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SOFC Anode

Hydrogen oxidation at porous nickel and nickel/yttria-stabilised zirconia cermet electrodes

Baukje de Boer


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Boer, Baukje de

SOFC Anodes : Hydrogen oxidation at porous nickel and nickel/yttria-stabilised zirconia

cermet electrodes

Thesis Enschede. With ref. With summary in Dutch.

ISBN 90-36511909

Copyright 1998 by B. de Boer, the Netherlands.


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SOFC ANODE

HYDROGEN OXIDATION AT POROUS NICKEL AND

NICKEL/YTTRIA-STABILISED ZIRCONIA CERMET ELECTRODES

PROEFSCHRIFT

ter verkrijging van

de graad van doctor aan de Universiteit Twente,

op gezag van de rector magnificus,

prof. dr. F.A. van Vught,

volgens besluit van het College voor Promoties

in het openbaar te verdedigen

op vrijdag 9 oktober 1998 te 16.45 uur.

door

Baukje de Boer

geboren op 8 oktober 1968

te Appelscha


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Dit proefschrift is goedgekeurd door de promotor

prof. dr. ir. H. Verweij

en de assistent promotor

dr. H.J.M. Bouwmeester


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I have yet to see any problem, however complicated,

which, when you looked at it in the right way,

did not become still more complicatedP. Anderson, 1926

(New scientist, 25 September 1969, p 639)


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The investigations described in this thesis were supported financially by the Netherlands Energy

Research Foundation, ECN.


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Summary

In the ongoing search for alternative and environmental friendly power generation facili-

ties, the fuel cell is a good candidate. There are several types of fuel cells with large differ-

ences in application, size, cost and operating range. The Solid Oxide Fuel Cell (SOFC) is a

high temperature fuel cell, interesting for decentralised generation of heat and power.

Nickel/yttria-stabilised zirconia cermet is the state-of-the-art material for use as anode in

SOFCs. This thesis describes a number of experimental studies on the anode of the SOFC.

The emphasis is on two important aspects of this type of electrode, the kinetics of the hy-

drogen oxidation reaction and the effect of the microstructure on the electrochemical per-

formance of the electrode. Insight in these two aspects will lead to a better understanding

and further improvements of the anode.

A general introduction in fuel cells is given in Chapter 1, followed by a brief review on

SOFC materials and SOFC anode kinetics as reported in literature. This last part gives a

review of different mechanisms proposed in literature from which it is evident that the ex-act nature of the reaction kinetics has still not been well established.

In Chapter 2 the experimental set-up and the design for the electrochemical cell is de-

scribed. Let it be a warning to every researcher who needs to do concessions when an ex-

perimental set-up is built to study only part of the system.

Porous nickel electrodes are the subject of Chapter 3 and 4. The choice to study this type of

electrodes is initiated because of their less complicated microstructure compared with the

Ni/YSZ cermet electrodes. The lithograpically prepared nickel pattern electrodes (Chapter

3) have a well defined microstructure in terms of electrolyte area covered with nickel and

the Triple Phase Boundary (TPB) line between electrolyte, electrode and gas phase. Linepatterns with variation in nickel line width from 10 to 75 m resulted after electrochemicalmeasurements in TPB lengths in the range 1.6 to 11.3 mcm-2. For electrochemical charac-terisation of the electrodes impedance and I- type of measurements are performed. Im-pedance measurements performed under standard conditions resulted in spectra, which

when analysed with an equivalent circuit, are built up out of three semicircles, one of these

dominating the overall behaviour. A relation between the TPB length and the overall elec-

trode performance seems apparent, but experimental data show large scatter.

The porous nickel electrodes described in Chapter 4 can be regarded as a nickel layer per-

forated with small holes. Although quantification of the microstructure of this type of

electrode is less straightforward compared with that of the lithographic electrodes, reason-

able results are obtained with image analysis. TPB lengths of this type of electrode are in

the range 45 to 61 mcm-2. Again a relation is obtained between the TPB length and theelectrode conductivity. At equilibrium the impedance diagram consists of one dominant arc

at the high frequency side of the spectra and a significantly smaller arc at the low fre-

quency side. Results of impedance and I- measurements as a function of pH2 and pH2Oare discussed in view of a tentative multi-step mechanism formulated for the electrode re-

action. The results indicate that a simple description in terms of a Butler-Volmer formal-

ism, based on a single rate determining step, is excluded. It is suggested that a strong

variation in the fractional coverage of adsorbed intermediates with overpotential on ei-


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ther nickel or yttria-stabilised zirconia surfaces must be taken into consideration to account

for the experimental data.

To study the effect of YSZ in cermet electrodes the surface of a porous nickel electrode

was modified with fine YSZ (Chapter 5). The surface modification leads to a significant

improvement in the electrochemical activity compared with that of bare porous nickelelectrodes, e.g. the total electrode resistance decreases with more than 50%. This effect is

ascribed to an increase in the number of reaction sites. The polarisation and impedance be-

haviour at different H2 and H2O partial pressures for the modified nickel electrodes is

found to be very similar to that observed for bare nickel electrodes.

In Chapter 6, the study on the nickel/yttria-stabilised zirconia cermet electrodes starts with

analysis of the microstructure. By using different ratios of fine to coarse YSZ powder dur-

ing the preparation of the electrodes seven different microstructures were obtained. The

microstructures was quantified by image analysis in terms of porosity, nickel particle size

and surface coverage of the interface between the electrode and the electrolyte. The rela-

tion between the electrochemical performance of the electrode and the surface coverage

indicates that, for coarse cermet structures, the electrode reaction is confined to the imme-

diate interface between the cermet electrode and the electrolyte. For fine cermet structures

on the other hand, the electrode reaction zone extends into the bulk of the cermet.

To obtain more insight in the spatial extension of the TPB perimeter in a direction perpen-

dicular to the electrolyte / porous electrode interface a ladder network (continuous trans-

mission line) model is involved in analysis of impedance data as discussed in Chapter 7.

For this network an analytical expression is derived for the impedance. The variables in the

model are the impedances associated with the transport of ionic and electronic charge car-

riers through both constituent phases of ceramic and metal, and that of the charge transferreaction at the TPB points. The results show that the replacement of coarse YSZ particles

by a corresponding fraction of fine YSZ particles in electrode preparation reduces the total

polarisation losses, but does not change the requirements regarding the thickness of the

electrodes in order to optimise their performance.

In Chapter 8 the kinetics of the hydrogen oxidation reaction at the cermet electrodes is

studied by impedance measurements as a function of pH2 and pH2O partial pressures, ano-

dic polarisation and temperature. The impedance spectra show a complex behaviour at the

low frequency side, which is not completely understood. This part consists of a small

semicircle with a large capacitance value at the low frequency side and an inductive loopin the mid-frequency region. The appearance of an inductive loop can be associated with

concentration relaxation of adsorbed intermediates. The high frequency arc can be related

with the TPB length and is therefore ascribed to charge transfer.

In the last chapter an evaluation of the results in this thesis work is given. As four different

types of electrodes, with their own specific microstructure are studied under the similar

experimental conditions, a perfect opportunity is created to obtain insight in the relation

between the microstructure and the electrochemical performance of the electrodes. The

electrodes cover a large range in the triple phase boundary length. For the nickel electrodes

a linear relationship exists between the TPB length, which is related with the active area

for the electrode reaction, and the total electrode conductivity. Based upon this linear rela-


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tionship it is concluded that for the cermet electrodes part of the bulk is active in the elec-

trode reaction, consistent with the results from transmission line modelling of the imped-

ance behaviour of the cermet. A transmission line model indicates that for all cermet elec-

trodes the bulk will be active. For increasing fractions of fine to coarse YSZ in the cermet

electrodes the TPB length per unit volume of the electrode increases, leading to a decreasein the electrode resistance. This result emphasises the importance of a fine and highly per-

colative structure for the cermet electrode

The combined results of reaction kinetic studies at the different types of electrode support

the conclusion that under the conditions covered by these experiments the electrode proc-

esses are governed by charge transfer.


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Samenvatting

In de voortdurende zoektocht naar alternatieve en milieuvriendelijke mogelijkheden voor

de opwekking van electriciteit, wordt de brandstofcel gezien als een goede kandidaat. Er

zijn verscheidende type brandstofcellen die zich onderscheiden op het gebied van toepas-

sing, grootte, kosten en werkgebied. De vaste oxide brandstofcel (SOFC) is een brandstof-

cel met een hoge werktemperatuur, deze cel is vooral interessant voor het gedecentrali-

seerd opwekken van warmte en electriciteit. Nikkel/yttria-gestabiliseerd zirconia cermet is

op dit moment het state-of-the-art materiaal voor de anode van de SOFC. Het accent van

dit werk ligt bij twee belangrijke aspecten van dit type electrode, de kinetiek van de water-

stof oxidatie reactie en het effect van de microstructuur op het electrochemisch gedrag van

de electrode. Inzicht in deze aspecten zal leiden tot een beter begrip en tot betere prestaties

van de anode.

Een algemene introductie in brandstofcellen wordt gegeven in Hoofdstuk 1. Dit wordt ver-

volgd met een kort overzicht van de SOFC materialen en de SOFC anode kinetiek zoalsgegeven in de literatuur. Uit de verschillende reactie mechanismen zoals deze zijn voorge-

steld kan geconcludeerd worden dat de eigenlijke aard van de anode reactie nog niet is

vastgesteld.

In Hoofdstuk 2 wordt de experimentele opstelling en het ontwerp van de electrochemische

cel beschreven. Laat het een waarschuwing zijn voor alle onderzoekers die concessies

moeten doen op het moment dat een meetopstelling wordt gebouwd waarin slechts een

deel van het uiteindelijke systeem wordt bestudeerd.

Poreuze nikkel electroden zijn het onderwerp van Hoofdstuk 3 en 4. De keuze om aan dit

type electroden te meten wordt ingegeven door de eenvoudige microstructuur van nikkelelectroden in vergelijking met die van de Ni/YSZ cermet electroden. De nikkel patroon

electroden die gemaakt zijn met lithografische technieken (Hoofdstuk 3) bezitten een goed

gedefinieerde microstructuur in termen van het met nikkel bedekte electrolyt oppervlak en

de drie-fase-grens (TPB) tussen het electrolyt, de electrode en de gas fase. Lijn patronen

waarbij de nikkel lijn breedte wordt gevarieerd tussen 10 en 75 m resulteert, na electro-chemische metingen, in een TPB lengte tussen de 1.6 en 11.3 mcm-2. Voor de electroche-mische karakterisering van de electroden wordt gebruik gemaakt van impedantie en I-metingen. Impedantie spectra die volgen uit impedantie metingen onder standaard condi-

ties worden geanalyseerd met een equivalent circuit. Deze fit bestaat uit drie halve bogen,waarvan n boog het totale gedrag domineert. Een relatie werd gevonden tussen de TPB

lengte en het totale electrode gedrag, waarbij vermeld dient te worden dat er in de electro-

chemische data van de patroon electroden een behoorlijke spreiding zit.

De poreuze nikkel electroden zoals beschreven in Hoofdstuk 4 kunnen worden beschouwd

als nikkel lagen met vele kleine gaatjes. De quantificering van dit type microstructuren is

minder eenvoudig dan die van patroon electroden. Met behulp van beeldanalyse technieken

kunnen redelijke resultaten worden verkregen. Voor dit type electrode ligt de TPB lengte

tussen de 45 en 61 mcm-2. Ook hier werd een relatie gevonden tussen de TPB lengte enhet totale electrode gedrag. In evenwicht bestaat het impedantie diagram uit n dominante

boog aan de hoog frequente zijde en een significant kleinere boog aan de laag frequentezijde. Resultaten van impedantie en I- metingen als functie van de pH2 en pH2O worden


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besproken in het licht van meer-stappen mechanisme voor de electrode reactie. De resul-

taten geven aan dat een eenvoudige beschrijving in termen van een Butler-Volmer mecha-

nisme, gebaseerd op n snelheidsbepalende stap, is uitgesloten. Er wordt gesuggereerd dat

er een sterke variatie bestaat in de fractionele bezetting van geadsorbeerde deeltjes op nik-

kel ofwel YSZ, als functie van de overpotentiaal .Om het effect van YSZ in cermet electroden te bestuderen wordt het oppervlak van eenporeuze nikkel electrode gemodificeerd met fijn YSZ (Hoofdstuk 5). De oppervlakte modi-

ficatie leidt tot een significante verbetering van de electrochemische activiteit. In vergelij-

king met een niet gemodificeerde poreuze nikkel electrode, neemt de totale electrode weer-

stand af met meer dan 50%. Dit effect wordt toegeschreven aan een toenemend aantal ac-

tieve plaatsen. Het polarisatie en impedantie gedrag voor de gemodificeerde nikkel elec-

trode als functie van de H2 en H2O druk vertoont overeenkomst met de nikkel electroden.

In Hoofdstuk 6 start het onderzoek aan de nikkel/yttria-gestabiliseerd zirconia cermet

electroden met een analyse van de microstructuur. Door gebruik te maken van verschillen-

de verhoudingen in de fracties fijn en grof YSZ poeder was het mogelijk om zeven ver-

schillende microstructuren te verkrijgen. De microstructuren werden gequantificeerd in

termen van porositeit, nikkel deeltjes grootte en oppervlakte bedekking van het grensvlak

tussen de electrode en het electrolyt. De relatie tussen het electrochemische gedrag van de

electrode en de bedekkingsgraad, indiceert dat voor grove cermet structuren de electrode

reactie wordt beperkt tot het directe grensvlak tussen de cermet electrode en het electrolyt.

Aan de andere kant breidt voor fijne cermet structuren de actieve laag zich uit in the bulk

van de cermet.

Om meer inzicht te krijgen in de ruimtelijke uitbreiding van de TPB grens in een richting

loodrecht op die van de poreuze electrode / het electrolyt wordt, in Hoofdstuk 7, een laddermodel (transmissie lijn model) betrokken bij de analyse van de impedantie data. Voor dit

netwerk is een analytische oplossing afgeleid voor de impedantie. De variabelen in dit mo-

del zijn geassocieerd met het transport van ionische lading dragers in het YSZ, met elec-

tronische lading dragers door het nikkel en de lading overdracht reactie of de TPB punten.

Het resultaat toont aan dat, wanneer grove YSZ deeltjes in het cermet vervangen worden

door een corresponderende fractie fijne YSZ deeltjes, dit leidt tot een verlaging van de po-

larisatie weerstand. De vereisten voor een optimale prestatie met betrekking tot de dikte

van de electroden verandert niet.

In Hoofdstuk 8 worden de resultaten gegeven van een studie naar de kinetiek van de water-stof oxidatie reactie aan de cermet electroden. Deze is uitgevoerd met behulp van impe-

dantie metingen als functie van de partile pH2 en pH2O druk, de anodische polarisatie en

de temperatuur. De impedantie vertoont een complex gedrag aan de laag frequente zijde,

welke niet geheel begrepen wordt. Dit deel bestaat voor de laag frequenties uit een kleine

halve cirkel met een hoge capaciteit waarde en voor het mid-frequentie gebied uit een in-

ductieve boog. De verschijning van de inductieve boog kan geassocieerd worden met

concentratie relaxatie van geadsorbeerde tussen producten. De hoog frequente boog kan

gerelateerd worden aan de TPB lengte en wordt daarom toegeschreven aan lading over-

dracht.


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In het laatste hoofdstuk wordt een evaluatie gegeven van de resultaten zoals beschreven in

dit proefschrift. Doordat vier verschillende types electroden zijn bestudeerd, die elk met

hun eigen specifieke microstructurele eigenschappen onder dezelfde experimentele condi-

ties zijn bestudeerd, is een perfecte mogelijkheid gecreerd om het inzicht te vergroten in

de relatie tussen de microstructuur en het electrochemische gedrag van de electroden. Deelectroden beslaan een groot bereik in TPB lengte. Voor de nikkel electroden is een lineai-

re relatie gevonden tussen de TPB lengte, gerelateerd aan het actieve gebied voor de elec-

trode reactie, en de totale electrode geleidbaarheid. Gebaseerd op deze lineaire relatie kan

geconcludeerd worden dat voor cermet electroden de bulk van de electrode actief moet

worden. Dit resultaat is overeenstemming met resultaten verkregen uit transmissie lijn mo-

dellering, welke indiceert dat voor alle cermet electroden de bulk actief zal zijn. Voor toe-

nemende fracties fijn YSZ in de cermet electroden neemt de TPB lengte per volume een-

heid van de electrode toe, dit leidt tot een afname van de electrode weerstand. Dit resultaat

benadrukt het belang van een fijne en hoog percolatieve structuur voor cermet electroden.

De gecombineerde resultaten van de studie naar de reactie kinetiek aan verschillende type

electroden onderschrijft de conclusie dat onder de toegepaste experimentele condities het

electrode proces wordt gedomineerd door lading overdracht.


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Table of contents

1 Introduction 11.1 The Fuel Cell 1

1.1.1 Definition of a fuel cell 11.1.2 Historical background 21.1.3 Types of fuel cells 21.1.4 Advantages and drawbacks of fuel cells 4

1.2 Solid Oxide Fuel Cell 51.2.1 Thermodynamic principles 51.2.2 Fuel Cell efficiency 61.2.3 SOFC materials 91.2.4 SOFC anode kinetics 11

1.3 Scope of this thesis 202 Experimental considerations 25

2.1 Introduction 252.2 The experimental set-up 252.2.1 The electrochemical cell 252.2.2 Sample holder 262.2.3 Gas conditioning and controlling system 272.2.4 Furnace and controller 272.2.5 Measurement Equipment 27

2.3 Sample choice in view of geometric requirements for the electrode configuration 282.3.1 Experimental 302.3.2 Results 312.3.3 Discussion 312.3.4 Comparison of results of different type of electrolytes for porous nickel and cermet

type of electrodes. 342.3.5 Conclusions 353 Hydrogen oxidation at nickel pattern electrodes on yttria-stabilised zirconia 37

3.1 Introduction 373.2 Experimental 38

3.2.1 Sample preparation 383.2.2 Characterisation of the electrode microstructure 383.2.3 Electrochemical characterisation 39

3.3 Results 403.3.1 Microstructure 403.3.2 Electrochemical performance 43

3.4 Discussion 50

3.4.1 Relationship of electrode performance with microstructure 503.4.2 Electrochemical measurements 513.4.3 Comparison with data from Mizusaki et al. 52

3.5 Conclusions 534 Hydrogen oxidation at porous nickel electrodes on yttria-stabilised zirconia 55

4.1 Introduction 554.2 Theory 56

4.2.1 Reaction scheme 564.2.2 Langmuir adsorption 574.2.3 I - relationship 58

4.3 Experimental 604.3.1 Sample preparation 604.3.2 Characterisation of the electrode microstructure 614.3.3 Electrochemical characterisation 61


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4.5 Discussion 704.5.1 Reaction mechanism 704.5.2 Relationship of the microstructure with electrode conductivity 754.5.3 Comparison with literature 75

4.6 Concluding remarks 785 Hydrogen oxidation at porous nickel electrodes on yttria-stabilised zirconia: Effect of surfacemodification with fine YSZ. 81

5.1 Introduction 815.1.1 Fabrication of electrolyte and electrodes 825.1.2 Electrochemical characterisation 83


5.3 Discussion 91

5.3.1 Microstructure 915.3.2 Polarisation and impedance characteristics 955.4 Conclusions 96

6 Cermet electrodes, relation between microstructure and performance 996.1 Introduction 996.2 Experimental 100

6.2.1 Sample preparation 1006.2.2 Electrochemical characterisation 1016.2.3 Microstructural characterisation 102

6.3 Results and Discussion 1026.3.1 Microstructure 1026.3.2 Electrochemical performance 106

6.3.3 Relation between microstructure and electrochemical performance 1106.4 Conclusions 1127 Impedance of porous cermet electrodes 115

7.1 Introduction 1157.2 Theory 1167.3 Discussion of the model 1187.4 Experimental 1207.5 Results and discussion 121

7.5.1 Impedance analysis using 'Equivalent Circuit' 1217.5.2 Impedance analysis using the ladder network model 123

7.6 Conclusions 1258 Investigation into the kinetics of hydrogen oxidation on the Ni/YSZ cermet electrode 129

8.1 Introduction 1298.2 Experimental 130

8.2.1 Sample preparation 1308.2.2 Electrochemical characterisation 130

8.3 Results 1318.3.1 Impedance measurements 1318.3.2 I- measurements 141

8.4 Discussion 1418.4.1 Processes observed on the cermet electrode 1418.4.2 Comparison with porous nickel electrodes 1458.4.3 Comparison with literature 1468.4.4 Overall comparison 148

8.5 Conclusions 1499 Evaluation 151


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9.1 Introduction 1519.2 Relationship of the microstructure with electrode resistance 1519.3 Hydrogen oxidation reaction at the anode 155

Dankwoord 159Levensloop 161


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1

1Introduction

Abstract

A general introduction is given on fuel cells. The history, different types, advantages and

drawbacks of fuel cells are discussed. The second part of this introductory chapter is con-cerned with the Solid Oxide Fuel Cell (SOFC), its principles, materials employed and

electrode kinetics. Particular attention is drawn to the presently available knowledge on

kinetics of the anodic reaction. At the end of the chapter the scope of this thesis is pre-

sented.

1.1 The Fuel Cell1.1.1 Definition of a fuel cellFuel cells are electrochemical devices that directly convert chemical energy, from a reac-tion between a fuel and an oxidant, into electrical energy. The basic elements of a typical

fuel cell, as depicted in Figure 1.1, consist of an electrolyte phase in intimate contact with a

porous anode (negative electrode) and a porous cathode (positive electrode). The fuel and

oxidant gases flow along the surface of the anode and cathode, respectively, and react

electrochemically in the three-phase-boundary region established at the gas / electrolyte /

electrode interface. A fuel cell can theoretically produce electrical energy for as long as

fuel and oxidant are fed to the porous electrodes, but the degradation or malfunction of

some of its components often limits the practical life span of al fuel cell.

Different fuels can be used, such as hydrogen, ethanol, methanol, or gaseous fossil fuelslike natural gas. Solid or liquid fossil fuels need to be gasified first before they can be used

Figure 1.1: Schematic representation of a fuel cell.


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1 Introduction

2

as a fuel. Oxygen or air can be used as oxidant.

1.1.2 Historical backgroundThe fuel cell concept dates from the beginning of the 19th century and is ascribed to Sir

Humprey Davy 0. The possibility of making it a reality was demonstrated by Sir WilliamGrove, who operated a successful hydrogen-oxygen cell in 1839, generally stated as the

start of fuel cell history. Grove built a cell in which the reaction of hydrogen and oxygen

produced water, and generated an electric current. He stated: A shock was given which

could be felt by five persons joining hands, and which when taken by a single person was

painful 0.

The history of the solid oxide electrolytes can be considered to commence at the end of the

19th century, when Nernst produces his glower 0. Nernst discovered that the very high

electrical resistance of pure solid oxides could be greatly reduced by addition of certain

other oxides. The most promising of these mixtures consisted mainly of zirconia (ZrO2)

with small amounts of added yttria (Y2O3). This is still the most widely used electrolyte

material in the Solid Oxide Fuel Cells (SOFC).

The first working SOFC was demonstrated by Baur and Preis (1937), using stabilised zir-

conia as electrolyte and coke and magnetite as a fuel and oxidant, respectively 0. At a cur-

rent density of approximately 0.3mA/cm2 the cell voltage was 0.65V. Although the opera-

tion of the first SOFC was demonstrated, the current output of this cell was too low for

practical purposes.

A first period of intense activity in SOFC research began in the early 1960s, with intensive

research programs driven by new energy needs mainly for military, space and transport

applications 000. At that time basic research dealt with the improvement of electrolyteconductivity and the first steps in SOFC technology. A second period of high activity be-

gan in the mid-1980s and goes on today, focussing on electrode materials and technology.

Efforts thus far have resulted in almost commercial units which are part of our power

generation facilities. Leading companies in SOFC commercialisation are Siemens and Sul-

zer (Europe) 0, Westinghouse Electrical Cooperation (USA) 0 and Fuji Electric Corporate

Research and Development, Ltd and Tokyo Electric Power Co. (Japan) 0.

In the Netherlands the first 100 kWe SOFC field unit (Westinghouse SOFC technology)

was put into operation at the end of 1997 as a demonstration project where 6 Danish Pro-

duction Companies names ELSAM and a Consortium of 5 Dutch Energy DistributionCompanies, EnergieNed and the Dutch Subsidiser NOVEM co-operate 0.

1.1.3 Types of fuel cellsThe various types of fuel cells are usually classified by the applied electrolyte (Table 1.1).

The following types are known

the polymer electrolyte fuel cell (PEFC); alkaline fuel cell (AFC); the phosforic acid fuel cell (PAFC);

the molten carbonate fuel cell (MCFC); the solid oxide fuel cell (SOFC).


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3

MCF

Ni-10%

Li-dope

62 Li2CK2C

0.1

65

H2 +CO

H2O + CO

O2 + 2CO22CO

PAFC

Pt/C

Pt/C

100% H3PO4

0.1-1

200

H2 2H+ + 2e-

O2 + 4H+ + 4e-

2H2O

AFC

b

Ni

Li-doped NiO

85% KOH

~0.4

260

H2 +2OH-

2H2O + 2e-

O2 + 2H2O + 4e-

4OH-

AFC

a

80% Pt 20% Pd

90% Au 10% Pt

35-45% KOH

0.4

80-90

H2 +2OH-

2H2O + 2e-

O2 + 2H2O + 4e-

4OH-

PEM

Pt black

or Pt/C

Pt black

or Pt/C

Nafionc

0.1-0.5

80

H2 2H+ + 2e-

O2 + 4H+ + 4e-

2H2O

Anode

Cathode

Electrolyte (mol%)

Abs pressure (Mpa)

Temperature (C)

Anode reaction

Cathode reaction

Table 1.1: Typical components, operating conditions and electrochemical reactions in Fuel Cells. a Spa

Program, c Fluorinated sulfonic acid, registered trademark of E.I. du Pont de Nemours & Com


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1 Introduction

4

Large differences exist in application, design, size, cost and operating range for the differ-

ent type of fuel cells. The fuel cells above are listed in order of increasing operating tem-

perature, ranging from ~80C for PEFC to 1000C for SOFC. The low temperature fuelcells (PEFC, AFC, PAFC) utilise aqueous electrolytes in which H+ or OH- ions are the

dominant ionic current carriers.At higher temperatures, CO32- ions in the molten salt electrolyte of the MCFCs and O2-

ions in the solid electrolyte of the SOFC are the ionic current carriers. The operating tem-

perature has consequences for design, the efficiency of the fuel cell, the choice of other

materials needed in and around the fuel cell and the kind of fuel that may be used. For low

temperature fuel cells (PEFC, AFC and PAFC) the operating temperature is too low to en-

able direct oxidation of hydrocarbon fuels like natural gas, therefore fuels like hydrogen

and methanol are used. Low temperature fuel cells are generally seen as interesting for

small scale applications, for example mobile applications like cars (PEFC0), notebooks,

phones etc.

For high temperature fuel cells (MCFC and SOFC) it is possible to use natural gas which

can be reformed internally into hydrogen and carbon monoxide (depending on operating

temperature a catalyst will be necessary). The high temperature fuel cells, but also PAFC,

are interesting for the decentralised generation of heat and power. 00000

1.1.4 Advantages and drawbacks of fuel cellsThe advantages and drawbacks of fuel cell systems are determined by their type and appli-

cation. As it can be useful to compare a SOFC or MCFC system with traditional genera-

tors, a small fuel cell developed for applications as notebooks or mobile phones should be

compared with traditional batteries. The advantages and drawbacks given here are mostlybased on SOFC systems, but in general part of it will be valid for other types of fuel cells.

The main advantages:

(1)High energy conversion efficiency. Because of the direct conversion of free enthalpyinto electrical energy the usual losses from fuel to electrical energy, due to the conver-

sion of fuel to heat, heat to mechanical energy and mechanical energy to electrical en-

ergy, is avoided. The efficiency is further improved when the by-product heat is fully

utilised.

(2)Environmental compatibility. Fuel cells are capable of using practical fuels as an en-ergy source with minor environmental impacts (less CO2 and NOx produced per kWattpower).

(3)Modularity. Fuel cells have the characteristics of modularity, i.e. cells can be made inmodular sizes. The size of a fuel cell can be easily increased or decreased and its elec-

tric efficiency is relatively independent of size.

(4)Siting flexibility. Because fuel cells can be made in a variety of sizes they can be placedat different locations with minimum siting restrictions. Fuel cell operation is quiet be-

cause a fuel cell has no moving parts. Consequently fuel cells can be easily located

near points of use such as urban residential areas.

0


21/182

1.2 Solid Oxide Fuel Cell

5

Unfortunately, there are some drawbacks which have caused a slow introduction of solid

oxide fuel cells on the energy market 0:

(1)Material problems in relation with costs: For SOFC there are roughly two designtypes, tubular 0 and flat plate 0. For the tubular cell material problems are less, but fab-

rication costs are high. For the flat plate design fabrication costs are less, but morematerial problems arise.

(2)Economics. Introduction on the energy market would presently involve a high capitalcost-to-performance ratio.

1.2 Solid Oxide Fuel Cell1.2.1 Thermodynamic principlesThe principle of an SOFC is illustrated in Figure 1.2, two electrodes (the anode and cath-

ode) being separated by a solid electrolyte. Oxidant is reduced at the cathode and fuel isoxidised at the anode.

If hydrogen and oxygen are used as fuel and oxidant, respectively, in an SOFC with an

oxygen ion conducting electrolyte the reactions in the fuel cell involve the oxidation and

reduction of oxygen at the electrodes. At the cathode the reduction of oxygen is given as

O e Oc e224 2+ = (1.1)

where the subscripts c and e refer to states at the cathode and in the electrolyte, respec-

tively.

At the anode the reverse reaction of (1.1) takes place:

2 42

2O O ee a

= + (1.2)where the subscript a refers to states at the anode. Consequently, the overall cell reaction

(which determines the cell voltage) can be represented by the following equationO Oc a2 2= (1.3)

The SOFC is therefore considered to be an oxygen concentration cell, and the electromo-

tive force (emf) or reversible (thermodynamic) voltage,Er, is given by the Nernst equation:

ERT

F

pO

pOr

c

a

=4

2

2

ln (1.4)

Figure 1.2: A Solid Oxide Fuel Cell.


22/182

1 Introduction

6

whereR is the gas constant, Tthe temperature, Fthe Faraday constant and pO2 the partial

pressure of oxygen at the electrode.

For a certain oxygen partial pressure at the cathode, pO2 c, the magnitude ofEr depends on

the anode oxygen partial pressure, pO2 a, and thus on the type and composition of the fuel

fed to the anode. For example, when H2 is fed to the anode, the following cell reaction

takes place:

H O H Oa cK

ai

212 2 2+ (1.1)

Where Ki is the equilibrium constant of (1.1). The equilibrium oxygen partial pressure at

the anode is given by

pOpH O

pH Ka

a

a i

22

2

2

=

(1.2)

Substituting the equation for the anode oxygen partial pressure (1.2) into (1.4) yields

E ERT

FpO

RT

F

pH

pH Or c

a

a

= + +0 22

24 2ln ln (1.3)

whereE0 is the reversible voltage at the standard state and is given as

ERT

FKi

0

4= ln (1.4)

At the standard state,Er equalsE0, and the following equation is established for any fuel

EG

zF

H T S

zF

00 0 0

= =

(1.5)

where G0 is the standard Gibbs free energy change of the combustion reaction of the fuel,H0 the standard enthalpy change, S0 the standard entropy change and z the number ofelectrons involved in the reaction to convert a single fuel molecule. The maximum energy

obtained in this case is given by -G0 and the ideal thermodynamic efficiency, T, repre-sented by G0/H0. Table 1.2 gives G0, H0,E0 and T for the combustion of H2 as fuel.0

1.2.2 Fuel Cell efficiencyThe overall efficiency of an SOFC, FC, is the product of the electrochemical efficiency, E,and the heating efficiency, H. The electrochemical efficiency is, in turn, the product of thethermodynamic efficiency, T, the voltage efficiency, V, and the Faradaic or current effi-ciency, J, of the fuel cell.Thus, FC E H T V J H = = (1.6)

T (K) G0 (kJ) H0 (kJ) E0 (V) T

1000 -192.5 -247.3 0.997 0.78

1250 -178.2 -249.8 0.924 0.71

Table 1.2: Thermodynamic data and efficiency (T) for the hydrogen oxidation reaction 0.


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7

1.1.1.1Heating efficiencyThe heating efficiency must be considered in cases where the fuel contains inert gases, im-

purities and other combustibles in addition to the electrochemically active species. The

heating value efficiency, H, is defined as:

Hcom

H

H=

0

(1.7)

where H0 represents the amount of enthalpy of fuel species available in the fuel cell togenerate electricity and Hcom represents the amount of enthalpy included in all combusti-ble species in the fuel gases fed to the fuel cell.

1.1.1.2 Thermodynamic efficiencyIn an SOFC the free enthalpy change of the cell reaction, G, may be totally converted toelectrical energy. Thus a fuel cell has an intrinsic (maximum) thermodynamic efficiency

given byT

G

H

T S

H= =

1 (1.8)

1.1.1.3 Voltage efficiencyIn an operating SOFC the cell voltage is always less than the reversible voltage. As the

current is drawn from the fuel cell, the cell voltage is reduced due to various losses. The

reduction in the cell voltage under current load depends on current density and several

factors such as temperature, pressure, gas flow rate, gas combustion and cell material. The

voltage efficiency, V, is defined as the ratio of the operating cell voltage under load, E, to

the equilibrium cell voltage,Er, and is given as

Vr

E

E= (1.9)

The difference between the operating cell voltage and the expected reversible voltage is

termed polarisation or overpotential and is presented as . The total polarisation of a cell isthe sum of four types of polarisation: charge transfer or activation polarisation, A, diffu-sion or concentration polarisation, D, reaction polarisation, R, and resistance or ohmicpolarisation, : = + + +A D R (1.10)

Polarisation cannot be eliminated but can be minimised by material choice and cell design.Temperature, pressure, electrolyte composition and electrode material naturally influence

cell polarisation.

1.1.1.3.1 Charge transfer or activation polarisation(Electro) chemical reactions involve an energy barrier that must be overcome by reacting

species. This energy barrier, called the activation energy, results in activation or charge

transfer polarisation, A. Activation polarisation is related to current density, i, by the fol-lowing equation:

i iF

RT

iF

RT

a A c A=

0 0exp exp

(1.11)


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1 Introduction

8

Where is the symmetry coefficient and i0 is the exchange current density. The symmetrycoefficient is considered as the fraction of the change in polarisation which leads to a

change in the reaction rate constant. The exchange current density is related to the bal-

anced forward and reverse electrode reaction rates at equilibrium. A high exchange current

density means a high electrochemical reaction rate and, in that case, a good fuel cell per-formance is expected. The exchange current density can be determined experimentally by

extrapolating plots of log i versus to =0. For large values of (either negative or posi-tive) one of the bracketed terms in (1.11) becomes negligible. After rearranging one ob-

tainsA a b i= log (1.12)

which is usually referred to as the Tafel equation. Parameters a and b are constants which

are related to the applied electrode material, type of electrode reaction and temperature.

1.1.1.3.2 Diffusion or concentration polarisationDiffusion or concentration polarisation, D, becomes eminent when the electrode reactionis hindered by mass transport effects, i.e., when the supply of reactant and/or the removal

of reaction products by diffusion to or from the electrode is slower than that corresponding

to the charging/discharging current i. When the electrode process is governed completely

by diffusion, the limiting current, iL, is reached. The limiting current can be calculated

from the diffusion coefficient of the reacting species, D, their concentration, cM, and the

thickness of the diffusion layer, , by applying Fick's law as

izFD c

LM=

(1.13)

For an electrode process free of activation polarisation, the diffusion or concentration po-

larisation can be expressed as

DL

RT

zF

i

i=

ln 1 (1.14)

In general, mass transport is a function of temperature, pressure and concentration of the

species involved. In SOFCs the reactants must diffuse through the porous anode and cath-

ode, emphasising the importance of the microstructure and design of electrodes.

1.1.1.3.3 Reaction polarisationThe reaction polarisation, R, appears when the rate of the electrode process is influencedby a chemical reaction. A possible reaction includes the incorporation of oxygen in the

oxide sublattice at the cathode.1.1.1.4Resistance or ohmic polarisationThe ohmic polarisation is caused by the resistance of the conducting ions (through the

electrolyte), electrons (through the electrodes and current collectors) and contact resis-

tances between cell components. The ohmic polarisation, , is given as = iRi (1.15)

whereRi represents the total ohmic cell resistance, including both ionic and electronic re-

sistances.


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9

1.1.1.5 Current efficiencyThe efficiency of a SOFC drops if all of the reactants are not converted to reaction prod-

ucts. For 100% conversion of a fuel, the amount of current density, iF, produced is given as

(Faraday's law)

i zFdf

dtF = (1.16)

where df/dt is the molar flow rate of the fuel. For the amount of fuel actually consumed,

the current density produced is given by

i zFdf

dt consumed=

(1.17)

The current efficiency, J, is the ratio of the actual current produced to the current available

from complete electrochemical conversion of the fuel

JF

i

i= (1.18)

In the case of fuel cells, the current efficiency is commonly expressed as fuel utilisation. 0

1.2.3 SOFC materialsThe basic components of a ceramic fuel cell stack are the electrolyte, the anode, the cath-

ode and the interconnect. Each component serves several functions in the fuel cell and has

to meet certain requirements. These requirements include: proper stability (chemical,

phase, morphological and dimensional) in oxidising and/or reducing environments, chemi-cal compatibility with other components and a proper conductivity. The electrolyte and

interconnect must be dense in order to separate the oxidant and fuel gases, whereas both

anode and cathode must be porous to allow gas transport to the reaction sites. An overview

of the state of the art materials is given in Table 1.3. Extended reviews are provided in the

literature 000. A short discussion about the electrolyte and cathode follows, more attention

is given to the anode.

1.1.1.6 The electrolyteYttria-stabilized zirconia ((Y2O3)0.08-(ZrO2)0.92 abbreviated as YSZ) is the most common

electrolyte in SOFCs because the material possesses an adequate level of oxygen-ion con-

component composition specific conductivity at1000C (S/m)

conductivity depending on

Anode Ni/YSZ cermet 400 - 1000 Ni/YSZ particle size ratio

Ni content

Cathode SrxLa1-xMnO3- 6 - 60 Porosity

Sr content

Electrolyte Y2O3-ZrO2 10 - 15 Density

Yttria content

Interconnect LaCrO3

Table 1.3: State of the art materials used in SOFC and relevant properties 00.


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1 Introduction

10

ductivity and exhibits desirable stability in oxidising and reducing atmospheres. ZrO2 in its

pure form does not serve as a good electrolyte because its ionic conductivity is too low.

Therefore it is doped with Y2O3, which means a direct substitution of a trivalent cation of

appropriate size, e.g. Y3+, for the host lattice cation Zr4+. The substitution not only creates a

large concentration of oxygen vacancies but also stabilises the cubic fluorite structure 00.1.1.1.7 The cathodeThe perovskite materials SrxLa1-xMnO3- (SLM) with x between 0.15 and 0.50 is consid-

ered as the standard SOFC cathode material. SLM is mainly an electronic conductor and

the oxygen reduction reaction is assumed to take place at the interface between cathode

material, electrolyte and the oxygen gas phase 0. Only at high overpotentials does SLM

show appreciable ionic conductivity 0.

1.1.1.8 The anodeThe nickel/yttria-stabilised zirconia (Ni/YSZ) cermet (ceramic-metal mixture) is at thismoment the state of the art material for the anode. Nickel is used because it is one of the

metals that is able to withstand the operating conditions of a SOFC: reducing conditions

and a temperature of 1000C. Other possible materials are cobalt and noble metals, but

taking into account volatility, chemical stability, catalytic activity and cost, nickel appears

to be the best candidate. Nickel plays the role of the electronic conducting phase and needs

to transport the electrons from the reaction site to the current collector. YSZ is added to

support the nickel-metal particles, to inhibit coarsening of the metallic particles and to pro-

vide a thermal expansion coefficient acceptably close to those of the other cell compo-

nents. The functional properties of the YSZ in the anode are a matter of discussion, it issuggested that it may play an active role by forming conductive paths for oxygen transport,

thereby enlarging the active area available for the electrode reaction. The third important

phase in the cermet structures is the porous phase. This phase is important for easy gas

phase diffusion to (hydrogen) and from (water) the active sites. 00000.

The most important requirements for the anode are:

Long term stability. Sinter activity during operation has to be virtually absent, meaningmaintaining good electronic and ionic conductive phases as well as porosity.

Low polarisation resistance of the electrode. Physical and chemical stability under a reducing atmosphere at high temperature.

Matching of the thermal expansion coefficient to the electrolyte to prevent the electrodefrom flaking of the electrolyte.

High catalytic activity to promote reaction of the fuel with oxide ions.00000

The above requirements cannot be satisfied only by choosing the appropriate composition.

The nature of the starting powder and the applied manufacturing technique has a strong

effect on the electrochemical properties of the electrode in the final assembly. For instance,

parameters like the particle morphology of the powder and the porosity of the sintered

electrode will influence the electrical conductivity of the electrode structure. These two

parameters also influence the total amount of electrode particles which occupy the elec-trolyte interface. Other parameters such as the sintering temperature and the sintering time


27/182


11

of the electrode will have an effect on the adherence of the electrode structure to the elec-

trolyte and possible formation of deleterious reaction products between the electrode and

the electrolyte. Hence, the manufacturing procedure is very important 0.

Other materials that have been studied as anode are:

Nickel without the addition of YSZ, especially for research purposes in order to performexperiments with simple geometrical structures 000. Ruthenium/stabilised ZrO2 cermet. This material shows a lower polarisation then that of

Ni/YSZ cermet. Ruthenium has the advantage of a better resistance to sintering and also

a higher reforming activity compared with nickel 00. Disadvantages are cost and evapo-

ration of ruthenium oxide.

(Ni-Mg)O-YSZ. The use of this material improves the overvoltage characteristics of theanode compared to Ni-YSZ cermet anodes which is attributed to a finer porous structure

that induces a greater specific surface area 0.

To decrease the anodic overpotential, it was suggested to insert a mixed conductor be-tween YSZ and the metallic conductors. A significant decrease of polarisation was

found when ceria-based solid solutions like (CeO2)0.6(LaO1.5)0.4 were used 0. This effect

was attributed to mixed conduction resulting from the partial reduction of Ce4+ to Ce3+

in the reducing operating conditons. A decrease in anodic polarization was also found

for other oxide components such as praseodymium oxide (PrOx), ceria (CeOx) and sa-

maria-doped ceria (SDC) 0. Other studies indicated a significant decrease of the ohmic

drop when YSZ was used with a thin modified layer of mixed conductors 0. A com-

parative study of ceria and titania doped YSZ has shown that additions in the order of

10 mol% titania increased the electronic conductivity of the solid solution 00. This sug-

gests that such solids solutions may be good candidates as anode cermet components forSOFC.

1.2.4 SOFC anode kineticsEven though studies on SOFC electrodes started back in 1960 and a lot of work has been

published ever since, the exact nature of the reaction kinetics is still not established. In this

section, an impression is given of what is known about the reaction kinetics on nickel

metal and Ni/YSZ cermet electrodes.

1.1.1.9Nickel electrodesStudies on nickel metal electrodes have been initiated because of their less complicated

microstructure compared with that of Ni/YSZ cermet electrodes. Using these electrodes it

should be possible to avoid structural limitations and measure purely chemical parameters

and relate them with the microstructure. Attempts to relate the polarisation resistance to the

length of the Triple Phase Boundary (TPB) have been done for simple geometries using

nickel wires and porous nickel layers 00, nickel stripes 000000 and a nickel ball 00. A sur-

vey of the results from measurements on various nickel electrodes is given in Table 1.4.


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12

Gas phase dependence ofelectrode conductivity (1/R)

1.697 pH2O1/2

0.016 pH2-1/2+0.396 pH2

1/2

pH2O1/2 (at low pH2)

pH2O1/2 (at low pH2)

pH2O1 (at high pH2)

pH2O0 (at low pH2 &


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13

Guindet et al. 0 studied H2 oxidation on a nickel ball pressed onto an YSZ disc at 960C.They paid special interest to polarisation curves, having selected points on the curve char-

acterised by impedance spectroscopy. A maximum current density was found for an anodic

polarisation close to 850 mV versus air. Since this value is close to that of the Ni-NiO

system, the formation of nickel oxide is suggested. For lower anodic potentials between 1000 and 850 mV versus air, hydrogen oxidation takes place on the nickel and the log i

versus curve follows Tafel behaviour (see Table 1.4). At anodic potentials between -850and 650 mV versus air NiO is formed. Hydrogen oxidation still takes place on nickel, but

now also on passivating NiO. The electrode resistance increases and a large capacitive ef-

fect is seen in the impedance plot.

For anodic potentials smaller than 650 mV versus air, hydrogen oxidation takes place

only on NiO and an inductive loop appears at the low frequency side of the impedance dia-

gram. In later work Guindet and co-workers 0 also studied the influence of H2 and H2O

partial pressures. The interfacial conductivity showed a minimum in pH2. For low pH2 val-

ues the conductivity was found to be proportional to pH2-1/2 and for high pH2 to pH21/2. At

fixed pH2, a pH2O1/2 dependence was found. Anodic polarisation curves, measured as a

function of pH2O, were analysed with Butler-Volmer type of equations, resulting in an ap-

parent anodic transfer coefficients, a, varying between 1.4 and 1.7. Based on analysis ofthe polarisation curves, the authors concluded that charge transfer is not the rate determin-

ing step in the anodic reaction.

Norby et al. 0 paid special attention to the relation between the TPB and the electrode per-

formance. As electrodes these authors used a nickel wire wrapped around an alumina rod

and an electroplated nickel layer (2m thick) with circular holes made by microlithogra-phy. For the nickel wire electrodes an impedance diagram with one semicircle was re-

corded. The resistance associated with this semicircle was attributed to charge transfer and

the Constant Phase Element (CPE) was attributed to the double layer capacitance. For the

porous nickel layer two semicircles were found in the impedance diagram. The semicircle

appearing at the low frequency side dominated the spectrum and was ascribed to a reaction

resistance and associated surface coverage of adsorbed intermediates. This resistance de-

creased with increasing pH2O, while increasing with pH2. The charge transfer resistance

varied approximately with pH2O-1/2 at low pH2 values. At moderate pH2 values, the charge

transfer resistance exhibited a inverse linear relationship with the TPB length.The general conclusion drawn by Norby et al. is that for electrodes with a small TPB

length and a large nickel and YSZ area available per unit TPB length, the rate is limited by

charge transfer, due to the restricted TPB length. On the other hand, the reaction resistance

dominates for the electrode with a large TPB length and a smaller accessible area of nickel

and YSZ per unit TPB length. The authors suggested that for the latter electrode and

probably for most of the Ni-YSZ cermet electrodes, the rates are limited by a too small or

inactive surface rather than by a small TPB length.

Mizusaki et al. 0000 and Yamamura et al. 00 studied nickel pattern electrodes. With pho-

tolithography, 16 pattern electrodes were prepared, different in the width and distance be-


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1 Introduction

14

tween the stripes (5-10-25-50m for nickel and/or YSZ). In 00, they present a linear rela-tion between the electrode conductivity and the TPB length. Impedance spectra consisted

of one semicircle. The associated resistance was found to be essentially independent of pH2and, at high vapour pressure, proportional to pH2O

-1, but becoming asymptotically constant

with decreasing pH2O.Based on these results and the polarisation curves measured as a function of pH2 and

pH2O, the following model was proposed 0. The major adsorbed species on the nickel sur-

face are assumed to be Oads, OHads, Hads and H2Oads. The coverages of these species are

taken to be low and, therefore, the portion of vacant adsorption sites, v close to 1. Be-tween YSZ and nickel, Oad adatoms exchange reversibly. The following equilibria are ex-

pected to hold near the TPBH O s H OHad e ad ad 2 + + (1.20)OH s H Oad e ad ad + + (1.21)O O sad ad YSZ e +, (1.22)

O e Oad YSZ Ox

, + 2 (1.23)

H O g s H Oe ad2 2 + (1.24)where se denotes a vacant site on the nickel surface. The reaction,2 22H H g sad e + (1.25)is assumed to be slow and does not reach equilibrium near the TPB.

Under virtual equilibrium condition for reactions (1.20) - (1.24), three possible rate deter-

mining reaction steps are considered:

(i) direct attack of H2(g) on Oad:

H g O H O i k pH FERT

k pH Oad ad 2 2 1 1 2 1 22 + = : exp' (1.26)

which process predominates under anodic polarisation.

(ii) surface diffusion of Had on the nickel surface towards reaction sites near the TPB

H H i k pH k pH OFE

RTad ad

TPB =

2 2 21 2

2 21 2/ ' / exp (1.27)

where the constants k2 and k2 comprise, among other factors, the ratio of a diffusion coef-

ficient over an effective distance in accordance with Ficks first law of diffusion. This pro-

cess would dominate under small cathodic polarisation.

(iii) dissociative adsorption of H2(g) near the TPB

H g V H i k pH k pH OFE

RTad ad 2 3 3 2 3 22 2

2 + =

: exp

'(1.28)

which process predominates under large cathodic polarisation.

All three rate-determining processes proceed in parallel. The magnitude of the rate con-

stants and the oxygen activity at the nickel/YSZ interface determine which process domi-

nates.

A kind of intermediate electrode between nickel electrodes and cermet electrodes was

studied by Nakagawa et al. 0. The investigated electrodes consisted of a sputtered nickel

layer having a thickness between 0.8 and 12.9 m, coated with a plasma-sprayed porous


31/182


15

YSZ layer (0-207m). The effect of the porous ceramic top layer appeared in the imped-ance diagram at equilibrium, but not under polarisation in dc experiments. For a 2 m thicknickel layer without a porous YSZ top layer, one depressed arc was found in the imped-

ance diagram. Upon coating with YSZ a second arc appeared. The associated resistance

was found to be proportional to the thickness of the applied porous YSZ layer and attrib-uted to the diffusion of H2O inside the porous YSZ layer. Because no effect was observed

on the electrode performance in the applied range of the nickel layer thickness, the effec-

tive reaction zone for the rate determining process on nickel was estimated to be less than

1m from the electrode/electrolyte interface. The results led the authors to the overall con-clusion that under polarisation an activation process and not a diffusion process is impor-

tant for the anodic reaction.

Table 1.4 summarises results on nickel electrodes obtained by different authors. Overall it

can be stated that comparison between the results is difficult mainly because different type

of electrodes are used by different authors, measurements being performed at different

temperatures and under different ambient conditions.

One of the reasons to use nickel electrodes in many investigations is to search for the ex-

pected inverse relationship between the specific resistance and the TPB length. Results are

compared in Table 1.5, where the specific resistances observed in different studies are

normalised for the TPB length of the electrode used in the investigation. Huge differences

can be seen in the normalised values. No corrections were made for differences in the gas

phase conditions employed by the various authors. It should however be mentioned that

such a correction cannot account for the observed scatter, so other factors need to be con-sidered.

1.1.1.10Cermet electrodesThe most widely studied anode for oxidation of hydrogen in SOFCs is the Ni/YSZ cermet

anode. Use of the cermet structure is a basic requirement to increase adhesion between an-

ode and electrolyte and stability of the electrode, compared to nickel. Another reason to

use a cermet structure is the possibility of increasing the number of reaction sites per

nominal electrode area. The complex microstructure of Ni/YSZ cermet anodes, the prob-

lem of establishing a quantitative description of such structures and the complex imped-ance spectra obtained have resulted in a significant number of studies on this system. A

survey of measurement results is given in Table 1.6 (impedance) and Table 1.7 (polarisa-

tion). Below some possible reaction mechanisms are given as described by various authors.

In 1993, Mogensen and Lindegaard 0 proposed a mechanism for the oxidation of hydrogen

on a Ni/YSZ cermet electrode. This model is probably most often referred to in literature.

It is based on impedance data where the polarisation resistance consisted of two separate

contributions, only one of these being strongly dependent on both the H2 partial pressure

and the ratio pH2/pH2O. The high frequency semicircle is assumed to arise partly from the

transfer of ions across the TPB line and partly from the resistance inside the electrode par-ticles. The associated CPE is probably connected with the accumulation of charge


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16

Eact

(kJmo

154.4

68.1

R

()

1970

65k

1.6*

10.5k

Impedance

(nr arcs)

1

1

2

1

Gas phaseconditions

(105 Pa)

pH2O

0.053

0.0063

0.0063

0.017

pH2

0.75

0.0012

0.0012

0.1

Temp

(C)

975

900

900

700

TPB

Length

(mcm2)

9.4mm

3.3mm

121 m

3.26

Type ofelectrode

Ni ball

Ni wire

Porous Nilayer

Ni stripe

Table 1.5: Comparison of resistance as function of TPB length for different nickel electrodesRr unknown.

Authors

Mohamedi-Bou-lenouar et al. 0

Norby et al. 0

Norby et al.0

Mizusaki et al. 0


33/182

17

Range (105Pa)

0.97-0.015

(pO2:4.5*10-18)

0.05-0.95

(pH2O: 0.01 or 0.03)

(* pH2 >0.5)

0.1-1

(pH2O: 0.02)

0.1-97

(pH2O: 0.03)

PH2 order dependence

order

0

1-2

0-0*

0.2-1*

0.1-1*

0

0.1

0.1

-

pH2O

0.03

0.03

0.02

0.03

0.03

0.03

0.03

Gas phase condition(105Pa)

pH2

0.97

0.97

0.5

0.3

0.97

0.97

0.97

R (cm2)

0.2()

0.2()

0.1

0.07

0.31

0.24

0.04

0.051.81

(total)

3.4a1

13.5a1

0.044a2

0.018a2

0.3b1

2.6b2

10.5(tot)b3

Impedance

arc

1

2

1

2

3

1

2

31

2

3

1

2

1

2

1

1

3

Temp

(C)

1000

1000

1000

1000

1000

1000

1000

Vol% nickel in

Cernet

NiO 55w/o

YSZ 45w/o

40

40

40

45

45

40

Table 1.6: Result from impedance analyses for cermet electrodes obtained by various researchers a1&2E0.21m; NiO: 12.5m; (a2) YSZ:3.1m; NiO: 3.1m; b1&2&3Effect of the sinter temperature number of arcs in the impedance spectra) (b1) sinter T=1500C; (b2) sinter T=1400C; (b3) s

Authors

Mogensen et al.0

Primdahl et al. 0

Aaberg et al. 0

Tjelle et al. 0

Lee et al. 0

Kawada et al. 0


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18

Validity of

Electrode ch

(only valid

725-825C:890C: =0

950C: =0

(a) =2

value of d

Current expression

i = i0(exp (2F/RT) - exp (-F/RT))

i = i0(exp (F/RT) exp (-F/RT))

ia = i0 exp (F/RT)

pH2O

0.03

0.05

0.02

Gas phase conditions

(105 Pa)

pH2

0.97

0.48

0.3

Temp

(C)

1000

725-950

1000

Vol% nickelin cermet

40

----

(a) 33

(b) 50

Table 1.7: Result from polarisation measurements for cermet electrodes obtained by various research

Authors

Kawada et al. 0

Divisek et al. 0

Aaberg et al. 00


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19

at the interface between the Ni and YSZ particles. It does not behave as an ideal capaci-

tance (CPE with n=0.67), which is attributed to a variation in grain size and orientation,

causing a distribution in relaxation times. The semicircle at low frequencies is attributed to

a reaction resistance associated with the formation of water (equation (1.32)), which is as-

sumed to occur at the YSZ surface. The CPE associated with the low frequency semicirclewould be related to the degree in coverage of protons and H2O on the Ni surface. Based on

the above interpretations the following mechanism was proposed:H Had Ni2 2 , (1.29)

2 ++ H H ead Ni ad Ni, , (1.30)

Diffusion of H to the Ni YSZ boundaryad Ni,+

2 2 + + H O OHad Ni YSZ YSZ , (1.31)

2 22

OH H O OYSZ YSZ + (1.32)

Recently this mechanism was reformulated 0, allowing incorporation of protons in the bulkof the Ni and YSZ as well as on the surface:

H g H eNi2 2 2 + (1.33)

H HNi i YSZ , (1.34)

2 2H O H O Vi Ox

ad YSZ O

+ +, (1.35)

H O H O gad YSZ2 2, (1.36)

Reactions (1.34) and (1.35) are assumed to be rate-limiting. This mechanism accounts for

two arcs in the impedance plot. Recent studies on Ni/YSZ cermet electrodes by Mogensen

and Primdahl 0 indicate that most structures are well described by three, more or less

overlapping arcs, implying that at least three (physical or chemical) processes contribute tothe limitation of the reaction rate. The high frequency arc is sensitive to the cermet struc-

ture (particle size) and relatively insensitive to atmospheric composition and overvoltage.

The related CPE is interpreted as a double layer capacitance at the Ni/YSZ interface. The

medium- and low frequency arc are sensitive to atmospheric composition and overvoltage.

The capacitance associated with the low frequency arc behaves ideal, is in the order 0.5-2.5

F/cm2 and is very dependent on pH2O. This magnitude of the capacitance suggests changes

in the bulk composition of the gas phase above the electrode, rather than surface adsorption

of charged species.

A more or less similar reaction scheme has been suggested by Aaberg et al. 00.H g Hads Ni2 2 , (1.37)

H H eads Ni ads YSZ , , + (1.38)

2 2H O H O Vads YSZ O O, + + (1.39)

Aaberg et al. assumed, in agreement with earlier findings by Skaarup et al. 0, that adsorp-

tion and mobility of hydrogen on nickel is fast. Consequently, it is reasonable to assume

that hydrogen adatoms are abundantly present at the Ni/YSZ boundary even when the

electrode is moderately polarised to anodic potentials. The electrochemical transfer of

protons from the metal to the electrolyte is assumed to be the potential dependent step,which is assumed to be close to equilibrium. The rate determining step will be the combi-


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1 Introduction

20

nation of protons and oxide ions on the electrolyte surface. This is probably a multi step

process. With the above assumptions and the assumption that (1.38) is not far from equilib-

rium, Aaberg et al. arrived at the following equation for the anodic current:

i Fk pH FE RT a = 2 23 2' exp ( / ) (1.40)

k3 is the rate constant. This is identical to the relation Mizusaki et al. 0 found for the ano-dic current under anodic polarisation. Experimentally the transfer coefficient was found to

vary between 1 and 2, depending on the microstructure.

An alternative route for (1.38) could be

H H eads abs Ni i/ , + (1.41)

and illustrates protons being absorbed in the electrolyte. If (1.39) is slow the interstitial

protons will accumulate in the electrolyte. Reaction (1.41) than acts merely as a large ca-

pacitance, but does not contribute much to the overall rate unless sub-surface diffusion of

protons extends towards the reaction zone. Its involvement may account for the large ca-

pacitance experimentally found at low frequencies, provided that the diffusion coefficientfor protons in zirconia is high. The solubility of hydrogen in zirconia, as reported earlier by

Wagner 0, is proposed to contribute to an extension of the reaction zone. Whether the pro-

tons are transferred across the TPB in an adsorbed state only, or additionally, across the

metal zirconia interface as hydrogen dissolved in the metal, is not clear.

Impedance analysis is a widely used tool for kinetic studies on electrodes. Very frequently

and as described above, the impedance data is fitted to a number of RQ-arcs, to which no

physical meaning can be ascribed in a simple manner. Especially for cermet electrodes,

containing both electronic and ionic conducting phases, the use of a chain ladder network

is of interest. With this type of network (also known as transmission line model) bulk ac-

tivity of the cermet is visualised. 000000

1.3 Scope of this thesisA number of studies on different types of nickel anode for SOFC are described in this the-

sis. Emphasis is put on the relation between the microstructure and the electrode perform-

ance. A broad range of microstructures is covered by starting with pattern electrodes and

porous nickel structures, finally arriving at the complex microstructures of Ni/YSZ cermet

structures. Attention is paid to the kinetics of the hydrogen oxidation reaction.The experimental set-up is described in Chapter 2, where the design of the electrochemical

cell is also discussed.

Nickel electrodes are the subject of Chapters 3 and 4. These types of electrode are not di-

rectly interesting as an anode for real SOFC applications, but are very interesting for re-

search purposes because of their uncomplicated geometry. This makes them indispensable

for studies on the relation between microstructure and electrochemical performance.

Lithographically prepared nickel pattern electrodes are the subject of Chapter 3. This type

of electrode results in relatively small nickel perimeters. Results from impedance and po-

larisation types of measurements are discussed. In Chapter 4 results on porous nickel elec-

trodes are described. These electrodes have a nickel perimeter about 5 times longer thanthe lithographically prepared electrodes. Besides impedance and polarisation measure-


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1.3 Scope of this thesis

21

ments under different conditions a model is proposed for the hydrogen oxidation reaction

at the nickel anode.

A kind of intermediate electrode between the nickel and cermet electrode is discussed in

Chapter 5. By modifying a porous nickel electrode with fine YSZ particles the effect of the

YSZ in a cermet is studied.The real state-of-the-art SOFC anodes are treated in the Chapters 6-8. The influence of the

microstructure on the electrode performance is described in Chapter 6. The microstructure

is affected by changing the particle size of the YSZ fraction in the preparation of the

Ni/YSZ cermet electrode. Coarse and fine YSZ powder were mixed in different ratios re-

sulting in seven different microstructures. Impedance measurements were performed to

study the electrode performance. In Chapter 7 impedance data are analysed with a network

model. Chapter 8 describes impedance and polarisation measurements obtained under dif-

ferent conditions aiming at kinetic parameters of the hydrogen oxidation reaction at the

cermet electrode.

A brief evaluation of the results obtained in this thesis is given in Chapter 9.

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409-445, CRC Press, Inc., 1997.N.Q. Minh, 'Ceramic Fuel Cells', J. Am. Ceram. Soc., 76 [3] 563-88, 1993.T. Shirakawa, S. Matsuda, and A. Fukushima, 'Characterisation of NiO/YSZ electrodes by Temperature Programmed

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F.H. van Heuveln, F.P.F. van Berkel, and J.P.P. Huijsmans, 'Electrochemical characterisation of porous electrodes andapplications in SOFC'; pp. 53-68 in: Ris High Temp. Electrochemical Behaviour of Fast Ion and Mixed Conduc-tors, 14th Ris Inter. Symp. On Material Science, 1993.

F.P.F. van Berkel, F.H. van Heuveln and J.P.P. Huijsmans, Status of SOFC Components Development at ECN, pp. 533-41 in: Proc. Of the 3rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.

K.C. Chou, S. Yuan, and U. Pal, 'Deposition, electrical property and direct porosity measurement of Ni-ZrO 2 cermetelectrodes'; pp 744-751 in: Proc. Of the 3rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.

M. Mogensen and T. Lindegaard, 'The kinetics of hydrogen oxidation on a Ni/YSZ SOFC electrode at 1000C'; pp. 484-93 in: Proc. Of the 3rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.

J. Divisek, A. Kornyshev, W. Lehnert, U. Stimming, I.C. Vinke and K. Wioppermann, Advanced characterisation tech-niques for Nickel-YSZ Cermet Electrodes used in Solid Oxide Fuel Cells, Electrochemical Proceedings Volume97-18, 606-16, 1997.

A. Ioselevich, A.A. Kornyshev and W. Lehnert, Degradation of SOFC Anodes due to Sintering of Metal Particles: Cor-related Percolation Model, J. Electroch. Soc., 144, 3010-19, 1997.

T. Kawada, N. Sakai, J. Yokokawa, M. Dokiya, M. Mori and T. Iwata, Structure and Polarisation Characteristics ofSolid Oxide Fuel Cell Anodes, SSI, 40/41, 402-06, 1990.

T. Norby, O.J. Velle, H. Leth-Olsen and R. Tunold, 'Reaction resistance in relation to three phase boundary length ofNi/YSZ electrodes', pp.473-78 in: Proc. Of the 3 rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993

J. Mizusaki, H. Tagawa, T. Saito, K. Kamitani, T. Yamamura, K. Hirano, S. Ehara, T. Takagi, T. Hikita, M. Ippommatsuand S. Nakagawa, 'Preparation of Nickel pattern electrodes on YSZ and their electrochemical properties in H 2-H2Oatmospheres'; pp. 533-41 in: Proc. Of the 3rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.

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Int. Symp. on Solid Oxide Fuel Cells, Athens, Greece, July 2-5, 1991.M. Suzuki, H. Sasaki, S. Otoshi, and M. Ippommatsu, Development of Ru/Y 2O3 SOFC anode', pp.585-91 in: Proc. of the

2nd Int. Symp. on Solid Oxide Fuel Cells, Athens, Greece, July 2-5, 1991.M. Suzuki, H. Sasaki, S. Otoshi, A. Kajimura and M. Ippommatsu, 'High power density solid oxide electrolyte fuel cells

using Ru/Y2O3 stabilised zirconia cermet anodes, Solid State Ionics, 62, 125-30, 1993.K. Okumura, Y.Yamamoto, T. Fukui, S. Hanyu, Y. Kubo, Y. Esaki, M. Hattori, A. Kusunoki and S. Takeuchi, 'Micro-

structure and overvoltage characteristics of the anode for Solid Oxide Fuel Cells', pp. 444-53 in: Proc. Of the 3 rd Int.Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.

C.S. Tedmon, H.S. Spacil and S.P. Mittof, General Electric, Rep. 69-C-056, 1969.K. Eguchi, T. Setoguchi, K. Okamoto and H. Arai, 'An investigation on anode materials and anodic reaction for Solid

Oxide Fuel Cell', pp. 494-503 in: Proc. Of the 3rd Int. Symp. On Solid Oxide Fuel Cells, Honolulu, Hawaii, 1993.H. Miyamoto, M. Sumi, K. Mori, I. Koshiro, F. Nanjo and M. Funatsu, Improvement of Anode Performance by YSZ

Surface Modification, in Proc. Of the Third Int. Symp of SOFC, pp 504-12, The Electrochemical Society, Pen-nington, NJ, 1993.

R.M.C. Marques, J.R. Frade, F.M.B. Marques, Ceramic materials for SOFC anode cermetsin Proc. Of the Third Int.Symp of SOFC, pp 513-22, The Electrochemical Society, Pennington, NJ, 1993.

M.T. Colomer, J.R. Jurado, R.M.C. Marques, F.M.B. Marques, Evaluation of Titania Doped YSZ for SOFC Anodes, inProc. Of the Third Int. Symp of SOFC, pp 523-32, The Electrochemical Society, Pennington, NJ, 1993.

M. Mogensen, S. Sunde and S. Primdahl, SOFC Anode Kinetics, pp 77-100 in: Proc. of the 17th Ris Int. Symp. onMaterial Science: High Temp. Electrochemistry: Ceramics and Metals, Roskilde, Denmark, 1996.

T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, M. Mori and T. Iwata, Characteristics of slurry-coated nickel zirconiacermet anodes for solid oxide fuel cells, J. Electrochem. Soc. 137, 10 , 3042-47, 1990.

J. Divisek, L.G.J. de Haart, P. Holtappels, U. Stimming and I.C. Vinke, Elelctrode kinetics of hydrogen oxidation onNickel/YSZ cermet anodes, pp 235-40 in: Proc. of the 17th Ris Int. Symp. on Material Science: High Temp.Electrochemistry: Ceramics and Metals, Roskilde, Denmark, 1996.

J. Mizusaki, T. Yamamura, H. Yoshitake, H. Tagawa, K. Hirano, S. Ehara, T. Takagi, M. Hishinuma, H. Sasaki, T. Sogi,Y. Nakamura, and K. Hishimoto, Kinetic studies on Ni/YSZ anode reaction of SOFC in H 2-H2O atmospheres bythe use of Nickel pattern electrodes, pp 363-68 in: Proc. of the 17th Ris Int. Symp. on Material Science: High

Temp. Electrochemistry: Ceramics and Metals, Roskilde, Denmark, 1996.J. Mizusaki, H. Tagawa, T. Saito, K. Kamitain, T. Yamamura, K. Hirano, S. Ehara, T. Takagi, T. Hikita, M. Ippommatsu,

S. Nakagawa, K. Hashimoto, Preparation of Nickel Pattern Electrodes on YSZ and Their Electrochemical Proper-ties in H2-H2O Atmospheres, J. Electrochem. Soc., 141, 2129-34, 1994.

J. Mizusaki, H. Tagawa, T. Saito, T. Yamamura, K. Kamitani, K. Hirano, S. Ehara, T. Takagi, T. Hikita, M. Ippommatsu,S. Nakagawa, K. Hashimoto, 'Kinetic studies of the reaction at the nickel pattern electrode on YSZ in H 2-H2O at-mospheres', Solid State Ionics, 70/71, 52-58, 1994.

P.A. Osborg, T. Norby, Characterisation of a H2+H2O/Ni/YSZ point electrode system by impedance spectroscopy,pp47-50, in 7th SOFC Workshop, theory and measurement of microscale processes in Solid Oxide Fuel Cells, Wa-dahl, Norway, 18-20 Jan., 1995.

T. Yamamura, H. Tagawa, T. Saito, J. Mizusaki, K. Kamitani, K. Hirano, S. Ehara, T. Takagi, Y. Hishinuma, H. Sasaki,T. Sogi, Y. Nakamura, K. Hashimoto, Reaction Kinetics at the Nickel Pattern Electrode on YSZ and its depend-ence on Temperature, Proc. of the 4th Int. Symp. On SOFC-IV, pp 741-49, Yokohama, Japan, 1995.

T. Yamamura, H. Yoshitaka, H. Tagawa, N. Mori, K. Hirano, J. Mizusaki, S. Ehara, T. Takagi, M. Hishinuma, H. Sasaki,

Y. Nakamura, K. Hashimoto, Experimental Evidence for Three Phase Boundary as Active Site on Nickel/YSZSystem, pp 617-25 in: Proc. Of 2nd European SOFC Forum, Oslo, Norway, 1996.


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23

F.Z. Mohamedi-Boulenouar, J. Guindet and A. Hammou, Influence of Water Vapour on Electrochemical Oxidation ofHydrogen at the Ni/Zirconia Interface, pp 441-50 in: Proc. of the 17th Ris Int. Symp. on Material Science: HighTemp. Electrochemistry: Ceramics and Metals, Roskilde, Denmark, 1996.

N. Nakagawa, H. Sakurai, K. Kondo and K. Kato, 'Study on the extension of reaction zone from Ni/YSZ interface byusing fixed film electrodes', pp 721-30 in Proc. of the 4 th Int. Symp. On SOFC-IV, pp 741-49, Yokohama, Japan,1995.

S. Primdahl and M. Mogensen, Oxidation of Hydrogen on Ni/Yttria-Stabilized Zirconia Cermet Anodes, J. Electro-

chem. Soc., 144, 3409-19, 1997.R.J Aaberg, R. Tunold, S. Tjelle, R. degrd, Oxidation of CO and H 2 on Ni/YSZ cermet electrodes, pp 511-16 in:

Proc. of the 17th Ris Int. Symp. on Material Science: High Temp. Electrochemistry: Ceramics and Metals, Ro-skilde, Denmark, 1996.

R.J Aaberg, R. Tunold, S. Tjelle, R. degrd, A Possible Reaction Mechanism for the Oxidation of H 2 on Ni/YSZ cer-met Electrodes, pp 557- 64 in: The Electrochemical Society, Aachen, 1997.

S. Skaarup, B. Zachau-Christiansen and T. Jacobsen, Surface Species and Surface Mobility on Ni/YSZ Anodes, pp 423-30 in: Proc. of the 17th Ris Int. Symp. on Material Science: High Temp. Electrochemistry: Ceramics and Metals,Roskilde, Denmark, 1996.

C. Wagner, Ber. Bunsenges. Physic Chem., 72, 778-81, 1968.S. Tjelle, R. Tunold and R. degrd, Oxidation of H2 and CO on Ni/YSZ cermet, pp 67-72 in: Proc. Of the 7

th SOFCworkshop, Advanced Fuel Cells Programme Annex II Modelling and Evaluation of Advanced SOFC, Wadahl,Norway, 1995.

C-H Lee, C-H Lee, H-Y Lee and S.M Oh, Microstructure and anodic properties of Ni/YSZ cermets in Solid Oxide Fuel

Cells, Solid State Ionics 98, 39-48, 1997.Ch. Bleise, J. Divisek, B. de Haart, P. Holtappels, B. Steffen and U. Stimming, Kinetic Modelling of the Anodic Reac-tion Considering the Porous Structure, pp 31-39 in: Proc. Of the 7 th SOFC workshop, Advanced Fuel Cells Pro-gramme Annex II Modelling and Evaluation of Advanced SOFC, Wadahl, Norway, 1995.

A. Hahn, H. Landes, Investigations into the kinetics of SOFC cathodes pp 595-605 in Proc. Of the 5 th Int. Symp. OnSOFC, Ed. U. Stimming, S.C. Singhal, H. Tagawa, and W. Lehnert, The Electrochemical Society, Aachen, 1997.

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25

2Experimental considerations

2.1 IntroductionIn this thesis interest is focused on the electrochemical performance of nickel and nickel /yttria-stabilised zirconia cermet anodes in the hydrogen oxidation reaction. The present

chapter describes:

the experimental set-up, the design considerations regarding the electrochemical cell.

2.2 The experimental set-upThe experimental set-up is schematically represented in Figure 2.1 and is suitable for:

impedance and polarisation measurements,

electrochemical measurements as a function of gas condition (pH2, pH2O), polarisationand temperature.Essential parts are described in more detail in the following sections.

2.2.1 The electrochemical cellOne of the important require-

ments for the electrochemical cell

is the possibility to perform half-

cell measurements. Half-cell

measurements allow the study ofone electrode, in this case the an-

ode. The consequence of half-cell

measurements is the need for an

additional electrode, the reference

electrode. The commonly used

terminology of electrodes is

working, counter and reference

electrode. The working electrode

is the electrode whose perform-

ance or characteristics are understudy. The counter electrode is Figure 2.1: Overview of the experimental set-up.


42/182

2 Experimental considerations

26

the other current carrying

electrode positioned on the

opposite site of the electro-

lyte. The reference electrode

is used for controlling andmeasuring the potential of

the working electrode and

does not carry any apprecia-

ble current. A schematic representation of the electrochemical cell used throughout this

thesis is given in Figure 2.2. A detailed discussion of the requirements for the electrode

configuration is given in section 2.3.

2.2.2 Sample holderThe ceramic sample holder is schematically shown in Figure 2.3.a. The holder has an open

structure to allow gas flow to and from the electrode. A Pt/Pt-10%Rh thermocouple is

placed near the electrode for temperature measurement. Pt grids are used as current col-

lector. An additional wire is led to the working electrode to monitor the potential of this

electrode. A weight of about 325 g is placed on the test cell to ensure good electrical con-

tact between electrode and current collector.

The ceramic sample holder is placed in a quartz tube through which gas of desired compo-

sition can flow (Figure 2.3.b). Such a measurement set-up, where the cell

brandstofcel

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