- ions, one with

16O

-, and one

with 18

O-. The images are presented as 1-D depth profiles in order to establish the location of

each of the cell components. Detailed analysis of 1-D profiles of the fraction of 18

O is presented

is Section 5.0.

The 2-D ToF-SIMS images of the key secondary ions collected are shown in Figures 4.2.1 and

4.2.3. In Panel A of both Figures 4.2.1 and 4.2.3 appear the summations of the LaO- and MnO

-

ions which arise from the cathode only. Panel B shows the summations of the various ZrO- ions

which arise from both the regions of the electrolyte and the cathode. The electrolyte area is

differentiated from the cathode by the sharp difference in intensity of the ZrO-

ions and the

decrease of MnO- and LaO

- ions. The images show that the cathode and electrolyte are distinct

regions without migration of material into the electrolyte. Panels C and D show intensity images

of the 16

O- ions and

18O

- ions respectively.

50

Figure 4.2.1: Images of ions from an exposed cross section collected using ToF-SIMS. Each image is 500μm x 500μm. Panel A: shows the sum of the LaO

- and MnO

- ions. B: sum of ZrO ions. C: Sum of

16O

-

D: 18

O-

The 500μm x 500μm ToF-SIMS images reveal the entire infused isotope profile. The

observations in Section 4.2 indicate that channeling may have occurred which would have

allowed the tracer to move through the cell with a faster . However, there are no observed signs

of lateral heterogeneity within this set or the higher resolution data for any of the secondary ions.

Thus any lateral heterogeneity must exist at a scale greater than the scale of the measured depth

profiles. Also, while the images show the 2-D distribution of the secondary ions, 1-D depth

profiles are generated to yield additional information.

Since it is not clear the exact position each cell component and the position of the 18

O profile, 1-

D profiles are produced from the images and imposed on each other. Each 1-D profile is

produced from a homogenous region selected in the set of ToF-SIMS images. The intensities of

the secondary ions are averaged within the selected area. The distinction between each of the cell

components is made clear when the set of 1-D profiles are compiled into a single graph. The 1-D

profiles are presented in Figure 4.2.2 .

51

Depth Profiles (500μm x 500μm)

0

2000

4000

6000

8000

10000

12000

14000

0 50 100 150 200 250 300 350 400 450 500

depth (µm)

inte

nsi

ty

LaMn

O16

O18

Zr

YO

Figure 4.2.2: 1-D profiles for a select set of secondary ions taken from the 500μm x 500μm ToF-SIMS images.

The position of each of the cell components is determined by examining the 1-D profiles

presented in Figure 4.2.2. The leftmost region is the conductive resin that is used to hold the

sample. The resin begins at 0μm, where all but the 16

O intensity is zero, and continues to the

edge of the cathode which begins at 75μm. The cathode region is indicated by the LaO- and

MnO- peaks that spans from 75 μm to 100 μm. The electrolyte region is indicated by both the

increases in the ZrO- ions as well as the sharp decrease in LaO

- and MnO

-. After identifying each

of the cell components, the majority of the 18

O profile is observed to lie within the electrolyte. It

observed that the 18

O profile reaches a relatively steady intensity into the electrolyte. The 18

O-

profile is examined in greater detail in Section 5.0. Determining the 18

O profile in the cathode is

difficult due to the limited number of sample points and the thickness of the cathode. The

60.5μm x 60.5μm data set reveals the cathode in greater detail.

Higher resolution 2-D ToF-SIMS images, analyzed from a sample area of 60.5μm x 60.5μm, are

presented in Figure 4.2.3. It is observed through visual inspection that there is no significant

increase of 18

O within the cathode. Panel A shows the cathode region in the cell indicated by

LaO- and MnO

- ions. Panel D shows the enhanced

1I 8O that begins at the LSM/YSZ and YSZ

52

interface, and continues through the remained of the images. It is observed in Panel C that the

16O

- intensity is greater in the cathode region.

Figure 4.2.3: Images of ions in exposed cross section collected using ToF-SIMS. Each image is 60.5 μm x60.5 μm. Panel A: shows the sum of the LaO

- and MnO

- ions. B: sum of ZrO ions. C: Sum of

16O

- D:

18O

-

Using similar methods applied to the lower resolution data, a homogenous area is

selected from the set of ToF-SIMS images and is used to produce 1-D depth profiles. The set of

1-D profiles are compiled into a single graph. Figure 4.2.4 shows the profiles imposed on each

other.

53

Depth Profiles (60.5μm x 60.5μm)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 5 10 15 20 25 30 35 40

depth (µm)

inte

nsi

ty

LaO+MnO

16O

18O

ZrO ions

Figure 4.2.4: 1-D profiles for a select set of secondary ions taken from the 60.5μm x 60.5μm ToF-SIMS images.

The 1-D depth profiles present a clear di stinction between each of the cell components. In figure

4.2.4 the cathode begins between 0-5 μm and ends at 15μm. At the interface the LaO- and MnO

-

decrease and the ZrO- and

18O

- ions increase. It is observed that the

18O and the ZrO

- increase at

roughly the same way. It is necessary for both sets of the ToF-SIMS to be analyzed in order to

understand the oxygen transport at the interface. The 1-D profiles are used to produce profiles of

18O fraction. These profiles are compared with 1-D simulations applying different oxygen

activation regimes in that cathode. This is discussed in greater detail in the following section.

54

Chapter 5 Discussion

5.1 Depth Profiles in Cell Y17

In this section 1-D depth profiles of the fraction of 18

O in Cell Y17 are presented at two

resolutions and compared to various 1-D simulated profiles. The measured set of 1-D profiles are

produced using the 18

O- and

16O

- secondary ions profiles from the ToF-SIMS analysis. These

profiles may provide information on oxygen activation distributions in the cathode by comparing

the profiles with simulated profiles of specific cases. It is shown that oxygen activation is likely

not restricted to the cathode and electrolyte interface. It is also shown that it is likely that an

oxygen activation profile exists within the cathode.

Before the comparisons are presented, the method used to produce the 18

O fraction depth profiles

is described. A profile is calculated by averaging the ion intensities of the secondary ions 18

O-

and 16

O-

within a homogeneous area of the cross-section. The intensity of the 18

O- indicates the

tracer fraction. The fraction is calculated using the sum of the oxygen (18

O- +

16O

-)

intensities the area. Raw intensity counts of both ions are used to determine the 18

O fraction. The

following equation determines 18

O fraction, where indicates the ion intensity measured by the

ToF-SIMS.

The two 18

O fraction profiles are presented in Figure 5.1.1 and Figure 5.1.2. The profiles

presented show the 18

O fraction within the cathode and electrolyte regions only. In both profiles

the region representing the resin is removed. At 0 µm the cathode region begins and extends to

15 µm. The remainder of the profile represents the electrolyte.

55

In the first profile, Figure 5.1.1, the entire 18

O wave is seen within the electrolyte. This is the

depth profile resulting from the 500µm x 500µm ToF-SIMS analysis. Across the cathode and

electrolyte interface the 18

O fraction increases sharply, with about a 3 fold increase within 2 µm.

Although it appears as though the 18

O fraction increases within the cathode there are too few data

points along the vertical axis to report on the profile in detail.

Figure 5.1.1: An 18

O fraction depth profile produced from 500µm x 500µm ToF-SIMS images

In the second profile, Figure 5.1.2, the 18

O fraction profile in the cathode is clearer. This depth

profile is produced from the 60.5µm x 60.5µm ToF-SIMS image. In this profile the 18

O fraction

increases immediately at the interface and continues throughout electrolyte region.

Figure 5.1.2: An 18

O fraction depth profile produced from 60.5µm x 60.5µm ToF-SIMS images

Interface

Interface

56

Below, the two profiles are compared with a series of simulated depth profiles produced using

the 1-D model. Since this is a preliminary study, an attempt is made to determine whether the

measured depth profiles are capable of discriminating between three oxygen activation scenarios.

The three oxygen activation scenarios, presented in Figure 5.1.3, are applied to the model in

order to; 1) rule out limiting cases to determine the possible activation scenario of the depth

profiles and 2) evaluate the information provided from the measured depth profile.

The three scenarios are as follows: (A) Oxygen is only reduced at the electrolyte and cathode

interface (B) Oxygen is reduced throughout the cathode uniformly (C) Oxygen is reduced at the

cathode and platinum contact interface. Figure 5.1.3 shows each of the scenarios

diagrammatically.

Figure 5.1.3: Oxygen activation scenarios. Sphere represents a random distribution of electrolyte and cathode phases. (Left) Scenario A: Oxygen is only reduced at the electrolyte / cathode interface (Middle) Scenario B: Oxygen is reduced throughout the cathode uniformly. (Right) Scenario C: Oxygen is reduced at the cathode / platinum contact interface.

Before the simulations are compared to the measured profiles, the model parameters that are

adjusted are discussed. The model parameters that are adjusted in order to produce the best fit are

the effective diffusion coefficient, and the total current. The effective diffusion coefficient is the

diffusion coefficient for oxygen in the cathode. Adjustments to this parameter correspond to

difficulties with oxygen diffusion through the cathode. This includes the effects of tortuosity and

porosity. The simulated profiles are calculated using Matlab software and the code presented in

Appendix A.2.

57

In the first comparison, the measured profile for the 500µm x 500µm data set is compared to a

profile generated by each oxygen activation scenario, presented in Figure 5.1.4. It is observed

that the simulated profiles for all scenarios have a greater intensity than the measured profile.

While Scenarios B and C produced profiles with a similar shape, this shape does not fit the

measured profile. Scenario A produces a profile that is the closest match to the shape of the

measured profile. All three scenarios are investigated further below.

Figure 5.1.4: Simulated depth profiles for three oxygen activation scenarios compared to the depth profile produced from the 500µm x 500µm ToF-SIMS images

It is observed that the most significant difference between the simulated depth profile of Scenario

A and the measured depth profile is the profile in the cathode. An attempt is made to determine

whether Scenario A could produce a satisfactory fit by modifying the effective diffusion

coefficient in the cathode.

Interface

58

Figure 5.1.5: Scenario A simulations with changes in E compared to the depth profile produced from the 500µm x 500µm ToF-SIMS images

The effective diffusion coefficient influences the diffusion through the cathode. As stated

in Section 2.7, the parameter ε is multiplied by the oxygen self-diffusion coefficient of YSZ to

calculate the effective diffusion coefficient. Simulations are produced with ε ranging from 0 to

0.3. In the case ε=0, it is assumed that no diffusion occurs through the cathode. It is observed that

this parameter cannot produce a profile that matches the distribution within the cathode. Since

this parameter makes the most significant difference in the cathode, it is unlikely that oxygen

activation occurred at the cathode and electrolyte interface only.

Scenario C is investigated next. The best fit result produced using Scenario C is presented

in Figure 5.1.6. In order for this scenario to fit the measured profile, the total current is reduced

to 100 mA from 185mA.

Interface

59

Figure 5.1.6: Best fit Scenario C simulation compared to the depth profile produced from the 500µm x 500µm ToF-SIMS images

There are three observable differences between the simulated and measured profiles in Figure

5.1.6. The first is that the simulated profile in the cathode begins at a lower fraction and has a

more distinct gradient. The second is the magnitude of the peak of the simulated profile within

the electrolyte has a higher intensity. The final is that the tail end of the simulated profile

decreases slightly faster than measured.

Since this scenario had a close fit, it is investigated further. It is possible that the quenching

process did not cool the sample fast enough. If this is the case, then the 18

O can continue to

diffuse at a significant rate. An additional simulation of the best fit conditions for Scenario C is

calculated with 45s of additional time for diffusion to occur and no conductive flux. Figure 5.1.7

shows this result.

Interface

60

Figure 5.1.7: Scenario C simulation with 45s of diffusion after quenching.

The profile in Figure 5.1.7 that diffuses for an additional 45s appears to match the measured

profile well. The additional time flattened the previous best fit in a way for the three differences

observed previously to resolve. In this case the simulated cathode profile matches the measured

profile. The magnitude of the simulated signal is decreased and the tail end of the profile

matches the measure profile. Since it is shown that Scenario C is capable of matching the profile

it cannot be ruled out as a possible scenario.

The comparison between the measured and simulated profile continues with a comparison

Scenario B. The best fit result for Scenario B is presented in Figure 5.1.8. Similar to Scenario C,

in order for this scenario to fit the measured profile the total current is reduced to 100 mA.

Interface

61

Figure 5.1.8: Best fit Scenario B simulation compared to the depth profile produced from the 500µm x 500µm ToF-SIMS images

There are two observable differences between the simulated profile and the measured profile.

The first is that the simulated cathode profile has a greater magnitude than predicted. The second

is that tail end of the simulated profile decays quicker than the measured profile. However, of the

three scenarios, Scenario B provided the best fit.

Although the simulated profile is able to adequately fit the measured profile, a total

current of 100mA is required. Based on the observations in Section 4.1.4 it is likely that there is

lateral heterogeneity with respect to oxygen activation. The RTD measurement indicated that the

expected current density is almost twice the measured current density. The current density

necessary to produce the depth profiles is low by 50%. Despite these differences, the 2-D images

showed no observable signs of lateral heterogeneity on the scale of 100 µm. It is likely then that

the heterogeneity occurs at a greater scale.

From the observations made from the comparisons above several recommendations are made.

The first recommendation is to apply a more comprehensive model to examine the profile within

the cathode. The simple 1-D model, while able to fit the profile, does not adequately describe the

oxygen transport process through the cathode for any firm conclusion to be made. A 2-D

percolation model would be ideal to account for diffusion between two phases in the cathode as

well as diffusion from the cathode to the electrolyte.

Interface

62

The second recommendation is to adjust the quenching procedure. It may also be worthwhile to

establish the effect of quenching by measuring a frozen wave deeper within the electrolyte. By

examining a profile within the electrolyte only, it may be possible to determine the additional

time for significant diffusion to take place.

A third recommendation is to measure depth profiles at different locations within the cell.

Comparisons between profiles in different regions of the cell may or may not confirm the

observed heterogeneity in oxygen transport through the cell. This may reveal cracks or variety in

pore structure that may be present in other regions of the cell.

63

Chapter 6 Conclusions

This thesis applied two isotope tracing experiments using 18

O signals to measure oxygen

transport through polarized solid oxide materials. In the first experiment RTDs indicated that the

oxygen transport was heterogeneous laterally across the cell. The mean residence times of the

measured RTDs were always shorter than predicted using the measured current densities as

reference. The heterogeneity was described as a reduction in area caused by delamination of the

counter electrode. This conclusion is supported by post-mortem examination of the cell which

revealed severe delamination of the counter electrode. As a result, these in-situ measurements

proved successful in identifying differences from ideal oxygen transport.

In the second experiment, SIMS analysis of an exposed cross section revealed an infused 18

O

wave across the cathode and electrolyte interface. When the profile was compared to a variety of

limiting oxygen activation cases, it was found that it is unlikely that the oxygen activation is

restricted to the cathode and electrolyte interface.

The analysis of the depth profiles also revealed that the oxygen activation is likely to have been

heterogeneous laterally. This conclusion arose from the observation that the total current

necessary for creating the simulated profiles is less than measured. However, the SIMS 2-D

images showed no signs of heterogeneity. This implies that the heterogeneity exists at a larger

scale than 500µm.

Regretfully, the RTDs and depth profiles were produced under conditions that a SOFC cell

would not normally experience. Without an additional supply of „button‟ cells for analysis, the

experiments simply showed they were capable of producing the desired set of information.

Future work is focused on applying the technique to different materials including mixed

conducting cathode materials, applying sophisticated 2-D models to simulate oxygen transport

within the cathode in greater detail, and repeating experiments at lower applied potentials.

64

Chapter 7 Future Work

The scope of the current work was limited to demonstrating an application of isotope tracing

techniques to study oxygen transport within polarized SOFC „button‟ cells. The exploratory

work developed an apparatus and procedure which has created opportunities for future work.

Future work expands the scope to include additional modern surface science analysis techniques

to study oxygen transport, in particular measuring reduced oxygen species on the surface of

polarized SOFC cathodes. The proposed set of experiments detail surface analysis techniques

that are unique compared to the majority of current research efforts.

7.1 Motivation

The goal of the future work is to obtain a better understanding of the oxygen reduction

process. While a variety of analytical techniques exist to study the oxygen reduction process,

current efforts appear to be limited to electrochemical analysis techniques. For example,

electrochemical impedance spectroscopy (EIS) measurements of dense thin film microelectrodes

with well defined geometries [37,38] can be used to compare polarization resistances between

different materials. Differential impedance spectroscopy measurements [39] and non-linear EIS

analysis modeling techniques [40] are becoming common methods to study SOFC cathodes.

Despite advances, current studies which used basic EIS measurements on porous electrodes

(LSCF) are based on assumptions made on the oxygen reaction mechanism [41].

Modern surface analysis techniques provide a means of gathering molecular information

at the surface and near-surface of a working cathode. XPS analysis of LSCF microelectrodes

revealed La2+

deficiency at the surface after strong cathodic polarization [42]. In-situ XPS

studies of LSM and YSZ materials have been conducted by Backhaus et al [6, 43]. These in-situ

studies showed that Si impurities migrate to the surface during cathodic polarization. After

65

“cleaning” impurities from the surface the cells exhibited improved performance. These

experiments also indicated that Mn3+

ions migrate across the electrolyte surface, revealing a new

oxygen incorporation pathway. Surface analysis provides additional information on the oxygen

reduction process not available through EIS measurements. The proposed techniques will

examine polarized SOFC cathodes to gather information on the bulk oxygen transport, thickness

dependence, location of active regions, surface composition, oxidation state, ion migration,

vacancy distribution, and distributions of adsorbed oxygen species. While the oxygen reduction

mechanism cannot be measured directly, these experiments will be able to rule out some

possibilities.

7.2 Current Work Continued

The current work revealed signs of lateral and vertical heterogeneity within the electrolyte and

cathode of symmetric „button‟ cells. A possible source of heterogeneity may be poor platinum

contact. Platinum can be painted in various patterns on the surface of the working electrode.

Lateral heterogeneity can be examined by varying the geometries of the platinum contact.

Isotope RTDs and depth profiles can be used to compare effect of platinum contact area.

„Button‟ cells with different cell architectures are available for analysis. These cells have 0.6mm

thick electrolytes (compared to the 0.9 mm thick electrolyte cells used in the current work) with

composite cathodes. These cells have lower total cell resistance allowing for measurable RTDs

closer to regular operating voltages.

High resolution SIMS images will be obtained to determine 18

O fractionation between the LSM

and YSZ within the composite cathodes. For example, a positive secondary ion spectrum reveals

the La18

O+ or Y

18O

+ intensities which can be used to determine which phase contains more

isotope. Positive spectrum measurements are required to obtain higher yields of metal oxygen

fragments.

66

7.3 Proposed Experiments

For convenience the oxygen transport effects studied are described in three relative scales;

Macroscopic, Mesoscopic and Microscopic.

1. Macroscopic effects describe oxygen transport at the scale of cathode architecture

2. Mescoscopic effects describe oxygen transport effects across active regions on the cathode or

TPB regions. This experiment focuses on examining the effect of polarization on oxygen

incorporation in cells with known cathode geometries using isotope tracing techniques.

3. Microscopic effects describe oxygen transport effects at a molecular scale. These

experiments focus on measuring reduced oxygen species on the surface cathode materials

and the polarization dependence.

7.3.1 ‘Macroscopic’ Effects (>10µm)

Oxygen transport will be studied at a scale of the cathode architecture. Bulk oxygen transport

will be examined in cathodes with various architectures i.e. cathodes with varying thickness.

Various studies have examined the effectiveness of cathode thickness on performance (17,18)

but are limited to electrochemical analysis techniques.

The current work developed a technique to form and study isotope profiles in the cathode and

cathode/electrolyte regions. The isotope gradients can be studied to examine the effect of cathode

architecture on oxygen incoporation by comparing measured profiles to profiles produced from

various isotope incorportation flux gradients. Figure 8.3.1.1 shows a cell with uniform isotope

incorportation flux in a porous cathode.

67

Figure 7.3.1.1: Schematic of a cell with a uniform isotope incorportation flux gradient across a porous cathode. The z-axis indicates the depth which starts at z=0, the surface of the cathode, and th is the thickness of the cathode. Isotope incorportation flux is indicated by arrows on the right hand side of the cell, and the magnitude of the flux is represented by the size of the arrow.

The isotope flux into the cathode may not be uniform. In the case of a thick composite cathode, a

gradient in polarization within the cathode may exist [44]. When a polarization gradient exists

across the cell, it was demonstrated that the oxygen dependence could be misinterpreted resulting

in false conclusions on the rate limiting mechanism. 7.3.1.2 shows a variety of possible isotope

incorporation flux gradients with corresponding isotope gradients measurement.

Figure 7.3.1.2: Overview of isotope depth profiles in cathodes produced from corresponding isotope incorportation flux gradients. The top row is a set of cells with different isotope incorporation flux gradients. The bottow row is a set of depth profiles which may develop in the cathode. At t=0, the isotope intensity is uniform and the depth profile is displayed as a thick solid line. At t=pulse the isotope dose makes first contact with the cathode and is displayed as a dotted line. At t=x1,x2,x3 some time has passed since the introduction of the pulse and is/are displayed as a thin solid line.

68

Figure 7.3.1.2 provides an example of cells with non-uniform incorporation flux

gradients and the expected results. Non-uniform flux gradients can occur for a variety of reasons

such as when cathodes have a composite architecture or are made with a poor ion conducting

material. Depending on the measured isotope distribution, the isotope incorporation flux gradient

can be infferred by comparing the results to models of oxygen transport in cathodes. In the event

that multiple cells with various cell thicknesses are used, the effectiveness of the cell thickness

can be compared using the isotope gradients.

7.3.2 ‘Mesoscopic’ Effects (1-10µm)

Isotope tracing will be used examin7e oxygen transport at a more fundamental level to

reveal the active regions of SOFC cells with MIEC materials. The effect of the active region and

TPB region will be examined at various polarizations using SIMS analysis.

The isotope tracing experiment developed in this thesis will be applied to cells with patterned

electrodes. Since porosity, geometry, composition, and fabrication techniques will affect the

oxygen incorporation, a model patterned cathode with defined geometries, at a micron scale, is

required. The model cathode will be an epitaxially grown thin film stripe with a known

thickness, surface area and TPB length. This experiment will determine the location of the active

regions for oxygen incorporation using surface analysis tools to examining isotope depth profiles

at different polarizations.

Polarization influences the limiting oxygen incorporation pathway as reported experimentally by

Horita et al [1,2,3,4] and through various EIS measurements [37,38,39]. In this case, isotope

tracing will be applied to MIEC materials such as LSCF or PBCO. It is expected that oxygen

incorporation will be localized around the TPB at higher polarizations. At lower polarizations a

uniform isotope infusion profile is expected. Figure 7.3.2.1 and 7.3.2.2 shows the experimental

configurations along with expected results.

69

Figure 7.3.2.1: Under low polarization (-0.1V) the oxygen transport is limited by oxygen migration through cathode. The thick line in the right hand cell indicates the penetration depth of the isotope.

Figure 7.3.2.2: Under high polarization (-0.3V) the oxygen transport is limited by oxygen reduction on the surface of the cathode. The thick line in the right hand cell indicates the penetration depth of the isotope.

7.3.3 ‘Microscopic’ Effects (0.1-1µm)

A third set of experiments examines molecular information of surfaces using in-situ surface

analysis of cells under electrochemical polarization. The primary experiment uses in-situ XPS

analysis to measure reduced oxygen species at the surface on cathodes at different polarizations.

The distribution of cations across the surface at the TPB region and through the surface and near-

surface bulk region will also be measured. The second experiment uses in-situ SIMS analysis to

examine issues similar to those in section 7.3.2. The goal of these experiments is to gather

information at a molecular level in order to gain a better understanding of the oxygen reduction

mechanism.

Experiment cells will need to be fabricated for in-situ analysis. A model cathode with known

geometry at a micron scale is required. A reference electrode will be made of palladium (Pd) and

palladium oxide (PdO). This cell set-up is similar to one used by Greg Vovk [45]. The schematic

for a sample cell is shown in Figure 7.3.3.1.

70

Figure 7.3.3.1: Schematic for an experimental cell.

The experimental cell is placed in a sample holder capable of heating the cell and

providing a potential to the cell. The working side of the cell is exposed for XPS and SIMS

analysis. The PdO:Pd reference electrode provides constant oxygen partial pressure at the

counter electrode as long as the devolution on the working electrode is slow. An OCV is applied

to halt O2-

current flow due a electrochemical potential applied by the difference in PO2 at each

electrode. The potential is related to the difference in PO2 across the cell according the Nernst

equation for the cell.

anodeO

cathodeO

P

P

zF

RT

zF

GVE

,

,

2

2ln)(

The reaction on both sides of the cell is the oxygen exchange between the environment

and the cathode:

2

2 24 OeO

The partial pressure of oxygen is determined by the equilibrium reaction at the reference

electrode between Pd, O2 and PdO:

PdOOPd 22

1

71

The first experiment will examine the surface to collect molecular information at the surface and

near surface bulk using XPS. A sample holder is available which can apply a voltage and heat

the sample for in-situ analysis. Figure 7.3.3.2 shows a simplified overview of XPS analysis.

Figure 7.3.3.2: Overview of XPS analysis. (Left) X-rays bombard the sample surface exciting electrons from a cell experiencing an applied potential. (Right) Excited electrons leave the sample.

In-situ XPS analysis will provide molecular information of both the surface and near

surface bulk of a cathode material. It will be possible to determine the extent of cation migration,

vacancy distributions, and surface species at various temperatures and polarizations.

Measurements of reduced surface oxygen species at different polarizations will be used to test

various hypotheses on the oxygen reduction mechanism.

As stated in section 7.1, Backhaus has demonstrated the value of in-situ XPS analysis

from studying the movement of cations during polarization. The proposed in-situ XPS study will

measure cation migration at the electrolyte/cathode interface at the TPB. Also, a comparison

study of surface polarization resistances of MIEC cathodes was produced by Fleig et al. [42]. It

was found that barium strontium cobalt ferrite (BSCF) exhibited a lower oxygen exchange

resistance compared to LSCF. The proposed in-situ XPS analysis technique described would be

capable of determining the difference in surface chemistry between materials under polarization.

The second experiment will measure the distribution of adsorbed oxygen species (i.e. peroxides)

on the surface of cathodes in-situ using SIMS. During analysis, the reference electrode will

provide a constant oxygen partial pressure on the counter electrode. Oxygen will move from the

reference electrode to the surface. When the ion transport is slow the working cathode will

experience steady state during which SIMS will sample the surface. At steady state the surface

72

will contain adsorbed surface species. It is expected that O2,ad will accumulate at active regions

depending on the applied potential. Figure 7.3.3.3 shows an overview of the experiment with

expected results.

Figure 7.3.3.3: Schematic of 1-D surface profiles produced from different polarization regimes. Left: Low polarization. Right: High polarization. Bottom: 1-D represented of 02,ad across the surface of the cathode

Figure 7.3.3.3 shows schematics for the cross section of a cell with expected 1-D O2,ad

distributions at two polarizations. SIMS analysis will reveal the surface distribution of adsorbed

surface species. It is expected that the TPB region will have an abundance of diatomic surface

species at high polarizations compared to the bulk cathode due to often reported increase in

oxygen incorporation at those conditions.

Isotope tracing could potentially be used to get a greater picture of where oxygen evolves from a

cathode under different polarizations. The Pd:PdO reference electrode would be doped with 18

O.

In that case it would be possible measure 18

O distributions across the surface with different

polarizations, which would reveal the preferred oxygen transport pathway.

73

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75

46.

Parameters:

Atot(cm2): total geometric area of cells used in current work

C(mol/cm3): molar concentration

oD (cm2/s): oxygen self-diffusion coefficient

d(cm): cell thickness

Eo (V): standard state cell voltage

E (V): voltage

F(96485 C/mol e-): Faraday‟s constant

if : mole fraction of species i

oG (J/mol): standard Gibb‟s Free Energy change

itot(A/cm2): total current density

Itot(A): total current

J(moles O2-

/ cm2 s): oxygen ion flux

(mol/cm2): moles per area

Pi: partial pressure of species i

R (8.314 J/mol•K): universal gas constant

T (K): absolute temperature

th (cm): cathode thickness

t(s): time

76

(min): mean residence time

x(cm): distance

z: charge transfer coefficient

ζi(S/cm): conductivity of species i

ρ(g/cm3): density

77

Appendix

A.1 EZ Solve Code

// RTD Simulation // Source of 18O is the Electrolyte / Cathode interface // f1 represents the cathode / electrolyte interface f1'=Jcond*(fin-f1)/(rhoO*x)-Do*(f1-f2)/x^2 // f2-f19 represents the bulk electrolyte f2'=Jcond*(f1-f2)/(rhoO*x)+Do*(f1-f2)/x^2-Do*(f2-f3)/x^2 f3'=Jcond*(f2-f3)/(rhoO*x)+Do*(f2-f3)/x^2-Do*(f3-f4)/x^2 f4'=Jcond*(f3-f4)/(rhoO*x)+Do*(f3-f4)/x^2-Do*(f4-f5)/x^2 f5'=Jcond*(f4-f5)/(rhoO*x)+Do*(f4-f5)/x^2-Do*(f5-f6)/x^2 f6'=Jcond*(f5-f6)/(rhoO*x)+Do*(f5-f6)/x^2-Do*(f6-f7)/x^2 f7'=Jcond*(f6-f7)/(rhoO*x)+Do*(f6-f7)/x^2-Do*(f7-f8)/x^2 f8'=Jcond*(f7-f8)/(rhoO*x)+Do*(f7-f8)/x^2-Do*(f8-f9)/x^2 f9'=Jcond*(f8-f9)/(rhoO*x)+Do*(f8-f9)/x^2-Do*(f9-f10)/x^2 f10'=Jcond*(f9-f10)/(rhoO*x)+Do*(f9-f10)/x^2-Do*(f10-f11)/x^2 f11'=Jcond*(f10-f11)/(rhoO*x)+Do*(f10-f11)/x^2-Do*(f11-f12)/x^2 f12'=Jcond*(f11-f12)/(rhoO*x)+Do*(f11-f12)/x^2-Do*(f12-f13)/x^2 f13'=Jcond*(f12-f13)/(rhoO*x)+Do*(f12-f13)/x^2-Do*(f13-f14)/x^2 f14'=Jcond*(f13-f14)/(rhoO*x)+Do*(f13-f14)/x^2-Do*(f14-f15)/x^2 f15'=Jcond*(f14-f15)/(rhoO*x)+Do*(f14-f15)/x^2-Do*(f15-f16)/x^2 f16'=Jcond*(f15-f16)/(rhoO*x)+Do*(f15-f16)/x^2-Do*(f16-f17)/x^2 f17'=Jcond*(f16-f17)/(rhoO*x)+Do*(f16-f17)/x^2-Do*(f17-f18)/x^2 f18'=Jcond*(f17-f18)/(rhoO*x)+Do*(f17-f18)/x^2-Do*(f18-f19)/x^2 f19'=Jcond*(f18-f19)/(rhoO*x)+Do*(f18-f19)/x^2-Do*(f19-f20)/x^2 // Represents O2 evolution f20'=Jcond*(f19-f20)/(rhoO*x) // Constants rhoO=0.093 // mol cm-3 rhoOCath=0.024 // mol cm-3 Jcond=i/(96485*2) // mol cm-2 s-1 I=0.260 // mA xcath=0.0015/2 x=0.065/19 // cm i=I/As // A/cm-2 As=0.9 //Area // cm-2 temp=723+273 // K R=8.314 // J/mol*K F=96485 // C/mol // Nernst Einstein cond=(7.92*10^6/temp)*exp(-111000/(temp*R)) Do=cond*(1/rhoO)*R*temp/(2*F)^2

78

// Dose Calculation qo=5 // cc pulse = qo/60 // cm3 s-1 length = 240 // cm diam = 0.15 // cm pvol= 3.14*((diam/2)^2)*length // cm3 ptime = pvol/qo fin=1-1*Step(t,ptime)

79

A.2 MATLAB Code:

File: PFmodeldepth1I.m

function datasim=PFmodel(inputArea,surfrac,ks)

% Assigning Variables

Itot=0.185; % Total Current A Area=inputArea; % Area // cm-2 Temp=723+273; % K rows=500; th=0.09; % cm

cathth=9; % number of index points time=585; % s for 10 mins fsx=surfrac; deltat=0.1; ks=ks;

% Number of boxes x=th/rows; % cm

% Assigning Constants rhoO=0.093; % mol cm-3 R=8.314; % J/mol*K F=96485; % C/mol rhoOcath=0.032648; % mol cm-3 eps=0.3; % porosity and phase factor

% Dose Calculation qo=5; % cc pulse = qo/60; % cm3 s-1 length = 240; % cm diam = 0.15; % cm pvol= 3.14*((diam/2)^2)*length; % cm3 ptime = pvol/pulse;

% Nernst Einstein and other Equations id=Itot/Area; % A/cm-2 cond=(7.92*10^6/Temp)*exp(-111000/(Temp*R)); Do=cond*(1/rhoO)*R*Temp/(2*F)^2; Jcond=id/(96485*2); % mol cm-2 s-1 io=0.006; Jo=io/(96485*2);

% Matrix PF Solver

Matx(1:rows,1)=0.004; Matx(1:rows,2)=0.004;

%For index initializing j=0; k=1; fin=0.002; jx=time/deltat; jpulse=ptime/deltat;

rhoOInt=0.5*rhoO+0.5*rhoOcath; % No conduction through the Cathode

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while j<(jx)

if (j<(jpulse)) fin=fsx; else fin=0.002; end

k=1;

while (k<(rows+1))

if k==1 & cathth>1

Matx(k,2)=((0)*Jcond*(fin-Matx(k,1))/(rhoOcath*x)... -eps*Do*(Matx(k,1)-Matx(k+1,1))/x^2)*deltat+Matx(k,1);

end

if k>1 & k<cathth

Matx(k,2)=((0)*Jcond*(fin)/(rhoOcath*x)... +(0)*Jcond*(Matx(k-1,1))/(rhoOcath*x)... -(0)*Jcond*(Matx(k,1))/(rhoOcath*x)... +eps*Do*(Matx(k-1,1)-Matx(k,1))/x^2 ... -eps*Do*(Matx(k,1)-Matx(k+1,1))/x^2)*deltat+Matx(k,1); end

if k==cathth

Matx(k,2)=((0)*Jcond*(fin)/(rhoOcath*x)... +(0)*Jcond*(Matx(k-1,1))/(rhoOcath*x)... -(0)*Jcond*(Matx(k,1))/(rhoOcath*x)... +eps*Do*(Matx(k-1,1)-Matx(k,1))/(x^2) ... -eps*Do*(Matx(k,1)-Matx(k+1,1))/(x^2))*deltat+Matx(k,1); end

if k==(cathth+1)

Matx(k,2)=((1)*Jcond*(fin)/(rhoO*x)... +(0)*Jcond*(Matx(k-1,1))/(rhoO*x)... -Jcond*(Matx(k,1))/(rhoO*x)... +eps*Do*(Matx(k-1,1)-Matx(k,1))/(x^2) ... -Do*(Matx(k,1)-Matx(k+1,1))/(x^2))*deltat+Matx(k,1); end

if k>(cathth+1) & k<rows Matx(k,2)=(Jcond*(Matx(k-1,1)-Matx(k,1))/(rhoO*x)... +Do*(Matx(k-1,1)-Matx(k,1))/x^2 ... -Do*(Matx(k,1)-Matx(k+1,1))/x^2)*deltat+Matx(k,1); end

if k==rows Matx(k,2)=(Jcond*(Matx(k-1,1)-Matx(k,1))/(rhoO*x)+Do*(Matx(k-1,1)-... Matx(k,1))/x^2)*deltat+Matx(k,1); end

k=k+1;

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end Matx(1:rows,1)=Matx(1:rows,2); j=j+1; Matout(j,1)=Matx(rows,1);

end

model=Matx; datasim=model(:,1);

filename = uiputfile('*.xls','Save Profile file As'); xlswrite(filename, datasim);

clear;

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A.3 Experimental Equipment

Equipment:

Mass Spectrometer Set-Up: Quadrupole Mass Spectrometer combined UTi precision gas

analyzer: Model 100C. SEIKO SEIKI STP Control Unit STP-300 controlling STP-300 Turbo

Molecular Pump. Edwards dry diaphragm pump as a roughing pump. PC combined with software

used to control the UTi controller.

Diamond Saw – Clemex Brillant 221

Potentiostat: Model 273 PAR Princeton Applied Research Potentiostat/Galvanostat

Flow Controller: Omega FMA 5400/5500 Mass Flow Controllers

Temperature Controller: Omega Temperature controller

ION TOF IV ToF-SIMS at SI Ontario