Electrode reaction and electrochemical performance:
Materials and technology development for low temperature SOFC
Ellen Ivers-Tiffée, Dirk Herbstritt, Axel Müller, André Weber
Institut für Werkstoffe der Elektrotechnik IWE, Universität Karlsruhe (TH)
Forschungszentrum Umwelt, Adenauerring 20, 76131 Karlsruhe, Germany
1. ABSTRACT
Fuel Cells are a forward looking technology for a highly efficient, environmental friendly power
generation. The electrochemically active single cell is a multilayer structure consisting of ceramic and
metallic materials with different electrical transport properties. Materials for electrodes should exhibit
electronic and ionic conductivity in combination with porosity and catalytic activity, whereas the
electrolyte has to be a purely ionic conducting and gas tight membrane. All components have to show
a well adjusted thermal expansion behavior, chemical compatibility of material interfaces and
chemical stability in the prevailing temperature and gas atmosphere. The performance and long term
stability of the single cell is significantly increased by the use of suitable materials, a proper design of
the cell and an optimized microstructure at the electrode/electrolyte-interfaces. The application of
appropriate technologies for the production of single cells with optimized microstructure becomes
even more important for highly efficient SOFCs operating in the medium and low temperature range.
2. INTRODUCTION
Starting from the different available fuels like natural gas or heating oil for stationary applications as
well as gasoline or diesel fuel for propulsion, Solid Oxide Fuel Cells offer significant advantages for a
high efficient electrochemical energy conversion. Due to their high operating temperatures in-between
600 °C and 1000 °C a wide variety of fuels can be processed1,2,3 . A substantial progress in materials,
fabrication and system-technology enabled field tests of different SOFC-systems within the last years.
Up to now only small stationary SOFC units in the power range of 1 kW to less than 300 kW have
been realized. But even the small systems are capable of electrical net efficiencies up to 50 % 4,5,6 and
overall efficiencies of about 70 - 90 % (including thermal power). Large pressurized SOFC/gasturbine systems in the MW-range are expected to achieve electrical net efficiencies up to 70 % 7. The
stationary application of SOFC in small combined heat and power generation systems for single
family houses is already on the way to the market, the setup of large scale production capacities within
the next years is expected to result in a significant cost reduction8,6.
41st Battery Symposium, Nagoya 2000
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development
field test
market
large stationary systems (Siemens-Westinghouse)
(2005)
2001*
small stationary systems (Sulzer-Hexis)
(2005)
mobile auxiliary power units (BMW-Delphi)
(2010)
universal SOFC-modules (DOE-SECA)
1970
1980
1990
2000
2010
(estimated)
*confirmed by Sulzer -Hexis
fig. 1: Development status and targets for different SOFC applications
Due to a missing hydrogen infrastructure resp. the technical effort for on board hydrogen production,
which is necessary for PEMFC powered electric vehicles, mobile applications of SOFC attracting
more and more interest. The development of SOFC-based auxiliary power units (APU) for cars, trucks
(BMW / Delphi)9,10 and military applications11 as well as the development of low cost, high power
density SOFC core modules which can be used in stationary as well as in a mobile systems (SECA:
Solid State Energy Conversion Alliance) broadens the areas of SOFC-application and enhances the
demands on SOFC- materials and components.
application:
stationary
mobile (traction)
power density
> 0.25 W/cm²
> 1 W/cm²*
operation temperature
700 to 1000 °C
500 to 700 °C
lifetime (operation)
> 40000 h
> 2000 h
degradation rate
< 1 µV/h
< 10 µV/h*
fuel utilization
> 80 %
> 80 %*
thermal cycles
> 100*
> 5000*
> 1 K/min
> 100 K/min*
natural gas, fuel oil
gasoline, diesel
air
air
< $500/kW
< $100/kW
heating rates
fuel
oxidant
system costs
* estimated
tab. 1: Target areas for mobile and stationary SOFC-systems
The high operating temperature of SOFCs is advantageous concerning the processing of common fuels
and is also required for combined SOFC/gas-turbine power plants. On the other hand significant
disadvantages concerning the long term stability and costs of materials and components as well as the
demand for a short startup time in mobile applications arise.
41st Battery Symposium, Nagoya 2000
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The operating temperature of a SOFC is restricted by thermal activated transport processes and
electrochemical reactions like the oxide ion conductivity of the solid electrolyte and different reaction
steps in the electrodes resp. at the electrode/electrolyte interfaces.
Decreasing the operating temperature generally results in a decreased power density resp. efficiency.
On the other hand, the long term stability of the system is improved and the system costs can be
reduced by using less costly metal alloys for interconnects and external components. Therefore the
improvement of cells and stacks for a medium temperature SOFC operating economically in-between
600 °C and 800 °C is required whereas for temperatures below 600 °C at first new materials, cell
concepts, production technologies and suitable stack design have to be developed.
800°C
high temperature
+ materials available
+ technology existent
+ cell performance
+ fuel processing
o long term stability
dynamic operation
system costs
600 °C
medium temperature
o materials available
o technology existent
o cell performance
o fuel processing
o long term stability*
o dynamic operation*
o system costs*
low temperature
materials available
technology existent
cell performance
fuel processing
+ long term stability*
+ dynamic operation*
+ system costs*
*estimated
fig. 2: Advantages and disadvantages of SOFCs for different temperature ranges
Stationary SOFCs operating in the high temperature range meet the requirements concerning cell
performance. Suitable materials and production technologies are available. The system costs have to
be decreased decisively and the long term stability of cells and stacks has to be improved. Dynamic
operation i.e. fast startup, load- and thermal-cycling as well as redox-cycles due to gas supply failures
are still unsolved problems. For medium operating temperatures (600 – 800 °C) the development
targets concerning power density and efficiency of single cells are almost fulfilled. 12,13,14
3. CELLS FOR HIGH OPERATING TEMPERATURES
A SOFC is a multilayer structure consisting of ceramic and metallic materials. There are different
types of SOFC concepts, basically tubular and planar ones which differ in the single cell design and
arrangement, interconnector materials and gas flow. SOFC materials have to fulfill different
requirements. The electrolyte has to be a gas tight and purely ionic conducting membrane whereas the
electrodes have to enable the transport of electrons as well as of gaseous reactants and reaction
products and therefore have to be porous. The electrode materials should exhibit a high catalytic
activity for the desired chemical and electrochemical reactions. The microstructure of the electrodes
should display a large number of active reaction sites. All materials and components of the stack have
to show chemical stability in the prevailing atmospheres whereupon changes in the gas composition
41st Battery Symposium, Nagoya 2000
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due to fuel and oxygen utilization, fuel supply failures and leakage in the stack have to be taken into
account. The compatibility in-between the different materials i.e. a well adjusted thermal expansion
behavior, chemical compatibility and good adhesion at the interfaces has to be fulfilled.
Materials
electrical
properties
thermomechanical
properties
porosity
strontium doped
lanthanum-manganite
(La,Sr)MnO3
yttria doped
zirconia
(YSZ)
cathode
electrolyte
anode
nickel-YSZ
cermet
el. conductivity
(σel + σion)
thermal expansion
coefficient
catalytic
activity
adhesion
ionic conductivity
σion >> σel
gas-tightness
mechanical stability
catalytic
activity
adhesion
el. conductivity
(σel + σion)
thermal expansion
coefficient
porosity
fig. 3: State of the art materials and required electrical and (thermo-) mechanical properties of SOFCsingle cells.
State of the art materials currently used in most SOFC systems are yttria stabilized zirconia (YSZ) as
the electrolyte which can be used either as TZP (3YSZ: ZrO2 doped with ~ 3 mol% Y2O3) or CSZ
(8YSZ: ZrO2 doped with ~ 8 mol% Y2O3). Although the oxide ion conductivity of TZP is significantly
lower, this material is advantageous because of its outstanding mechanical stability. Strontium doped
lanthanum manganite (LSM) is used as the cathode and nickel/YSZ-cermets as the anode..
cathode
cathode
100
cell 2.1
electrolyte: 8YSZ, 150 µm
cathode: LSM, 30 µm
anode: Ni/8YSZ-cermet, 30 µm
electrode area: 10 cm²
nernst efficiency / %
95
90
85
80
75
0,00
temperature: 950 °C
fuel: H2 + H2O, 0.5 l/min
oxidant: air, 0.5 l/min
fuel utilization:
20 %
30 %
40 %
50 %
60 %
70 %
0,05
5 µm
electrolyte
electrolyte
0,10
0,15
0,20
power density (W/cm²)
anode
anode
0,25
50 µm
5 µm
fig. 4: Nernst efficiency vs. power density at different fuel utilization and SEM-images of an electrolyte
supported single cell with monolayer electrodes.
41st Battery Symposium, Nagoya 2000
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The electrical efficiency (nernst efficiency: cell voltage / open circuit voltage, fuel efficiency not
included) as a function of power density of a planar single cell with monolayer electrodes is given in
fig. 4. This type of cell just fulfills the targets concerning power density (tab. 1).
An investigation of the loss mechanisms in this type of single cells by in situ impedance spectroscopy
as well as the use of reference electrodes revealed that the main part of the losses is governed by the
cathode. In case of pure electronic conducting electrode materials like metals or some perovskite type
oxides (LSM), the electrochemical reactions are almost restricted to the triple phase boundaries (tpb).
The transport of oxide ions within the electrode material is advantageous concerning the number of
possible reaction pathways. Therefore electrodes should be either a composite consisting of an
electronic and an ionic conducting phase or a mixed conducting metal oxide to enlarge the active area
into the electrode volume (fig. 5).
cathode reaction: ½ O2 + 2 e- → O2-
O2
pure
pureelectronic
electronicconductor
conductor
electrons
oxygen
oxygen ions
O2
mixed
mixedelectronic
electronic- -ionic
ionicconductors
conductors
oxygen
oxygenreduction
reductionatatthe
the
triple
phase
triple phaseboundaries
boundaries
oxygen
oxygenreduction
reductionall
allover
over
the
cathode
surface
the cathode surface
fig. 5: Oxygen reduction at a pure electronic, composite and mixed conducting cathode
To increase the cell performance of an electrolyte supported single cell the polarization losses of the
electrodes, mainly of the cathode, have to be decreased. On the one hand the electrochemical
properties of the cathode can be influenced by choosing an appropriate composition. Different
perovskite type oxides ABO3 exhibiting lanthanum and strontium resp. calcium as a dopant on the Asite and transition metals like Cr, Mn, Fe, Co and/or Ni on the B-site have been investigated with
respect to their suitability as a cathode material15,16. For example in the La0.8Sr0.2Mn1-xCoxO3 solid
solution (LSMC), the electrical and oxygen ion conductivity can be increased significantly by
substituting a part of the manganese with cobalt17,18 (fig. 6). Therefore the use of this type of cobalt
containing cathode materials should result in a decreased cathode resistance. On the other hand, a high
amount of cobalt results in an increased thermal expansion coefficient resulting in a delamination at
the cathode/electrolyte interface resp. a cracking of the electrolyte.
41st Battery Symposium, Nagoya 2000
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thermal expansion coefficient
-5 20
-6
12,0
-10
oxygen diffusion coefficient
16
-6 12
8,0
-12
4,0
-14
0,0
0,0
α / (10-6/K)
0,2
0,4
0,6
0,8
8
D / (cm²/s) at 1000°C
TEC / 10-6 K
-8
log (k / (cm/s))
16,0
log (D / (cm ²/s))
electrical conductivity / (104 S/m)
20,0
electrical conductivity
σ / (S /m) at 1000°C
oxygen surface exchange coefficient
k / (cm/s) at 1000°C
-7 4
1,0
X in La0.8Sr0.2Mn1-xCoxO3-d
fig. 6: Electrical and thermomechanical properties of the La0.8Sr0.2Mn1-xCoxO3 solid solution
A well adapted thermal expansion behavior can be reached by using either an alternative mixed
conducting cathode material (fig. 7) or a composite consisting of a mixed conducting cathode material
and an appropriate amount of electrolyte material.
22
TEC / 10-6/K
20
18
16
La0.8Sr0.2Mn1-xCoxO3
La0.8Ca0.2Fe1-xCoxO3
La0.8Ca0.2Mn1-xCoxO3
La1-xSrxCoO3
La1-xCaxFe0,7Co0,3O3
La0,8Ca0,2Mn1-xFexO3
La1-xCaxMnO3
14
12
10
-0,1
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
1,1
x
fig. 7: Thermal expansion coefficient (TEC) of various cathode materials.
An additional problem of LSMC- and similar cathodes is the insufficient chemical compatibility
between these cobalt containing cathode materials and YSZ19. Sintering of the cathode-layer onto the
electrolyte-substrate at temperatures > 1100 °C leads to the formation of secondary phases like
lanthanum- and strontium-zirconate (fig. 8). These insulating interlayers lead to a significant increase
of the cathodic polarization resistance20.
41st Battery Symposium, Nagoya 2000
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Interface
InterfaceLSM/8YSZ
LSM/8YSZ
Interface
InterfaceLSMC/8YSZ
LSMC/8YSZ
(LSM:
)
(LSM:La
La0.80.8Sr
Sr0.20.2MnO
MnO3+δ
3+δ)
(LSMC:
)
(LSMC:La
La0.80.8Sr
Sr0.20.2Mn
Mn0.50.5Co
Co0.50.5OO3+δ
3+δ)
LSMC
LSMC
cathode
LSM
cathode
La2Zr2O7
SrZrO3
8YSZ
electrolyte
1 µm
300 nm
high
highcathode
cathoderesistance
resistance
due
duetotoinsulating
insulating
secondary
secondaryphases
phases
8YSZ
electrolyte
fig. 8: TEM images of cathode/electrolyte-interfaces. The interface between LSM and 8YSZ shows no
secondary phases at all whereas in-between the cobalt containing LSMC cathode and the 8YSZ
electrolyte the formation of secondary phases (lanthanum- and strontium-zirconate) takes place.
screenprinted
LSM-layer
MOD
thin-film
(~ 100 nm)
screenprinted
YSZ-particles
YSZ-electrolyte
10 µm
nanoporous
MOD thin film
(LSM, LSC, LSCM)
8YSZelectrolyte
200 nm
fig. 9: Improved cathode/electrolyte-interface: the effective electrolyte surface area is enlarged by
a) structuring the substrate surface with screenprinted 8YSZ-particles, b) coating the whole electrolyte
surface with a nanoporous electrochemical active MOD (metal organic deposition) LSM, LSCM or
LSC thin film. The screenprinted LSM-layer on top performs only as a current collector and gas
distribution layer.
41st Battery Symposium, Nagoya 2000
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Other possibilities to decrease the cathodic polarization losses are the improvement of the
microstructure at the cathode/electrolyte-interface as well as the enlargement of the effective
cathode/electrolyte interface area. A significant increase of the cell performance is possible using an
improved interface as shown in fig. 921. The use of MOD (metal organic deposition) technology for
applying nanoporous thin film cathodes offers significant advantages. Due to the low processing
temperatures and times (<< 1000 °C, rapid thermal annealing process), the formation of secondary
phases in between cathode an electrolyte is prevented. The application of mixed conducting cathode
materials like strontium doped lanthanum cobaltate (LSC) becomes possible 22 .
The performance of electrolyte supported single cells made of a 150 µm thick 8YSZ-substrate and a
Ni/YSZ-anode with different types of cathodes is shown in fig. 10. In spite of the advantageous
material properties of LSC, which exhibits a significantly higher electronic and ionic conductivity than
LSM (fig. 6), the cell with a monolayer LSC-cathode shows lowest performance. Applying a
structured electrolyte surface and a LSM MOD thin film cathode a decisive enhancement in
performance was achieved. In case of a LSC MOD thin film cathode, an additional increase in
performance is possible.
temperature: 950 °C
fuel: H2, 0.5 l/min
oxidant: air, 0.5 l/min
electrolyte: 8YSZ, 150 µm
anode: Ni/8YSZ-cermet, 30 µm
electrode area: 10 cm²
efficiency (Ucell/Uth) / %
1,00
0,80
0,60
screenprinted LSC-cathode on smooth YSZ-surface
screenprinted LSM-cathode on smooth YSZ-surface
screenprinted LSM-cathode on structured YSZ-surface
LSM-MOD-cathode on structured YSZ-surface
LSM-MOD-cathode on structured YSZ-surface
0,40
0,20
0,0
0,2
0,4
0,6
0,8
1,0
1,2
power density / (W/cm²)
fig. 10: Efficiency vs. power density of electrolyte supported single cells with different cathodes
Besides an increase in performance, single cells with this type of optimized cathode/electrolyteinterface show an enhanced long term- and thermocycling stability. Whereas a monolayer cathode
shows extensive delamination after a few thermocycles, the three dimensional penetration structure
between electrolyte and cathode inhibits any significant delamination. The thermocycling stability i.e.
the increase in cell resistance due to thermal cycling is shown in fig. 11.
41st Battery Symposium, Nagoya 2000
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current density at 0.7 V cell voltage
0,8
improved cathode
standard cathode
cathode
0,6
electrolyte
10 µm
0,4
cross section of a single cell with monolayer
cross section of a single cell with monolayer
LSM-cathode after 5 thermocycles
LSM-cathode after 5 thermocycles
0,2
cathode
electrolyte
anode
0,0
0
2
4
6
thermocycle
8
200 µm
10
fig. 11: Decrease in cell performance due to thermal cycling of single cells during operation. Whereas
the cell with a standard monolayer cathode fails after 5 thermocycles, the improved cathode shows an
enhanced stability. (The decreased cell performance includes degradation of electrolyte and anode.)
Nevertheless degradation is a serious problem at high operating temperatures. Depending on the
operating conditions, a gradient in oxygen chemical potential at the cathode/electrolyte-interface
(LSM-cathode/YSZ-electrolyte) might induce a demixing of the materials resulting in microstructural
changes and a minor adhesion. Even at moderate operating conditions (850 °C, 200 mA/cm² for 4500
hours) a depletion of yttrium and an interdiffusion of manganese as well as the formation of
micropores at the LSM/YSZ-interface was observed 23.
Zr
Zr
ZrOx
ZrOx
3
4
O
6
YSZ
YSZ
3
5
Zr
Mn
Zr
Zr
4
Zr
Zr
Mn
LSM
O
Zr
6
5
200 nm
Zr
Zr
O
Mn
cell: LZ 6
4500 h at 200 mA/cm², 850 °C
Y
Zr
O
Mn
Y
Zr
fig. 12: TEM and EDX-analysis of the LSM/8YSZ-interface after 4500 h of operation at 850 °C and
200 mA/cm² revealed Mn-diffusion and Y-depletion at the surface of the 8YSZ-electrolyte.
41st Battery Symposium, Nagoya 2000
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4. CELLS FOR MEDIUM AND LOW OPERATING TEMPERATURES
Due to significant problems concerning long term stability of cells and other stack components for
SOFC-systems operating at high temperatures, an operating temperature in between 600 °C and
800 °C exhibits many advantages. The temperature dependence of cell voltage and losses of an
electrolyte supported single cell are shown in fig. 13. This type of standard cell, consisting of an 8YSZ
electrolyte with screenprinted LSM monolayer cathode and Ni/YSZ cermet anode, exhibits
insufficient performance operating at temperatures < 850 °C.
1,0
electrolyte supported
electrolyte supported
single
singlecell
cell
cell voltage
(current density: 100 mA/cm²)
LSM-cathode: 50 µm
voltage / V
0,8
(LSM: (La,Sr)MnO3)
8YSZ-electrolyte: 150 µm
• increased long term stability
• decreased system costs
0,6
Ni/8YSZ-anode: 50 µm
(8YSZ: 8 mol% Y2O3 doped ZrO2)
0,4
∆Ucathode
0,2
0,0
950
∆Uanode
∆Uelectrolyte
900
850
800
750
700
650
temperature / °C
fig. 13: Cell voltage of an electrolyte supported single cell (150 µm 8YSZ electrolyte substrate,
screenprinted monolayer electrodes) at a constant current density of 100 mA/cm² as a function of
operating temperature (a sufficient power density is achieved at an average current density of about
300 mA/cm²).
Operating temperatures below 800 °C lead to an intolerable increase in the internal cell resistance. For
reduced operating temperatures the polarization losses, especially at the cathode, and the ohmic losses
in the electrolyte have to be to be lowered substantially.
The electrolyte resistance can be reduced using an alternative electrolyte material with a higher oxide
ion conductivity than YSZ or/and an electrode supported thin film electrolyte. Beneath a high oxide
ion conductivity additional requirements for medium and low temperature solid electrolytes have to be
fulfilled. The material must exhibit a pure oxide ion conductivity i.e. no electronic conductivity and
structural as well as mechanical stability in a wide temperature and oxygen partial pressure range. In
fig. 13 the conductivity of various candidate materials for solid electrolytes which have been studied
by many research groups intensively is given as a function of temperature.
41st Battery Symposium, Nagoya 2000
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T / °C
σ / (S/m)
100
1100 1000
900
800
700
600
500
LSGM, (La,Sr)(Ga,Mg)O3
ScSZ, Sc doped ZrO2
GCO, Gd doped CeO2
8YSZ, 8 mol% Y2O3 doped ZrO2
3YSZ, 3 mol% Y2O3 doped ZrO2
10
1
0,7
0,8
0,9
1,0
1,1
1,2
1,3
1000 K / T
fig. 14: Oxide ion conductivity of different electrolyte materials 24,25
Ce-based oxide ion conductors like Gd doped CeO2 (GCO) offer a significantly higher ionic
conductivity but they become mixed conductors in the anode environment resulting in an internal
short circuit of the cell. Therefore a decreased open circuit voltage (OCV) and an additional fuel
utilization even under OCV-conditions takes place resulting in a decreased system efficiency26.
Nevertheless the application of GCO-electrolytes at operating temperatures < 700 °C is possible. Due
to the excellent chemical compatibility with mixed conducting cathode materials, the power density
and efficiency of a GCO-electrolyte based cell might be sufficient (fig. 15).
0.600
start
169 hours
0.85
0.80
0.500
0.75
Voltage (V)
0.70
0.400
0.65
0.60
0.300
0.55
0.50
0.200
0.45
0.40
Cell test
Anode
Cathode
Electrolyte
T-sinter
Area
0.100
0.35
0.30
0.000
0.0
0.2
0.4
0.6
0.8
1.0
Current density (A/cm²)
fig. 15: C/V-characteristics of a GCO-electrolyte based single cell at 700 °C (ECN)
41st Battery Symposium, Nagoya 2000
page 11of 16
:
:
:
:
:
:
LT cell 82
commercial, SP
RP LSCF FC-7 SP
10GCO 1300/8 177µm
1200°C
9.6 cm2
Temperature : 700°C
Oxidant
: Air, 500 ml/min
Fuel
: Hydrogen, 375 ml/min
Power density (W/cm2)
0.90
An other possibility is the use of double- or multilayer electrolytes. To take advantage of the
compatibility of Ce-based electrolytes with mixed conducting cathodes materials and the mixed
conducting properties of doped CeO2 in an anode environment, interlayers resp. composite or cermet
electrodes containing doped CeO2 can be used (fig. 16) 27,28.
temperature / °C
1
1000
950
900
850
800
750
700
1 µm
LCFC/GCO-cathode
log (σcathode ⋅ Ωcm²)
LSM-cathode
LCFC-cathode
GCO-coating
8YSZ electrolyte substrate
Ni/8YSZ-anode
LC
cat FC
hod
e
GC
elec O
trol
yte
0
LSM-cathode
8YSZ electrolyte substrate
Ni/8YSZ-anode
-1
0,80
0,85
0,90
0,95
1,00
1,05
LCFC: (La,Ca)(Fe,Co)O3
LSM: (La,Sr)MnO3
1000 K / T
fig. 16: Temperature dependence of the cathode resistance of single cells with electronic conducting
cathode (screenprinted LSM-layer on 8YSZ-substrate) and mixed conducting cathode (screenprinted
LCFC-layer + GCO-interlayer on 8YSZ-substrate)
YSZ
GCO
LSGM
ScSZ
excellent stability in
good compatibility with
good compatibility with
excellent stability in
oxidizing and reducing
cathode materials
cathode materials
oxidizing and reducing
environment
mixed electronic-ionic
environment
excellent mechanical
conductor at low pO2
better long term stability
stability (3YSZ)
(application in anode
than 8YSZ ?
> 40.000 h of fuel cell
cermets)
operation possible
high quality raw
materials available
low ionic conductivity
electronic conduction at
Ga-evaporation at low
availability and price of
(especially 3YSZ)
low pO2 → low OCV
pO2
scandium
incompatible with some
mechanical stability
incompatible with NiO
cathode materials
mechanical stability
tab. 2: Advantages and disadvantages of possible electrolyte candidates for SOFC
Lanthanum Gallate (LSGM: (La,Sr)(Ga,Mg)O3) based electrolytes exhibit the highest oxide ion
conductivity of possible electrolyte materials for SOFC so far29,30. LSGM also shows an excellent
41st Battery Symposium, Nagoya 2000
page 12of 16
chemical compatibility with mixed conducting cathode materials. The long term stability due to Gaevaporation in a reducing atmosphere as well as the compatibility to Ni resp. NiO seems to be a severe
problem at high operating temperatures31. The use of LSGM-electrolyte-substrates contains problems
due to the low mechanical stability of LSGM and the high costs of gallium. LSGM might be an
interesting electrolyte material for low operating temperatures (< 600 °C).
With respect to all secondary requirements for an electrolyte material, it is still questionable if zirconia
based electrolytes can be replaced. A way out might be doping zirconia with scandium instead of
yttrium. Scandium doped ZrO2 (ScSZ) exhibits a significantly higher ionic conductivity than YSZ.
Sc-doped ZrO2 electrolytes are the most promising alternatives for medium and even low operating
temperatures32. But up to now the price and availability of scandium is uncertain and there is no high
quality ScSZ-powder available yet.
Another possibility to increase the cell performance at decreased operating temperatures is a reduction
of the electrolyte thickness. For electrolyte supported cells the minimum thickness is in between
150 µm and 250 µm for CSZ and ceria based materials. Due to the outstanding mechanical stability of
TZP, a minimum thickness of only 50 µm for TZP seems to be possible. In case of an supported thin
film electrolyte, a thickness of a few microns is possible even for large cell areas12. The voltage losses
in the electrolyte at an acceptable current density of 300 mA/cm² as a function of temperature and
electrolyte thickness are shown fig. 17 for 8YSZ and LSGM.
8YSZ
350
LSGM*
1 µm
m
1µ
100
µm
150
10
15
0
µm
15
0
200
10 µ
m
µm
250
(300 mA/cm²)
voltage losses / mV
300
maximum
50
∆Uelectrolyte
0
1000
900
800
700
600
500
400
300
temperature / °C
* conductivity values: T. Ishihara et al., Proc.
5th
Intl. Symp. SOFC, The Electrochemical Society, 301-310, (1997).
fig. 17: Electrolyte losses as a function of operating temperature and electrolyte thickness for 8YSZ
and LSGM. (current constriction ignored)
Accepting electrolyte losses < 50 mV, the lowest operating temperature of electrolyte supported single
cells (LSGM or ScSZ electrolyte substrate, 150 µm thickness, ) is about 750 °C whereas a supported
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thin film electrolyte theoretically exhibits an undermost operating temperature of less than 500 °C.
Including the polarization losses at the electrodes a sufficient cell performance can be achieved at
about 650 °C to 750 °C using the state of the art materials (fig. 18).
T:
FZ Jülich
T:650
650°C
°C
LSM / 8YSZ / Ni-8YSZ-cermet
anode supported thin film electrolyte (~20 µm)
substrate size: 50x50 mm²
electrode area: 16 cm²
cell voltage / V
1,2
1,0
fuel: 1 l/min H2 + 30 ml/min H2O
oxidant: 1 l/min air
0,8
Mo578
0,6
0,4
fuel: 0.5 l/min H2
oxidant: 0.5 l/min Air
0,2
0,0
Z1.4
0,1
Siemens St. 2.4.1 PEN
LSM / 8YSZ / Ni-8YSZ-cermet
electrolyte supported (~150 µm)
substrate size: 50x50 mm²
electrode area: 10 cm²
0,2
0,3
0,4
current density / (A/cm²)
fig. 18: Current / voltage characteristics of electrolyte and anode supported single cells consisting of
state of the art materials (LSM / 8YSZ / Ni-8YSZ) at 650 °C.
The different electrolyte and electrode supported planar single cell concepts are shown in fig. 19.
800
800- -1000
1000°C
°C
600
600- -800
800°C
°C
600
600- -800
800°C
°C
interconnector: metal
interconnector: metal
interconnector: metal
electrolyte
electrolytesupported
supported
anode
anodesupported
supported
cathode
cathodesupported
supported
cathode:
50 µm
electrolyte: >150 µm
anode:
50 µm
50 µm
cathode:
electrolyte:
anode:
< 20 µm
500 - 1500 µm
anode:
electrolyte:
50 µm
< 20 µm
cathode: 300 - 1000 µm
fig. 19: Planar single cell concepts
The most common is the anode supported one (Allied Signal, FZ-Jülich and many others), the most
advanced tubular Siemens-Westinghouse concept is based on a cathode supported electrolyte. An
advantage of all cathode supported single cells is a three dimensional penetration structure in between
cathode and electrolyte which is achieved during electrolyte deposition. Even with state of the art
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materials in combination with a ScSZ thin film electrolyte, polarization losses can be reduced
significantly at medium operating temperatures.
5. CONCLUSION
The optimum choice of materials, technology and single cell design for SOFC depends significantly
on the stack design and the system requirements. Even at high operating temperatures, the well proven
tubular Siemens Westinghouse concept has to be followed by an alternative SOFC concept due to the
demands on cost reduction and power density increase. For medium and low operating temperatures in
small scale stationary and mobile application the following issues can be stated:
§
With respect to the electrical performance, electrolyte supported single cells with ScSZ electrolyte
and custom tailored electrode/electrolyte interfaces using well established anode cermets and
mixed conducting cathode materials are promising to temperatures as low as 750 °C.
§
Below 750 °C thin film electrolytes (probably still zirconia based) have to be applied.
§
The interfaces between both electrodes and the thin film electrolyte have to be optimized for low
polarization losses independent of the chemical composition of the electrode materials.
§
Long term degradation effects due to interdiffusion and demixing will be less important but have
to be considered as well.
§
With respect to thermomechanical failures, all kinds of thin films, electrode/electrolyte-interfaces
and support materials have to be well adapted in their thermal expansion behavior and mechanical
properties.
§
The dynamic properties of cells and stacks i.e. electrical loading, fast startup/cool down
procedures and the redox stability of the anode has to be improved.
§
The requirements for single cells and stacks are determined by the application. Stationary and
mobile SOFC systems have to fulfill different properties, materials, cells and stacks have to be
custom tailored.
6. ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support from the Deutsche Forschungsgemeinschaft
(DFG). The MOD thin films for the improved cathode/electrolyte-interface have been developed by
Dr. Uwe Guntow from the Fraunhofer Institute ISC in Würzburg. Discussions with Prof. Yamamoto
and Prof. Ishihara about oxide ion conductors have been very helpful.
7. REFERENCES
1
K. Ahmed, K. Föger, Fourth European Solid Oxide Fuel Cell Forum, 69 (2000)
2
R.H. Cunningham, R. Ormerod, Fourth European Solid Oxide Fuel Cell Forum, 77 (2000).
3
A. Schuler et al., Fourth European Solid Oxide Fuel Cell Forum, 107 (2000).
4
J. Sukkel, Fourth European Solid Oxide Fuel Cell Forum, 159 (2000).
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5
F. Foger, Fourth European Solid Oxide Fuel Cell Forum, 167 (2000).
6
R. Diethelm et al., Fourth European Solid Oxide Fuel Cell Forum, 183 (2000).
7
S.C. Singhal, The Electrochemical Society, 99-19, 39 (1999).
8
SMM Schweizer Maschinenmarkt, 28 (1999)
9
http://www.delphiauto.com/index.cfm?location=1831
10
J. Trachtler, SAE 2000 World Congress, 1505 (2000).
11
H.H. Dobbs et al., Fourth European Solid Oxide Fuel Cell Forum, 85 (2000).
12
G. Schiller et al., Fourth European Solid Oxide Fuel Cell Forum, 37 (2000).
13
K. Honegger et al., Fourth European Solid Oxide Fuel Cell Forum, 29 (2000).
14
G. Maxwell et al., Fourth European Solid Oxide Fuel Cell Forum, 3 (2000).
15
O. Yamamoto et al., Solid State Ionics 22, 241 (1987)
16
H. Fukunaga et al., Fourth European Solid Oxide Fuel Cell Forum, 601 (2000).
17
M. Schiessl et al., Materials Science Monographs 66, 2607 (1991).
18
B.C.H. Steele, Solid State Ionics, 86-88 6S, 1223 (1996).
19
H. Yokokawa et al., Solid State Ionics 52, 43 (1992)
20
E. Ivers-Tiffée et al., Proc. 14th Risoe Int. Symp. on Mat. Science, 69 (1993).
21
D. Herbstritt et al., The Electrochemical Society, 99-19, 972 (1999).
22
D. Herbstritt et al., Fourth European Solid Oxide Fuel Cell Forum, 697 (2000).
23
A. Weber et al., The Electrochemical Society of Japan, Vol. 64 No. 6, 582 (1996).
24
R. Männer et al., 2nd Int. Symp. on SOFC - CEC Publ. EUR 13546 EN, 715 (1991).
25
E. Ivers-Tiffée et al., The Electrochemical Society, 95-1, 1039 (1995).
26
M. Gödikemeier et al., J. Electrochem. Soc. 145-2, 414 (1998).
27
C.C. Chen et al., The Electrochemical Society, 93-4, 598 (1993).
28
E.P. Murray et al., Nature 400 - 6745, 649 (1999).
29
M. Feng et al., Eur. J. Solid State Inorg. Chem. 31, 663 (1994).
30
T. Ishihara, J. Electrochem. Soc. 141, 3444 (1994).
31
K. Yamaji et al., Solid State Ionics 121, 217 (1999).
32
O. Yamamoto, Electrochimica Acta 45, 2423 (2000).
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