singhal, s. c. (2002). high temperature solid oxide fuel cells - funda

High Temperature
Solid
Oxide Fuel Cells
Fun dam en tals, Desig
and Apdirations
J;
2
'
cubhash
r
cinghal and Kevin Kendal


h
Temperature
Solid
Oxide
Fuel
Cells:
Fundamentals, Design and Applications

~
High Temperature
Solid Oxide Fuel Cells:
Fundamentals, Design and Applications
Edited
by:
Subhash
C
Singhal
and

Kevin
Kendall
ELSEVIER
UK
USA
JAPAN
Elsevier Ltd, The Boulevard, LangfordLane, Kidlington, Oxford
OX5
IGB,
UK
Elsevier Inc, 360 Park AvenueSouth, New York,
NY
10010-1710,
USA
Elsevier Japan, Tsunashima Building Annex, 3-20-12 Yushima.
Bunlryo-ku, Tokyo
11
3, Japan
Copyright
0
2003 Elsevier Ltd.
All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system or transmitted in any form
or
by anj7 means: electronic,
electrostatic, magnetic tape, mechanical, photocopying, recording or
otherwise, without prior permission in writing from the publishers.
British Library Cataloguing
in
Publication Data

High temperature solid oxide fuel cells: fundamentals,
design and applications
1.
Solid oxide fuel cells
I. Singhal, Subhash C.
62 1.3’12429
11.
Kendall, Kevin, 1943-
ISBN 1856173879
Library
of
Congress Cataloging-in-Publication Data
High temperature solid oxide fuel cells: fundamentals, design and
applications
/
edited by Subhash
C.
Singhal and Kevin Kendall.
p. cm.
Includes bibliographical references and index.
ISBN 1-85617-387-9 (hardcover)
1.
Solidoxidefuelcells. I. Singhal, SubhashC. II.Kendal1, Kevin, 1943-
TIC2931 .H54 2002
62
1.3 1’2429-dc2
1
2002040761
No
responsibility is assumed

by
the Publisher for any injury and/or damage
to persons or property as a matter of products liability, negligence or
otherwise,
or
from any use or operation of any methods, products,
instructions or ideas contained in the material herein.
Published by
Elsevier Advanced Technology, The Boulevard, Langford Lane, Kidlington
Oxford
OX5
lGB,
UK
Tel.:
+44(0)
1865
843000
Fax: +44(0) 1865 843971
Typeset by Variorum Publishing Ltd, Lancaster and Rugby
Printed and bound
in
Great
Britain
by
MPG
Books
Ltd,
Bodmin,
Cornwall
Contents

List of Contributors
Preface
Chapter
1
Introduction
to
SOFCs
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.13
Background
Historical Summary
Zirconia Sensors for Oxygen Measurement
Zirconia Availability and Production
High-Quality Electrolyte Fabrication Processes
Electrode Materials and Reactions
Interconnection for Electrically Connecting the Cells
Cell and Stack Designs
SOFC Power Generation Systems
Fuel Considerations

Competition and Combination with Heat Engines
Application Areas and Relation to Polymer Electrolyte
Fuel Cells
SOFC-Related Publications
References
Chapter
2
History
2.1
2.2
2.3
2.4 Progressin the 1960s
2.5
The Path to the First Solid Electrolyte Gas Cells
From Solid Electrolyte Gas Cells to Solid Oxide Fuel Cells
First Detailed Investigations of Solid Oxide Fuel Cells
On the Path to Practical Solid Oxide Fuel Cells
References
Chapter
3
Thermodynamics
3.1 Introduction
3.2 The Ideal Reversible
SOFC
3.3. Voltage Losses by Ohmic Resistance and
by
Mixing
Effects by Fuel Utilisation
xi
xv

1
2
4
5
7
8
11
12
14
15
17
18
19
19
23
26
29
32
40
44
53
56
62
vi
High
Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications
3.4
3.5
3.6
3.7 Summary

Thermodynamic Definition
of
a Fuel Cell Producing
Electricity and Heat
Thermodynamic Theory of SOFC Hybrid Systems
Design Principles of SOFC Hybrid Systems
References
Chapter
4
Electrolytes
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Introduction
Fluorite-Structured Electrolytes
Zirconia-Based Oxide Ion Conductors
Ceria-Based Oxide Ion Conductors
Fabrication of ZrOz and Ce02-Based Electrolyte Films
Perovskite-S tructured Electrolytes
4.6.1 LaA103
4.6.2
4.6.3
Oxides with Other Structures
4.7.1 Brownmillerites (e.g. Ba2InzO6)

4.7.2 Non-cubic Oxides
Proton-Conducting Oxides
Summary
References
LaGa03 Doped with Ca,
Sr
andMg
LaGaOs Doped with Transition Elements
Chapter
5
Cathodes
5.1 Introduction
5.2 Physical and Physicochemical Properties of
Perovskite Cathode Materials
5.2.1 Lattice Structure, Oxygen Nonstoichiometry,
5.2.2 Electrical Conductivity
5.2.3 Thermal Expansion
5.2.4
Reactivity of Perovskite Cathodes with Zr02
5.3.1 Thermodynamic Considerations
and Valence Stability
Surface Reaction Rate and Oxide Ion
Conductivity
5.3
5.3.1.1 Reaction ofperovskites with the
Zirconia Component in
YSZ
5.3.1.2 Reactionofperovskite with the
yttria (dopant) component in YSZ
5.3.1.3 Interdiffusion between Perovskite

and Fluorite Oxides
5.3.2 Experimental Efforts
5.3.3 Cathode/Electrolyte Reactions and Cell
Performance
66
69
77
80
81
83
83
89
92
94
96
97
99
104
106
106
108
110
112
112
119
120
120
123
125
126

130
130
130
130
131
132
134
5.3.4 Cathodes for Intermediate Temperature SOFCs 13 6
Contents
vii
5.4
5.5
5.6
Compatibility of Perovskite Cathodes with
Interconnects
5.4.1
Compatibility of Cathodes with Oxide
Interconnects
5.4.2
Compatibility of Cathodes with Metallic
Interconnects
Fabrication
of
Cathodes
Summary
References
Chapter
6
Anodes
6.1

Introduction
6.2
Requirements for an Anode
6.3
6.4
Cermet Fabrication
6.5
6.6
6.7
6.8
6.9
6.10
Summary
Choice of Cermet Anode Components
Anode Behaviour Under Steady-State Conditions
Anode Behaviour Under Transients Near Equilibrium
Behaviour of Anodes Under Current Loading
Operation
of
Anodes with Fuels other than Hydrogen
Anodes for Direct Oxidation of Hydrocarbons
References
Chapter
7
Interconnects
7.1
7.2
7.3
7.4
7.5

Introduction
Ceramic Interconnects (Lanthanum and
Yttrium Chromites)
7.2.1
Electrical Conductivity
7.2.2
Thermal Expansion
7.2.3
Thermal Conductivity
7.2.4
Mechanical Strength
7.2.5
Processing
Metallic Interconnects
7.3.1
Chromium-Based Alloys
7.3.2
Ferritic Steels
7.3.3
Other Metallic Materials
Protective Coatings and Contact Materials for
Metallic Interconnects
Summary
References
Chapter
8
Cell and Stack Designs
8.1
Introduction
8.2

Planar
SOFC
Design
8.2.1
Cell Fabrication
138
138
139
142
143
143
149
150
151
153
156
158
160
164
165
168
169
173
174
174
177
178
178
179
181

181
182
186
187
189
190
197
197
205
viii
High
Temperature Solid
Oxide
Fuel
CeJIs:
Fundamentals,
Design
and Applications
8.2.1.1
Cell Fabrication Based on
8.2.1.2
Cell Fabrication Based on
Particulate Approach
Deposition Approach
8.2.2
Cell and Stack Performance
8.3.1
Cell Operation and Performance
8.3.2
Tubular Cell Stack

8.3.3
Alternative Tubular Cell Designs
8.4.1
Microtubular
SOFC
Stacks
References
8.3
Tubular SOFC Design
8.4
Microtubular SOFC Design
8.5
Summary
Chapter
9.
Electrode Polarisations
9.1
Introduction
9.2
Ohmic Polarisation
9.3
Concentration Polarisation
9.4
Activation Polarisation
9.4.1
Cathodic Activation Polarisation
9.4.2
Anodic Activation Polarisation
Measurement
of

Polarisation
(By
Electrochemical
Impedance Spectroscopy)
References
9.5
9.6
Summary
Chapter
10
Testing
of
Electrodes, Cells and Short Stacks
10.1
Introduction
10.2
Testing Electrodes
10.3
Testing Cells and 'Short' Stacks
10.4
Area-Specific Resistance (ASR)
10.5
Comparison of Test Results
on
Electrodes and
on Cells
10.5.1
Non-activated Contributions to the
10.5.2
Inaccurate Temperature Measurements

10.5.3
Cathode Performance
10.5.4
Impedance Analysis of Cells
The Problem of Gas Leakage in Cell Testing
10.6,l
References
Total
Loss
10.6
10.7
Summary
Assessment of the Size
of
the Gas Leak
Chapter
11
Cell, Stack and System Modelling
1
1.1
Introduction
11.2
Flow
and Thermal Models
205
206
208
210
213
2

14
216
219
222
224
22 5
230
232
233
237
243
249
251
257
257
261
262
267
2 72
277
280
2 80
281
282
283
284
286
286
293
294

1
1.2.1
Mass Balance
11.2.2
Conservation ofMomentum
1
1.2.3
Energy Balance
Chemical Reactions and Rate Equations
Cell- and Stack-Level Modelling
1
1.3
Continuum-Level Electrochemistry Model
11.4
1
1.5
11.6
System-Level Modelling
11.7
Thermomechanical Model
1
1.8
Electrochemical Models at the Electrode Level
Contents
ix
295
295
296
299
303

308
314
315
318
11.8.1
11.8.2
11.8.3
11.8.4
Fundamentals and Strategy of
Electrode-Level Models
319
Electrode Models Based on a Mass
Transfer Analysis
321
One-Dimensional
Porous
Electrode
Models Based on Complete
Concentration, Potential, and
Current Distributions
322
Monte Carlo or Stochastic Electrode
Structure Model
324
11.8.4.1
Electrode or Cell Models Applied to
Ohmic Resistance-Dominated Cells
324
1
1.8.4.2

Diagnostic Modelling of
Electrodes to Elucidate Reaction
Mechanisms
Electronic Conducting
(MIEC)
Electrodes
11.8.4.3
Models ofMixed Ionic and
1
1.9
Molecular-Level Models
1
1.10
Summary
References
Chapter
12
Fuels
and Fuel Processing
12.1
Introduction
12.2
RangeofFuels
12.3
Direct and Indirect Internal Reforming
12.3.1
Direct Internal Reforming
12.3.2
Indirect Internal Reforming
Reformation of Hydrocarbons by Steam,

COz
and Partial Oxidation
Direct Electrocatalytic Oxidation of Hydrocarbons
12.4
12.5
12.6
CarbonDeposition
12.7
Sulphur Tolerance and Removal
12.8
Anode Materials in the Context of Fuel Processing
12.9
Using Renewable Fuels in SOFCs
12.10
Summary
References
324
325
325
326
327
333
335
338
340
341
342
346
347
351

352
3 54
355
356
x
High
Temperature
SoIid
Oxide Fuel Cells: Fundamentals, Design and Applications
Chapter
13
Systems
and
Applications
13.1
Introduction
13.2
13.3
13.4
Trends in the Energy Markets and SOFC Applicability
Competing Power Generation Systems and SOFC
Applications
SOFC System Designs and Performance
13.4.1
13.4.2
13.4.3
Pressurised SOFC/Turbine Hybrid Systems
13.4.4
System Control and Dynamics
13.4.5

SOFC System Costs
13.4.6
Atmospheric SOFC Systems for Distributed
Power Generation
Residential, Auxiliary Power and Other
Atmospheric SOFC Systems
Example of
a
Specific SOFC System
Application
13.5
SOFC System Demonstrations
13.5.1
Siemens Westinghouse Systems
13.5.1.1
100
kWAtmospheric
13.5.1.2 220
kWPressurised
13.5.1.3
Other Systems
13.5.2
Sulzer Hexis Systems
13.5.3
SOFC Systems ofother Companies
References
SOFC System
SOFC/GT Hybrid System
13.6
Summary

3 63
365
367
3 70
3 70
3 73
3 74
3 76
3 78
3
79
380
3 80
380
383
384
385
386
388
389
Index
393
List
of
Contributors
Harlan
U
Anderson
Electronic Materials Applied Research Center,
104

Straumanis Hall, University
ofMissouri-Rolla, Rolla,
MO
65410-1240,
USA

Rob
J
F
van Gerwen,
KEMA
Power Generation
&
Sustainables, KEMA Nederland
BV,
PB 9035,
6800
ET Arnhem, The Netherlands

Peter Vang Hendriksen
Materials Research Department, Risra National Laboratory, DK-4000 Roskilde,
Denmark

Teruhisa Horita
National Institute
of
Advanced Industrial Science and Technology, AIST
Tsukuba CentralNo. 5, Higashi
1-1-1,
Tsukuba, Ibaraki 305-8565, Japan

t. horita@ aist.go.jp
Tatsumi Ishihara
Department
of
Applied Chemistry, Faculty
of
Engineering, Kyushu University,
Hakozaki
6-10-1,
Higashi-ku, Fukuoka 812-8581, Japan

Ellen Ivers-Tiffee
Institut fur Werkstoffe der Elektrotechnik
IWE,
Universitat Karlsruhe
(TH),
Adenauerring 20, 7613
1
Karlsruhe, Germany

Kevin Kendall
School
of
Chemical Engineering, The University
of
Birmingham, Edgbaston,
Birmingham B15 2TT,
UIC

xii

High Temperature SoIid Oxide
Fuel
Cells:
Fundamentals. Design and Applications
Mohammad A Khaleel
Pacific Northwest National Laboratory, PO Box 999, Richland, WA 993 52, USA
moe .khaleel@pnl. gov
Augustin
J
McEvoy
Institute of Molecular and Biological Chemistry, Faculty
of
Basic Sciences, Ecole
Polytechnique Fedkrale de Lausanne, CH-1015 Lausanne, Switzerland
augustin.mcevoy @epfl.ch
Nguyen
Q.
Minh
General Electric Power Systems, Hybrid Power Generation Systems, 193
10
Pacific Gateway Drive, Torrance,
CA
90502-103
1,
USA

Hans-Heinrich Mobius
Ernst-Moritz-Arndt-Universitat
Greifswald (Emeritus)
Rudolf-Breitscheid Strasse 25,

D
17489 Greifswald, Germany

Mogens Mogensen
Materials Research Department, Riser National Laboratory, DK-4000 Roskilde,
Denmark

R
Mark Ormerod
Birchall Centre for Inorganic Chemistry and Materials Science, School of
Chemistry and Physics, Keele University, Staffordshire ST5 5BG,
UK

Nigel
M
Sammes
Connecticut Global Fuel Cell Center, University of Connecticut, 44 Weaver Road,
Unit-5233, Storrs,CT06269-5233, USA

J
Robert Selman
Center for Electrochemical Science and Engineering, Illinois Institute of
Technology, Chicago, IL 60616, USA

Subhash
C
Singhal
Pacific Northwest National Laboratory, PO Box 999, Richland, WA 993 52, USA
singhalo pnl. gov
Frank Tietz

Forschungszentrum Jiilich GmbH, Institut fur Werkstoffe und Verfahren der
Energietechnik
(IWV-l),
D-5242
5
Julich, Germany

List
of
Contributors
xiii
Ani1
V
Virlrar
Department
of
Materials Science and Engineering, University
of
Utah, Salt Lake
City,
Utah 84112, USA
anil.virkar
@
m. cc .Utah. edu
Wolfgang Winkler
Fuel Cells Laboratory, Hamburg University of Applied Sciences, Faculty
of
Mechanical Engineering and Production, Berliner Tor
2
1,

20099 Hamburg,
Germany

Osamu Yamamoto
Aichi Institute
of
Technology, 13-1,I.amo-gome, Chailri-cho, Ichinomiya, Aichi
491-0801, Japan
osyamamo
@
alles .or.
j
p
Harumi Yolrokawa
National Institute
of
Advanced Industrial Science and Technology, AIST
TsukubaCentralNo
5,
Higashi
1-1-1,
Tsukuba, Ibaraki
305-8565,
Japan


PREFACE
High temperature solid oxide fuel cells (SOFCs) are the most efficient devices for
the electrochemical conversion of chemical energy of hydrocarbon fuels into
electricity, and have been gaining increasing attention in recent years for clean

and efficient distributed power generation. The technical feasibility and
reliability of these cells, in tubular configuration, has been demonstrated by
the very successful operation of a
100
ItW
combined heat and power system
without any performance degradation for over two years. The primary goal now
is the reduction of the capital cost
of
the SOFC-based power systems to effectively
compete with other power generation technologies. Toward this end, several
different ceIl designs are being investigated and many new colIaborative
programs are being initiated in the United States, Europe, and Japan: noteworthy
among these are the Solid State Energy Conversion AlIiance (SECA) program in
the United States, the Framework
6
programs in the European Union, and the
New Energy and Industrial Technology Development Organization
(NEDO)
programs in Japan. The funding for
SOFC
development worldwide has
risen dramatically and this trend is expected to continue for at least the next
decade.
In
addition to cost reduction, these development programs are also
investigating wider applications
of
SOFCs in residential, transportation and
military sectors, made possible primarily because of the fuel flexibility of

these cells. Their application in auxiliary power units utilizing gasoline or diesel
as fuel promises to bring
SOFCs
into the ‘consumer product’ automotive and
recreational vehicle market.
This book provides comprehensive, up-to-date information on operating
principle, cell component materials, cell and stack designs and fabrication
processes, cell and stack performance, and applications of
SOFCs.
Individual
chapters are written by internationalIy renowned authors in their respective
fields, and the text is supplemented by a large number of references for further
information. The book is primarily intended for use by researchers, engineers,
and other technical people working in the field of SOFCs. Even though the
technology is advancing at a very rapid pace, the information contained in most
of the chapters
is
fundamental enough for the book to be useful even as a text for
SOFC technology at the graduate level.
xvi
High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and
Applications
As
in any book written by multiple authors, there may be some duplication
of information or even minor contradiction in interpretation of various
electrochemical phenomena and results from chapter to chapter. However,
this has been kept to a minimum by the editors. Also, in the interest of making
the book available in a reasonable time, it has not been possible to provide
uniformity
in

the nomenclature and symbols from chapter to chapter: we
apologise for that.
Many of our colleagues in the SOFC community provided useful comments and
reviews on some ofthe chapters and we are thankful to them. The encouragement
and financial support of the United States Department of Energy-Fossil Energy
(through
Dr.
Mark Williams, National Energy Technology Laboratory) to one of
the editors (SCS) is deeply appreciated. We are also grateful to
Ms.
Jane Carlson,
Pacific Northwest National Laboratory, for her administrative support during
the editing of the chapters.
Subhash
C.
Singhal
Richland,
Washington,
USA
Kevin Kendall
Birmingham,
UK
September
2003
Chapter
1
Introduction to
SOFCs
Subhash
C.

Singhal
and Kevin Kendall
1.1
Background
Solid oxide fuel cells (SOFCs) are the most efficient devices yet invented for
conversion of chemical fuels directly into electrical power. Originally the basic
ideas and materials were proposed by Nernst and his colleagues [l-31 in
Gottingen at the end of the nineteenth century, as described in Chapter
2,
but considerable advances in theory and experiment are still being made over
100 years later.
Figure
1.1
shows an SOFC scheme. It contains a solid oxide electrolyte made
from a ceramic such as yttria-stabilised zirconia
(YSZ)
which acts as a conductor
of
oxide ions at temperatures from
600
to 1000°C. This ceramic material allows
oxygen atoms to be reduced on its porous cathode surface by electrons, thus
being converted into oxide ions, which are then transported through the ceramic
body to a fuel-rich porous anode zone where the oxide ions can react, say with
hydrogen, giving up electrons to an external circuit as shown in Figure
1.1.
Only
five components are needed to put such a cell together: electrolyte, anode,
cathode and two interconnect wires.
Oxygen

Fuel (e
g
hydrogen)
Solid oxide
electrolyte
Porous
anode
\
A
fuel
oxidation
HI
+
0
-
2
e+H10
/Porous
cathode
Oxysen
reduction
0+2e
+
0
Electrons
to
external circuit
Electrons from external circuit
Figure
1.1

Schematicofsolidoxide fuel cell
(SOFC).
2
High
Temperature
Solid Oxide Fuel Cells: Fundamentals,
Design
and Apjdications
This is almost magical in its elegance and simplicity, and it is astonishing that
this process has not yet been commercialised to supplant the inefficient and
polluting combustion heat engines which currently dominate our civilization.
Largely, this failure has stemmed from a lack
of
materials knowledge and the
absence
of
chemical engineering skills necessary to develop electrochemical
technology. Our belief is that this knowledge and expertise is now emerging
rapidly. The purpose of this book is to present this up-to-date knowledge in order
to facilitate the inventions, designs and developments necessary for commercial
applications of solid oxide fuel cells.
An essential aspect of
SOFC
design and application is the heat produced
by
the
electrochemical reaction, not shown in Fig.
1.1.
As
Chapter

3
shows, heat is
inevitably generated in the
SOFC
by ohmic losses, electrode overpotentials etc.
These losses are present in all designs and cannot be eliminated but must be
integrated into a heat management system. Indeed, the heat is necessary to
maintain the operating temperature of the cells. The benefit of the
SOFC
over
competing fuel cells is the higher temperature of the exhaust heat which makes
its control and utilization simple and economic.
Because both electricity and heat are desirable and useful products of
SOFC
operation, the best applications are those which use both, for example residential
combined heat and power, auxiliary power supplies on vehicles, and stationary
power generation from coal which needs heat for gasification.
A
residential
SOFC
system can use this heat to produce hot water, as currently achieved with simple
heat exchangers.
In
a vehicle the heat can be used to keep the driver warm.
A
stationary power system can use the hot gas output from the
SOFC
to gasify coal,
or to drive
a

heat engine such as a Stirling engine or a gas turbine motor.
These ideas, from fundamentals of
SOFCs
through to applications, are
expanded in the sections below to outline this book’s contents.
1.2
Historical
Summary
The development
of
the ideas mentioned above has taken place over more than a
century. In
1890,
it was not yet clear what electrical conduction was. The
electron had not quite been defined. Metals were known to conduct electricity in
accord with Ohm’s law, and aqueous ionic solutions were known to conduct
larger entities called ions. Nernst then made the breakthrough of observing
various types of conduction in stabilised zirconia, that is zirconium oxide doped
with several mole per cent of calcia, magnesia, yttria, etc. Nernst found that
stabilised zirconia was an insulator at room temperature, conducted ions in red
hot conditions, from
600
to
1000°C
and then became an electronic and ionic
conductor at white heat, around
1500°C.
He patented an incandescent electric
light made from a zirconia filament and sold this invention which he had been
using to illuminate his home

[l-31.
He praised the simultaneous invention
of
the
telephone because it enabled him
to
call
his
wife to switch on the light device
while he travelled back from the university. The heat-up time was a problem
even then
[4].
Introduction to
SOFCs
3
The zirconia lighting filament was not successful in competing with tungsten
lamps and Nernst's invention languished until the late 1930s when a fuel cell
concept based on zirconium oxide was demonstrated at the laboratory scale by
Baur and Preis
[SI.
They used a tubular crucible made from zirconia stabilised
with 15 wt% yttria as the electrolyte. Iron or carbon was used as the anode and
magnetite (Fe304) as the cathode. Hydrogen or carbon monoxide was the fuel on
the inside of the tube and air was the oxidant on the outside. Eight cells were
connected in series to make the first SOFC stack. They obtained power from the
device and speculated that this solid oxide fuel cell could compete with batteries.
But several improvements were necessary before this would be possible. For
example, the electrolyte manufacturing process was too crude and needed
optimising, especially to make the electrolyte thinner to reduce the cell
resistance from around 2

Q.
In addition, the electrodes were inadequate,
especially the cathode Fe304 which readily oxidised.
Also,
the power density was
small with the stacking arrangement used, the connections between many cells
had to be developed, and the understanding of fuel reactions and system
operation needed much attention.
It was not until the 19 50s that experiments began on pressed or tape-cast discs
of stabilised zirconia when a straightforward design of test system was developed
which is still in use today. The essentials of the apparatus are shown in Figure
1.2a.
A
flat disc of stabilised zirconia, with anode and cathode on its two sides,
was sealed to
a
ceramic tube and inserted in a furnace held at red heat
[6].
A
smaller diameter tube was inserted into the ceramic tube to bring fuel to the
anode, and another tube brought oxidant gas to the cathode side. Current
collector wires and voltage measurement probes were brought out from the
electrode surfaces. Once a flat plate of electrolyte had been used, it was easy to see
how the flat plate voltaic stack could be built up with interconnecting separator
plates to build a realistic electrochemical reactor, as shown in Figure 1.2b. The
interconnect plate is essentially made from the anode current collector and the
Zirconia
+
electrodes Furnace
Fuel

4
r'
Oxidant
Anode Cathode leads
a
Interconnects
V
b
Figure
1.2
(a)
Flatplate test
cell;
(b)planarstackofcellsandinterconnects.
4
High
Temperature
Solid
Oxide
Fuel
Cells:
Fundamentals,
Design
and
App!ications
cathode current collector joined together into one sheet, thus combining the two
components. Additionally, the interconnector can contain gas channels which
supply fuel to the anode and oxidant t.o the cathode as well as electrically
connecting the anode of one cell to the cathode of the next.
It turned out that there were several problems with flat plate stacks as they

were made larger to generate increased power, including sealing around the
edges and thermal expansion mismatch which caused cracking. Consequently,
tubular designs have had greater success in recent years. However, the
configuration of Figure
1.2
has been prominent in zirconia sensors, discussed in
the next section, which are now manufactured in large numbers.
1.3
Zirconia Sensors for Oxygen Measurement
An
SOFC
in reality already exists on every automobile: it is the oxygen sensor
device which sits in the exhaust manifold in order to control the oxygen content
of the effluent mixture entering the exhaust catalyst. The composition of the
effluent mixture must be controlled to near stoichiometric if the catalyst is to
operate at its optimum performance. Yttria-stabilised zirconia
(YSZ)
is generally
used as the electrolyte because this uniquely detects oxygen, and platinum
is normally painted on its surface using proprietary inks to provide the electrodes.
In the original design, which was used from the
1970s
the configuration was
similar to that of Baur and Preis
[5].
A
thimble of
YSZ,
containing typically
8

wt%
yttria, was pressed from powder and fired to 1500°C to densify it. Platinum
electrodes were applied and the unit then fixed in a steel plug which could be
screwed into the car exhaust manifold,
so
that the YSZ +anode was protruding
into the hot gases. Air was used as the oxygen reference on the cathode side.
A
wire connection supplied the voltage from the inner electrode to the engine
management system, while the other electrode was grounded to the chassis.
Once the exhaust warmed up, above 600"C, the voltage from the sensor
reflected the oxygen concentration in the exhaust gas stream. This voltage
varied with the logarithm of oxygen level, giving the characteristic 1-shaped
curve of voltage versus oxygen concentration, hence the name 'lambda sensor'.
The control system then used the oxygen sensor signal to manage the engine
so
that the exhaust composition was optimised for the catalyst. Various
improvements have been made to this basic system over the years; for example, a
heating element can be built within the thimble, in order to obtain a rapid
heating sensor.
The major improvement introduced by Robert Bosch GmbH in
1997
was
to
redesign the zirconia sensor and
to
manufacture it by a different method. Instead
of
pressing a thimble from dry powder, a wet mix of zirconia powder with
polymer additives was coated and dried like a paint film on a moving belt in a

tape-casting machine. The
film
dried to a thickness of around
100
pm and could
be screen printed with the platinum metallisation before pressing three or four
sheets together to form a planar sensor array which was fired and then sectioned
to size before inserting in the metal boss which screwed into the engine manifold.
lntroduction
to
SOFCs
5
This process was rather like the ceramic capacitor process developed for the
electronic ceramics industry.
There were several benefits of this new device:
0
0
0
There was much less zirconia in it, about
2.8
g;
The thinner ceramic electrolyte gave much faster response:
Heaters and other circuits could readily be printed onto the flat sheets.
An
immediate bonus of this technology was the possibility of producing linear
response sensors as opposed to the logarithmic response of the thimble type,
so
as
to match the electronic control system more easily. This was achieved by setting
the oxygen reference by using one of the sheets as an oxygen pump which could

then leak from the cathode compartment through a standard orifice.
Oxygen sensors are now widely used in food storage, in metal processing
and in flame controIIers, but the main market is automobiles. Zirconia
technology for sensors has been very successful in the marketplace, and
it
has
pushed forward the development
of
solid oxide fuel cell materials. The main
difference is that the power output of sensors is
low
so
that partially stabilised
zirconia can be used. At higher power, fully stabilised zirconia must be used if
the electrolyte is
to
remain stable for long periods. The supply of this electrolyte
material is discussed next.
1.4
Zirconia Availability and Production
The main electrolyte material used in
SOFCs
at present is
YSZ,
as described more
fully in Chapter
4.
Although many other oxide materials conduct oxide ions,
some rather better than zirconia, this material has a number of significant
attributes which make it ideal for this application, including abundance,

chemical stability, non-toxicity and economics. Against these one can mention
several drawbacks, including the high thermal expansion coefficient, and the
problems of joining and sealing the material.
Low-grade stabilised zirconia already commands a large market, especially in
refractories, pigment coatings and colours for pottery, but it is only recently that
technical-grade zirconias have been produced for applications such as thermal
barrier coatings on gas turbine components, hip joint implants and cutting
tools. Much of this technology has stemmed from the study
of
pure zirconia and
the effects of small amounts of dopants on the crystal structure and properties.
Large effects were seen in the early
197Os,
pointing the
way
to substantial
applications of this material
[7].
Figure
1.3
shows the trend in worldwide production levels of ionic conductor-
grade yttria-stabilised zirconia over time. It is evident that in
1970
there was
very small production at a rather high price. However, the introduction of the
zirconia lambda sensor to control the emissions of automobiles in the
1970s
had
a large effect on the production rate, and price has dropped steadily since that
time. The price in

2000
was about
$50
per kg in
50
kg lots but this is expected to
6
High Temperature Solid Oxide Fuel Cells: Fundamentak, Design and Applications
1970 1980 1990
2000
2010
2020
year
Figure
1.3
Trend in theproduction ofionic conducting
yttrin-stabilisedzirconinpowder.
fall steadily with time towards
$13
per kg in
2020
as production rises to many
thousands of tons per year. In
2000,
the sensor application
of
YSZ was dominant
with an estimated world production
of
500

metric tons, but it is expected that
fuel cell power systems will rapidly rise to overtake sensors in demanding
YSZ
by
about
2
0
10,
There is little doubt that large quantities of zirconia will be needed for
SOFC
applications in the years to come, with annual requirements rising to more than
1
Mte per year, rather as titania expanded in the last century for pigment
applications. Fortunately, zirconia is one of the most common materials in the
earth’s crust, being much more available than copper or zinc, for example. Large
deposits exist in Australia, Africa, Asia and America, usually as the silicate,
zircon (ZrSi04). In terms of cost, the greatest difficulty is purifying this raw
material, especially to remove SiOz which tends to block the ionic and electron
paths in fuel cell systems. A typical zirconia powder for electrolyte application
should contain less than
0.1%
by weight of silica, and the highest quality YSZ
electrolytes contain only
0.005%
by weight. Other impurities, like alumina and
titania, can be useful in gettering the damaging silica,
so
that levels of
0.1%
by

weight are normal. The main impurity, hafnia, is usually present at several wt%
but causes no problem because it is an ionic conductor itself. Often, zirconia
contains small amounts
of
radioactive
a
emitter impurities, and this could pose
a
potential health problem during processing, but otherwise there are no
significant toxic hazards
known.
Yttria is the principal stabiliser used at present, though both the more expensive
scandia and ytterbia give better ionic conductivity. Typically, yttria is added at
13-16%
by weight
(8-10.5
mol%) to give
a
fully stabilised cubic material.
Details of these materials are given in Chapter
4.
Supply of scarce dopants such
as scandia could be a problem in future. However, a more significant issue is the
processing of the electrolyte material into a functional device.