Ishihara T. (Ed.) Perovskite Oxide for Solid Oxide Fuel Cells (Fuel Cells and Hydrogen Energy)
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15.5 Lattice Expansion During Reduction and Temperature
Change
Doped lanthanum chromites show lattice expansion under high-temperature redu-
cing atmospheres [32–34] because of the formation of oxygen vacancies in the lattice
under reducing atmosphere. In planar-type solid oxide fuel cells, the LaCrO
3
-based
interconnect plate is placed in a large oxygen potential gradient (air and fuel condi-
tions). The lattice expansion and deformation of the LaCrO
3
plate are significant
under operating conditions, and this phenomenon has been reported in real stacks
and modules [1–3]. A numerical model was proposed, and deformation was calcu-
lated from the profiles of oxygen vacancy concentration. Static and transient stress
calculation was carried out for plate and rectangular specimens. When the plate size
is 100 100 3 mm, the warp of the plate is calculated by a numerical model. The
maximum displacement of the center part normal to the originally flat surface was
calculated to be about 0.77 mm. The calculated tensile stress coming from the
transient deformation was as high as 50 MPa or more [32].
Because interconnects contact with the other cell components, the thermal expan-
sion behaviors should be matched within the acceptable level. Doping of Ca increases
the thermal expansion coefficient (TEC) values from 8.5 10
6
/K to 10.0 10
6
/K
(at x ¼0.3 in La
1x
Ca
x
CrO
3
) [1–5]. The TEC value of Ca-doped LaCrO
3
is close to
that of YSZ (about 10 10
6
/K), which can reduce the thermal stress during heating
and fabrication. Doping of Sr is also effective to increase the TEC values to match
the TEC values of YSZ. The thermal expansion behavior is dependent on oxygen
partial pressure as predicted by the defect chemistry. For example, Mori [35]
measured the TEC values for several kinds of La
0.8
Sr
0.2
Cr
0.8
M
0.2
O
3
(M ¼dopants)
in air and in hydrogen. Figure 15.7 shows thermal expansion behavior of Sr-, Ti-,
and V-doped LaCrO
3
[35]. Doping of V decreased the TEC values to be matched
with YSZ. In reducing atmosphere, the TEC values are a little bit larger than those in
air atmospheres. The lattice expansion in the reducing atmosphere is attributed to the
expansion of chromium ion because of the difference of ionic radius between
Cr
3þ
(VI) (0.0615 nm) and Cr
4þ
(VI) (0.055 nm).
15.6 Mechanical Strength
Mechanical strength is one of the critical issues for using LaCrO
3
-based oxide
ceramics. During operation, temperature change of the SOFC system is inevi-
table because of the starting and stopping of the system. Als o, under operating
conditions, an oxygen potential gradient is generated between air and fuel
atmospheres. Thus, the mechanical strength of LaCrO
3
should be high enough
under thermal stress and reducing conditions. The reported mechanical
strengths are 20–130 MPa for La
1–x
Ca
x
CrO
3
(x ¼0.1–0.3), 50–80 MPa for
La
1–x
Sr
x
CrO
3
(x ¼0.1–0.5), and 80–170 MPa for LaCr
0.9
Mg
0.1
O
3
at 1273 K
in air. Generally, the mechanical strength of Sr-doped LaCrO
3
showed a higher
15 LaCrO
3
-Based Perovskite for SOFC Interconnects 293
value than that of Ca-doped LaCrO
3
in air. How ever, for reasons of processing
of LaCrO
3
powders and sintered bodies, Ca-doped LaCrO
3
or Mg-doped
LaCrO
3
is adopted in the developing cells and stacks.
15.7 Summary
LaCrO
3
based perovskite is one of the promising materials for interconnects of
solid oxide fuel cells (SOFCs). Some cations doping into the A site or B site of
LaCrO
3
increased the sintering properties, electrical conductivity, and mechan-
ical strength to the applicable level. Gas tightness can be achieved by the
sintering of LaCrO
3
. However, electrochemical oxygen leak should be consid-
ered under a large oxygen potential gradient. Although the measured oxygen
leak value is negligible in a real operation condition, some oxygen can permeate
through the LaCrO
3
plate during operation via oxygen vacancies. Defect
chemistry of LaCrO
3
-based perovskite is introduced because it correlates with
electrical conductivity, lattice expansion in reducing atmosphere, thermal
expansion, and oxygen permeation.
Fig. 15.7 (a) Thermal
expansion behaviors of
some doped LaCrO
3
in air
and in the H
2
atmosphere in
comparison with 8YSZ
electrolyte. (b) Thermal
expansion coefficients of
La
0.8
Sr
0.2
Cr
0.9–x
Ti
0.1
V
x
O
3
in
the temperature range
508–10008C in air or in the
H
2
atmosphere.
(Reproduced by permission
of The Electrochemical
Society [35])
294 T. Horita
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296 T. Horita
Index
A
Acceptor-doped material, 102–103, 234
Activation analysis, 51
Activation energy, 19, 21, 37, 67, 75–76,
105–106, 108, 110–111, 113, 126, 171,
223–225, 229, 235, 244, 267, 288, 292
Activation energy of tracer diffusion, 106
Activation enthalpy, 98, 107, 224–225, 261,
266–270
Agglomeration, 169
Akbay, T., 183
Ambipolar diffusion mechanism, 256
Anisotropic transport of oxide ions, 141
Anisotropy, 8, 123, 129, 132–133, 135, 137
Anode polarization resistance, 176–177
Anomalous oxidation, 34
Arrhenius behavior, 21
Arrhenius plot, 69–70, 74, 89, 105, 109, 122,
246, 251
of diffusion coefficient, 105
of electrical conductivities, 74
Association enthalpy, 107
Auxiliary power units (APU), 18, 40
B
Back-diffusion, phenomenon of, 201
Berenov, A., 95
B–O bonds, 178, 266
Bond-breaking and forming, barriers for,
267
Bond valence sum (BVS), 134
Bragg–Brentano geometry, 119
Bragg intensities, 121, 123, 129, 133, 135
Brouwer diagram, 100–101
Brownmillerite, 6, 53–54, 89, 228, 230
-type stoichiometries, 228
Bulk diffusion path, 160
Burumauer-Emmott-Teller (BET), 12
C
Calorimeter, 221
Carbonation reaction, 248–249, 253–254
Carbon deposition, 29–31, 41, 168, 213
Carnot
cycles, 24
efficiency, 23–24
Catalytic activity, 7, 10, 15, 95, 148–149, 185,
188
Catalytic partial oxidation (CPOX), 211
Cathode for high-temperature, 156–160
chemical and morphological stability, 158
transport properties and electrochemical
reaction, 157–158
Cathode material, 148–153
catalytic activity, 148
chemical stability, 152
electronic conductivity, 149
morphological stability, 152
oxygen transport, 151
Cathode polarization, 279–280
Cathode reaction, 153–156
cathode–electrolyte interface, 154–156
oxygen electrode process, 153–154
Cation
deficiency, 2
drift, 152, 158
vacancies, 5, 99, 157, 159, 165, 233
Cell development, 184–189
anode, 185
cathode, 188
electrolyte, 184
Ceramic membrane reactor, 55
Ceramic proton conductor, 218, 237
Ceria-based oxides, 164
Ceria-perovskite assemblage, 174
Cerium-doped anode, 174
Charge-compensating defect, 219, 224, 231
Chemical capacitance, 154–155, 162, 164
297
Chemical diffusion coefficient, 96
Chloride ion conduction, 61
Chromium poisoning, 34
Combined heat and power (CHP), 183, 196
Compensation law, 111
Computational fluid dynamics (CFD)
analysis, 198
Conductivity and binding energies, 67
Conductivity isotherm, 255
Conversion efficiency, 17, 21–24, 36, 38, 65,
76, 87, 193, 197
Correlation factor, 96, 106, 112
Cracking, 118–119, 168, 177
Crystallographic
parameters, refined, 122, 129, 132
shears joining, 174
Crystal structure, 131
Current-potential curves, 157
Current tap, 200
Cyclic power output durability test, 193
D
Daily start-and-stop (DSS) operation, 39
Defect chemistry, 147, 168–170, 172, 228,
231, 238, 288–289, 293
Density functional theory (DFT), 221
Diffractometer, 119–120
Diffusion coefficient, 105
Diffusion path of oxide ions, 121–126
Diffusivity of oxide ions, 95–112
defect chemistry, 99
defect equilibria, 99
definitions of diffusion coefficients, 96
mixed electronic-ionic conducting oxides
(MEICs), 102–108
oxygen diffusion, 108–112
oxygen tracer diffusion coefficient, 96
oxygen transport, 99
surface exchange coefficient, 98
Dimethyl ether (DME), 206
Diphosphate groups, hydrolysis of, 235
Disk-type planar geometries, 184
Distortions of perovskite structure, 4
Doped lanthanum
chromite, 293
cobaltite, 55
Doping, 170
Doping–proton association, 238
Double cathode structure, 98
Dream reactions, 10
Drift velocity, 96
Dynamical hydrogen bond, 266–267
E
Electrical conductivity, 7, 9–10, 15, 67–69,
71–72, 75–76, 78, 81, 84, 99, 101, 152,
156, 174, 185, 207, 245, 252, 288, 294
Electrochemical vapor deposition (EVD), 17
Electrode
conductivity, 158
polarization resistance, 178
reaction, 147, 153, 157–158, 160, 191, 247
Electrolyte diaphragm, 46
Electromotive force, 46, 50, 88, 255
Electron
density map, 119
probe micro-analyzer (EPMA), 207
Electroneutrality, 169, 172, 220, 231
Electronic blocking electrochemical method, 292
Elucidation of diffusion paths, 120
Enthalpy-based conversion rate, 23
Entropy and enthalpy, 112, 223
Environmental catalysis, 10
Equilibrium constant, 55, 57, 231, 244, 246,
289–290
Evaporation, 118–119, 218, 233, 236, 249
Ewald method, 8–9
F
Faraday constant, 79, 291
Ferroelectricity, 7–8, 95
Ferromagnetism, 95
Fick’s second law, 96
Film electrolyte, conductivity of, 281–283
Flattened tubes, 35
Fourier transform, 121
Fuel cell (FC) vehicles, 273
Fuel cells
characterization of, 277–278
operation and evaluation of, 279–282
preparation of, 277
Fuel flexibility, 211
Fuel utilization, 31, 196, 200, 210, 218
G
Gas chromatograph, 214
Gas flow controllers, malfunctioning, 198
Gas tightness, 294
Gd-doped ceria (GDC), 23, 28
Giant magneto-resistive effect, 95
Gibbs–Du
¨
hem equation, 152
Gibbs energy, 22–25, 37
Grain growth, 118–119, 286
Green densities, 184
298 Index
H
Hebb–Wagner theory, 80
HERMES diffractometer, 121, 126, 131
See also Diffractometer
Hermetic seal, 191
High conversion efficiencies, 18, 38
Higher heating value (HHV), 18, 193
High-temperature neutron powder
diffractometry, 118
High-temperature proton conductors
(HTPCs), 244, 256
Horita, T., 285
Hydration
reaction, 219, 230, 233, 244, 246
thermodynamics, 221, 227–228, 238
Hydrocarbon fuels, 41, 168
Hydrogen
burning fuel cells, 237
economy, 273
membrane fuel cell (HMFC), 276–273
permeation, 256
potential gradient, 256
sensor, 227, 245, 255
Hydrogenation/dehydrogenation
electrochemical reactors, 217
Hydroxide orientation, 268
Hyperstoichiometric material, 100
I
Infrared
absorption spectra, 253
emission spectroscopy, 149
Intelligent catalyst, 11–12
Interdiffusion, 20, 28, 31, 161, 286
Inter-octahedra transfer, 267
Ionic conduction, 45–61, 68, 72, 117, 141,
169, 184
conduction behavior, 46
early studies on ionic conduction, 49
halide ion conduction, 60
lithium ion conduction, 59
oxide ion conduction, 52
proton conduction, 55
silver ion conduction, 61
IR spectroscopy, 221
Irvine, J. T. S., 167
Ishihara, T., 1, 65
Isothermal diffusivities, 103
Isotope
diffusion, 148, 161
exchange depth profiling technique
(IEDP), 98
oxygen exchange method, 292
Isotropic atomic displacement parameters,
122, 129, 133, 136, 140
Ito, N., 273
Iwahara, H., 45
J
Joule effect, 22–23
K
Kawada, T., 147
Kawakami, A., 205
Kilner, J. A., 95
Kreuer, K. D., 261
Kro
¨
ger–Vink notation, 66, 97, 219, 244
Kyocera, 18, 35, 36, 38–39, 42
L
Langmuir adsorption model, 158
Lanthanum gallate-based
compounds, 117
Lanthanum strontium gallium magnesium
oxides (LSGM), 18
Lattice expansion, 152, 293
Lattice relaxation, 82, 264–265
Lead zirconate titanate (PZT), 50
Linear regression, 221–222
Liquid petroleum gas (LPG), 206
Liquid-phase
chromates, 286
formation, 286
preparation method, 14
synthesis method, 13–14
Lithium ion conduction, 59
Lower heating value (LHV), 17
Lower-valent cation, 243
LSGMC electrolyte, 18
LSGM electrolyte system, 77–87
minor carrier behavior, 79
phase diagram, 77
reactivity with SOFC component, 77
single cell using LSGM electrolyte,
performance, 84–87
thermal expansion behavior, 78
M
Magnetron sputtering, 161
Marcus theory, 261
Matsumoto, H., 243
Index 299
MEM analysis, 120–121, 123, 129, 131, 138
MEM-based pattern fitting (MPF), 120–121
Methane reforming, 24, 177
Meyer–Neldel rule, 111, 113
Micro tubes, 35, 38
Micro-tubular cell development, 206
Migration enthalpy for oxygen vacancies, 106
Mixed electronic and oxide ionic
conductors, 68
Mixed ionic-electronic conductors
(MIECs), 255
Mobile oxide ion vacancies, 66
Mobile oxygen vacancies, 84, 113
Mobile vacancy concentration, 97, 106, 109
Module development, 192
Molten carbonate fuel cells (MCFCs), 18
Monotonical relation, 254
Morphological instability, 159
Multivalence transition elements, 177
N
National Aeronautics and Space
Administration (NASA), 237
Navier–Stokes, 198
NEDO project, 205
Nernst equation, 47, 50–51, 85, 176, 210, 255
Neutron diffraction data, 119, 131–132, 134,
136, 138–139
Neutron powder diffraction data, 126, 131, 138
Nickel coarsening, 187
Nickel sintering, see Sintering
Nonstoichiometry, 102, 156, 158, 160, 168,
170, 229–230, 233
Norby, T., 217
Nuclear density distribution, 120–121, 123,
125, 129–130, 133, 136, 138, 141
O
Occupancy factor, refined, 134
Octahedra tilting, 268
O–H bands, 253
On-board fuel processor, 274
Open circuit voltage (OCV), 88, 176, 185,
209, 281
Orthorhombic lattice distortion, 270
Oxidation reactions, 7
Oxide ion
conductivity, 65, 67–69, 71, 74–75, 77, 79,
81, 83, 85, 87, 89, 91
migration, 134, 137, 142, 175
mobility of, 70, 82
Oxide lattice vibrations, 224
Oxygen
deficiency, 6, 10–11, 56, 96, 132, 135, 228,
235
dissociation, 15, 78
electrochemical leak, 289
excess nonstoichiometry, 157
exchange reaction rate, 149
hypostoichiometry, 104
ion vacancies, 171
nonstoichiometry, 9, 153–154, 156,
162, 292
permeation effect, 22
sensor, 52, 72
separation membranes, 99
tracer diffusion, 96, 103–106,
110–113, 157
vacancies, 6, 10, 45, 53–54, 56–57, 66–67,
69, 71–72, 82, 84, 89–90, 96–97,
99–105, 107–108, 110–111, 136, 160,
170–171, 173, 177, 219–222, 224–231,
233, 235, 243, 289–290, 292–294
P
Palladium, 11
Parabolic rate law, 164
Percolation theory, 59
Perovskite chemistry, 169
Phosphonated polybenzimidazole (PBI), 236
Phosphoric acid fuel cells (PAFC), 18, 147
Planar cells, 18, 35
PLNCG sample, 138
Polarization
curves, 162–163
losses, reduced, 179
method, 80–81
resistance, 176–177, 279
Polymer electrolyte membrane fuel cell
(PEFC), 273
Polymer membrane fuel cells (PEMFCs), 236
Polymorph, 3
Polymorphic structures, 2
Porous cathode, 147, 189
Potential tap, 200
PRIMA program, 120, 122
Proton activation energy, 224
Proton concentration, master curves of, 228
Proton conducting electrolytes, 218
Proton conducting fuel cells (PCFCs), 236
Proton conducting solid oxide fuel cells, 236
Proton conduction
activation/deactivation of electrodes, 247
300 Index
complications, 268
hydration of ordered oxygen deficiency, 230
mechanisms of, 261–264
nomenclature of disordered intrinsic
oxygen deficiency, 231
order–disorder reactions, 232
proton hole mixed conduction, 255
stability, 248
Proton conductivity, 219–230
charge mobility and conductivity of
protons, 224
diffusion, 222
effects of defect–acceptor interactions, 228
grain boundaries, 229
hydration of acceptor-doped perovskites, 219
in oxides, 219
Protonex Technology Corporation, 216
Protonic conduction, 56, 58, 237, 243–244,
246, 251
Protonic defects, 218, 225, 262, 267–268,
270–271
mobility of, 264, 266, 270
Proton mobility, 218, 224–226, 228, 238, 261,
267, 269–271
Proton-phonon coupling, 261
Proton self-localization, 264
Proton solvation shell, 261
Proton transport, 223, 281
Pseudo-cubic lattice, 74
Pseudo-fluorite lattice, 122
Pulsed laser deposition (PLD), 87, 161
Q
Quantum chemistry, 82
Quasi-elastic neutron scattering (QNS), 263, 265
R
Random walk theory, 83, 96
Rapid thermal cycling, 211
Rate-determining step, 155–156, 161–162, 280
Reaction cages, 267
Redox
cycles, 19, 30
property, 11
Reformate gas, 206, 211, 213, 216
REMEDY cycle, 120–121, 123, 129
RIETAN-2000 program, 120, 122, 131
Rietveld analysis, 120–121, 123, 127, 129,
132–136, 138, 140
Rokko Testing facilities, 196
Rossiny, J., 95
Rotational diffusion step, 268
Ruddelsden-Popper compounds, 6, 110, 228
S
Samarium-doped ceria (SDC), 25, 185
Scanning electron microscopy (SEM),
185, 277
Schottky defects, 173
Schottky equilibrium, 99, 233
Sealless planar, 35
Secondary ion mass spectrometry (SIMS),
29, 32, 98, 161
Self-diffusion coefficient, 96–97, 105, 111
Self-thermal sustainability, 38
Short-circuited proton conductor, 276
Shorting effect, 22–23
Sintering, 19, 28–29, 31–32, 36, 78, 118–119,
152, 185, 188–189, 206–207, 219, 233,
257, 285–286, 294
Slater theory, 9
Solid acid fuel cells (SAFCs), 236
Solid oxide fuel cells (SOFCs)
all-perovskite, 15, 179
anode materials for, 168
characteristic features, 18
demerits, 18, 20
first generation, 25, 29, 35
fuel flexibility, 41
high power, 16
hybrid systems, 41
issues for intermediate-temperature
SOFCs, 20
merits, 18, 20
monolithic, 18
stack design, 35
stationary, 40
thin-film, 80, 275
tubular type, 15
zirconia-based, 168
Solid-state reaction method, 12, 131, 138
Space charge layer (SCL), 229–230
Stable oxide scale, 33
Stack development, 190–192, 214–215
Stack modeling, 198–202
Steam-to-carbon ratio, 199
Sticking probability, 223
Stoichiometric compositions, 286
Stoichiometric ratio, 48
Stoichiometric vacancy concentration, 97,
106–108
Strontium, 160, 172, 174, 183, 189
Strontium-doped samarium cobaltite
(SSC), 189
Structural disorder of oxide, 131, 137
Structure diffusion, 137, 261–262, 265
Sulfur poisoning, 21, 29–31, 41, 167–168
Index 301
Superconductivity, 7, 9
Surface exchange coefficient, 98,
104–105, 111
T
Thermal expansion, 118
Thermal expansion coefficient (TEC), 19, 28,
32–33, 79, 152, 158, 161, 169, 293
Thermal stability, 72, 152
Thermodynamic
enhancement factor, 96
parameters, 221
Thermogravimetric analysis (TGA), 107
Thermogravimetry, 221, 230,
253–254
Three-dimensional lattice diffusion, 223
Titanates, 50, 111, 172, 174, 176, 267
Tolerance factor, 3, 47, 110, 152, 226–227
TOTO, 38, 205–206, 208, 216
Tracer diffusion coefficient, 96–98, 104,
109–112
Transmission electron microscopy (TEM),
188, 278
Transport barrier, 162
Triple-phase boundary (TPB), 27, 151, 153,
158, 187
Tungsten bronze anode material, 178
U
Uncombusted hydrocarbons, 11
Undoped perovskites, hydration of, 233
V
Vacancy diffusion coefficient, 97, 104, 106,
108–109, 292
Valence stability, 26–27, 152
Vibrational frequencies, 223
Volumetric power density, 39–40, 205
W
Wagner polarization method, 79
X
X-ray
absorption spectroscopy (XAS), 257
diffraction (XRD), 118–119, 207, 249
power diffractometry, 118
scattering factor, 118
Y
Yashima, M., 117
Y-doping, 253–254
Yokokawa, H., 17
Yttria-stabilized zirconia (YSZ), 17, 20, 109,
151, 167
302 Index