1. Chap.12 Solid Oxide Fuel Cells (SOFCs)

2. 12.1 Introduction
SOFC
- The highest operating temperature (about 1000℃)
- Coal-derived gases and natural gas as the primary fuel
- “The third-generation fuel cell technology”
- The first solid-state oxygen ion conductor : (ZrO2)0.85(Y2O3)0.15
discovered by Nernst
- Research breakthrough by Westinghouse electric Corp (1958).
- In 1980s, monolithic cell structure and planar cell structure were
developed and demonstrated for higher power densities than the tubular
design.
- High operating temperature → to ensure adequate ionic and electronic
conductivity
- Not only oxygen ion conducting but also proton conducting is possible

3. - Advantages
٠ All solid component a simpler concept, design and construction
electrode-electrolyte interface (two phase) no electrolyte depletion
and no severe corrosion of the cell/stack
٠ Because of high operating temp., electrochemical kinetics at electrode
fast → non-noble metal catalyst can be used.
٠ High temp. → make it possible for the internal reforming of methane
٠ Ability to tolerate the presence of impurities in the reactant gas streams
٠ The high operating temperature provides a better system; it provides
high-quality waste heat for co-generation applications and bottoming
cycles utilizing conventional steam or gas turbines.
→ high-energy conversion efficiency (over 70 %)
- Disadvantages
٠ high temp. → Low value of reversible cell potential
very few appropriate materials

4. ٠ The current trend is to lower the operating temperature.
low temperature : 550 ~ 650 ℃
intermediate temperature : 650 ~ 850 ℃
The overall energy conversion efficiency is greater than 50 %;
the efficiency as high as 65% is possible, corresponding to cell
performance of 0.75 V without the conventional bottoming turbine systems.
SOFC can be used primarily for electric utility applications, and is also being
considered as power systems for trains and large surface ships.

5. 12.2 Basic Principles and Operations
Fig 12.1 Schematic of a solid oxide fuel cell (SOFC) illustrating its operational principle.
O2 at the cathode reacts with electrons to form oxide ions
1/2O2 + 2e- → O2=
The oxide ions transport through the electrolyte and reach the anode
At the anode, H2 reacts with oxide ions to form water and release electrons
The electrons migrate through the external circuit to reach the cathode.
H2 + O2= → H2O +2e-
The overall cell reaction is given by
1/2O2 + H2 → H2O + waste heat + electric energy

6. If CO is supplied at the anode,
CO + O2= → CO2 + 2e-
then, overall cell reaction becomes
CO + 1/2O2 → CO2 + waste heat + electric energy
Clearly in SOFC, CO is utilized as a fuel.
Coal-derived gases or natural gases are used as the fuel.
aH2 + bCO + (a+b)O2= → aH2O +bCO2+2(a+b)e- : anode reaction
1/2(a+b)O2 + 2(a+b)e- → (a+b)O2= : cathode reaction
½(a+b)O2 + aH2 + bCO → aH2O + bCO2 + waste heat + electric energy
: overall reaction
For solid oxide fuel cell, catalysts are not required for the steam reforming
of natural gas. However, in reality water vapor is required to avoid carbon
formation via the mechanisms of Boudouard reaction.

7. The actual reaction can occur at the electrode-electrolyte interface.
Fig 12.2 Schematic of charge transfer process in two types of electrode: (a) electronically conducting and (b) mixed conducting cathodes.
The actual reaction can occur at the electrode-electrolyte interface.
However, if the electrode is made to conduct both electronically and ionically,
the reaction can also occur throughout the entire porous electrode.
(mixed conducting electrode).

8. 12.3 Cell components and configurations
Single cell
The SOFC consists of a solid electrolyte and two electrodes.
Because all the cell components are solid, the SOFC can be made into rich
geometrical configurations (tubular, monolithic, and planar).
• Tubular SOFC
Fig 12.3 Schematic of tubular solid oxide fuel cells : (a) current collection at the cylindrical base and (b) current collection along the circumferential cylindrical surface.

9. High operating temperature, thermal cycling → the cell components should
have similar thermal expansion coefficient to avoid thermal crack.
Cylindrical shape of the cell → better resistance to the thermal stress
damage to the composite structure.
The corresponding current collection is made either at the base of tube
or along the curved cylindrical surface.
▪ For base current collection design
Large diameter tube can be used. However, the tube length is limited to avoid
excessive ohmic polarization. → Low power density (typically 1 cm)
With this design, maximum power density: (a) 0.3 W/cm2 using hydrogen, oxygen
(b) 0.2 W/cm2 using reformate gas, air
▪ For circumferential current collection design
The current follows the path of the cylindrical curved surface. → Tube diameter
should be small (typically 1.5 cm)
- The electrode layers can not be too thin, in order to keep ohmic loss

10. While tube can be very long, over 1 m.
Have higher volumetric power density than the base current collection design.
The corresponding current collection is made either at the base of tube
or along the curved cylindrical surface.
The power density in terms of the active electrode surface area → not increased.
A thick support tube on the inner side is employed to provide the structural support
However, the thick support tube is heavy →affects the specific power density in terms of volume, weight, and high cost

11. the electrode thickness.
• Monolithic SOFCs
- Electrons may flow a significant length along the electrode instead of across
the electrode thickness.
- In this design, no support material or structure is needed.
- The ceramic cell components support each other mutually.
Consequently, cell performance on a unit active surface-area basis may not
be improved significantly compared to the tubular design.
Fig 12.4 Schematic of co-flow type
monolithic SOFC.

12. - The power density on a volumetric basis → very high
(because of the highest active surface area density per-unit volume)
Power density of monolithic SOFCs (8.08 kW/kg, 4000 kW/m3)
(while tubular SOFCs : 0.1 kW/kg, 140 kW/m3)
For the monolithic design, the reactant flow can be easily arranged in
co-flow and crossflow.
• Planar SOFCs
Flat cell structure (able bipolar arrangement) → so-called planar design
The bipolar current collection provides the least ohmic resistance to the
transport of oxide ions and electrons.
→ Best cell performance per active cell-surface area
Power density : (a) 0.48 W/cm2 (hydrogen, air)
(b) 0.9 W/cm2 (hydrogen, oxygen)
A volumetric basis the performance of planar design is
between tubular and monolithic design.

13. → a large ohmic overpotential
(∵ the volumetric active cell surface area density is between the other two
designs.)
The thick electrolyte layer → the high resistance to the oxide ion transport
→ a large ohmic overpotential
Electrode-support cell structures → high resistance to mass transport
→ large concentration overpotential
Table 12.1 Comparison for the three different SOFC cell configurations

14. Table 12.2 A summary of typical dimensions of cell components for three different configurations of solid oxide fuel cells
Table 12.3 A comparison of planar SOFC cell configurations for high- and lower temperature SOFCs.

15. Stack
The cell to cell electrical connection is quite different, depending on the cell
structure described earlier.
• Tubular SOFC stack
- The cell-to-cell electrical connection : interconnection contact
All the cells in the same rows are connected in parallel between the anodes
by using a nickel felt
Parallel-series connection increases the stack reliability, preventing the stack
total failure due to the failure of individual cells in the stack.
Tubular cell stack design avoids the difficult task of gas-tight sealing at such
high temperature.
The oxidant preheat is achieved by burning off the exhaust fuel stream
before the cell inlet.
- Fuel supply : through external manifolding, oxidant supply : internal manifolding

16. Fig 12.5 Schematic of tubular SOFC involving circumferential current collection: (a) The cross-sectional view of the stack and (b) side view of a tubular cell in the stack.

17. Fig 12.6 Schematic of tubular SOFC stack showing the manifolding arrangement for the fuel and oxidant stream.

18. • Monolithic and Planar SOFC stack
In both monnolithic and planar SOFC, the reactant steams pattern:
(a) co-flow (b) crossflow
Co-flow arrangement : 1. easy for internally manifolded stack
2. more elaborate for external manifolding
Due to ceramic components, in planar or monolithic configuration
→ require exact matching of thermal expansion between the electrode and
electolyte layers.
- Stack length limitation : 20 ~ 25 cm
Fig 12.7 Planar SOFC stack (Photo Courtesy of Versa Power Systems).

19. System
BOP in SOFC power system : fuel processing, oxidant conditioning,
thermal management, and power conditioning unit.
Fuel processing is simple (compared to the lower-temperature fuel cells)
(∵ due to high tolerance to various impurities.)
Oxidant conditioning : the preheating of the air from the ambient to the fuel cell
inlet temperature.
Thermal management : primarily based on the process air flow
1.waste heat in the exhaust air stream heat form the combustion
with the anode exhaust gas
Used for incoming air-stream preheating
Heat exchanger should be integrated!!!

20. 12.4 Materials and manufacturing
The integration of the SOFC with conventional steam or gas turbine power
generation system → extremely high energy efficiency
12.4 Materials and manufacturing
Cathode
The cathode material : La1-xSrxMnO3, x= (LSM)
LSM : 1. p-type semiconductor
2. high electronic conductivity for low ohmic polarization
3. bulk electronic conductivity of La0.5Sr0.5MnO3 : 294 S/cm at 1000 ℃
4. effective electronic conductivity of LSM : 100 S/cm at 1000 ℃
(∵porous nature of the electrode structure)
5. Good catalytic property and dimensional stability during the fabrication
6. thermal expansion coefficient : 1.2 ×10-5 cm/(cm∙℃), larger than
electrolyte.

21. Efforts are being made to reduce this difference in the thermal expansion
coefficients in order to reduce the thermal stress and thermal cracking
Other materials
Metals may be a choice. However only noble metals may be used as metal
cathode due to the highly oxidizing environment of the cathode.
▪ gold and silver : melting and sintering problem
▪ palladium : high vapor pressure in SOFC operating temperature
▪ platinum remains a suitable metallic material → serious cost implications
Another alternative is to embed metal wires in a matrix of an oxide material
(metal wire : current collector, porous oxide material : the path for oxygen
transport and acts the electrocatalyst)
▪ Metal wires are invariably made of noble metals such as platinum wire
embeded in porous zirconia → cost problem

22. - Most attractive cathode electrode materials: the oxides with mixed conduction
▪ For such electrodes, the entire electrode structure the electrocatalytic sites
for the electrode charge transfer reaction → electrode polarization ↓
▪ In this sense, LSM is the best available cathode material.
(it conducts electrons and oxygen anion)
Anode
The anode material : a nickel-zirconia cermet or a mixture of nickel and YSZ
Ni+YSZ : 1. a high tolerance to sulfur impurity in the fuel stream
2. 20 ~ 40% porosity for the mass transport of reactant and product
3. Ni : electrocatalyst for anode reaction, electronic conductor for
the electrons produced at the anode
4. YSZ : porous support, sintering inhibitor for the nickel metal
conducts oxygen oxides → making the entire anode electrode
with mixed conduction and increasing significantly the active
anode surface area → minimizing anode electrode polarization

23. 5. Nickel in the mixture must be at least 30% by volume
→ in order to maintain electronic conductivity
6. Nickel has a high thermal expansion coefficient (50% higher)
7. Optimal nickel content : 35 vol%
8. Anode electronic conductivity (Ni-YSZ) : about 1000 S/cm at 1000 ℃
9. Noble metal (platinum) can be used as anode material,
not practical from economic considerations.
Electrolyte
The electrolyte material : YSZ (ZrO2 doped with 8-10 mol% Y2O3)
YSZ : 1. conductivity for oxygen oxides over a wide range of O2 partial
pressures without electronic conductivity ( atm)
2. resists the interdiffusion of cations, Mn and La, into the electrolyte
structure.
3. very dense with low gas permeability to prevent reactant gas crossover

24. 4. YSZ layer with 92~93% theoretical bulk density has a hydrogen
permeability of less than 10-8 cm2/s.
5. YSZ is highly stable in both reducing and oxidizing environment
6. YSZ has low ionic conductivity , about 0.02 S/cm at 800 ℃ and
0.1 S/cm at 1000 ℃.
7. SOFC electrolyte must be made as thin as possible
to keep the ohmic loss at acceptable levels.
8. Electrolyte structure can be made about 40 μm thick by EVD, tape casting et.
9. Due to the low ionic conductivity of the electrolyte, SOFC is operated at
a high temperature.
10. A lower-operating temperature (600~800℃) can still provide fast electrode
kinetics and the potential for internal reforming of hydrocarbon fuels
→ The lower-temperature solid oxide fuel cell represents the direction for
current and future development.

25. Interconnection
For tubular SOFCs, bipolar current collection is not possible,
the edge current collection or cell-to-cell electrical connection is accomplished by
the so-called interconnection (Fig. 12.3b)
- The material for the cell interconnection : LaCr1-xMgxO3 (x= )
Lanthanum chromite perovskite phase is stable at a high temperature of around
1000 ℃ and for a wide range of oxygen concentrations (for oxygen partial
pressure PO2 in the range of bar).
Pure LaCrO3 : 1. p-type conductor
2. a low electronic conductivity (0.6 S/cm at 1000 ℃ in air)
3. By small proportion Mg doping, the electronic conductivity
increases significantly ( only 2 S/cm at 1000 ℃)
Noble metal → expensive, other metal →some nickel alloys → thermal expansion
coefficient problems.

26. Table 12.4 Evolution of typical material for tubular SOFC components

27. Table 12.5 Evolution of typical material for tubular SOFC components

28. 12.4.5 Fabrication techniques
For tubular SOFC configuration
1. The support tube is first made of calcia-stabilized zirconia by extrusion into a
cylindrical shape.
2. One end of the tube is plugged to seal the end.
3. Followed by sintering
4. On this support, a slurry of Sr-doped LaMnO3 power is deposited by using
slurry coating.
5. The resulting composite tube is sintered to produce a thin layer of cathode
electrode with a desired thickness.
6. The cell interconnection is deposited on the support tube-cathode structure
by EVD.
7. The electrolyte layer is made through EVD by providing the zirconium-and
yttrium-chloride vapor in a desired ratio to the outer surface of the cathode
layer.

29. 8. On the outer surface of the dense and uniform electrolyte layer nickel powder
slurry is deposited by slurry coating.
9. Yttria-stabilized zirconia is impregnated by EVD to form the nickel-zirconia
cermet anode layer.

30. Table 12.6 Summary of components, structural characteristics, materials and fabrication processes for the tubular SOFC

31. Fig. 12.8 Flowchart for the fabrication process of the tubular SOFC.

32. For monolithic SOFC configuration
▪ For the tape-casting process
1. A slurry for each cell component is made by mixing the ceramic powder with
organic binders, plasticizers, and solvents.
2. A thin layer of each cell component is cast one layer on top of another and
the layer thickness is controlled by using the knife edge of a doctor blade.
▪ For the tape-calendaring process
1. A thin, flat tape for each cell component is produced by rolling
through a two-roll mill
2. The resulting three tapes for the anode, electrolyte and cathode are laminated
by rolling through a second two-roll mill.
3. The composite tape is corrugated through molding and stacked into
the desired fuel cell stack configuration (Fig. 12.4).
4. Sintering the stack in one piece at a desired temperature completes the stack
fabrication process.

33. For planar SOFC configuration
1. Each cell component is tape cast and sintered separately at the respective
optimal temperature.
2. The cell components are assembled into the cell or stack unit as desired.
※ Fabrication process for the planar SOFC is much easier, without the risk
posed by the co-firing of all the cell components together.
※ However, the assembly process becomes more time-consuming and complex.
Fig Flowchart for the fabrication process of the planar SOFC.

34. 12.5 Performance of SOFCs
Performance may differ significantly for the three SOFC configurations.
SOFC performance :
(12.9)
Where the reversible cell potential is given by Nernst eq.
(12.10)
Where n=2 represents the number of electrons transferred during cell reaction.
The reversible cell potential would be the mixed electrode potential due to
the electrooxidation of hydrogen and carbon monoxide at the anode.
The electrooxidation of carbon monoxide is fairly slow, and the oxidation of
carbon monoxide is mainly via the water-gas shift reaction to convert into
hygrogen gas.

35. At the SOFC operating temperature of 1000 ℃,
٠The polarization due to the electrode activation
and mass transport (small)
٠The ohmic polarization (large)
∵ The polarization curve for the SOFC operating near the temp. of 1000 ℃ is
almost linear for the cell IV relation.
Electrochemical reaction at both the anode and cathode can be represented by
the Butler-Volmer equation with a symmetry factor of about 0.5. Then the
activation polarization for the anodic and cathodic reaction can be written, as
(12.11)
The exchange current density is estimated to be (with hydrogen, air)
(12.12)

36. cell operating temperature.
Eq would imply that the activation overpotential would increase with
cell operating temperature.
The ohmic polarization is contributed by the resistance of the cathode,
anode, electrolyte, and interconnection.
ex) tubular SOFC
٠ large contribution (65%) from cathode (∵long current path along the surface,
about 1.1 cm)
٠ second large contribution (25%) from anode (∵long current path, about 0.8 cm)
٠ small contribution from electrolyte, interconnection (∵respective thickness)
Table 12.7 Relative contribution of cell component to total ohmic polarization at 1000 ℃ for tubular SOFC

37. the thickness of each layer.
ex) planar SOFC
٠ Since for planar geometry, the current density can be taken as uniform and along
the thickness of each layer.
٠ The large resistivity of the electrolyte dominated the ohmic polarization.
٠ Total area specific resistance : 0.33 Ω·cm2 for tubular configuration
→ Ω·cm2 for planar configuration
(∵shortening of the path length for the current)
Table 12.8 Relative contribution of cell component to total ohmic polarization at 1000 ℃ for planar SOFCs

38. 2. the change in various polarization terms
Effect of temperature
The effect of temperature : 1. the change in the reversible cell potential
2. the change in various polarization terms
The reversible cell potential ↓with cell operating temperature increase
(For the SOFC, cell potential is reduced at small current densities)
- The effect of temperature at high current density is dominated by the changes in the various polarizations.
- The cell potential as a function of temperature
(12.13)
The change in the activation polarization from eq
(12.14)
Where ΔT = T - T1

39. In practice, a cell is operated at a nominal design current density J1.
ΔJ = J - J1
- To linearize the current density dependence in Eq
(12.15)
The exchange current changes is neglected in this analysis.
(∵Exchange current dependence on the temperature is not available)
[sinh-1(x)]’=(x2+1)-1/2, substituting Eq into Eq
For J1 = 0.16 A/cm2, the anodic and cathodic activation polarization change with
temperature as
The negative sign above two eq. (activation overpotential is reduced, when T ↑)

40. change of cathode overpotential > change of anode overpotential
- Total activation overpotential
The change in the ohmic overpotential can be expressed as
Where the ohmic loss in the cell component is related to the resistivity as

41. The resistivity as a function of temperature for each cell component
(12.20)
Where the constants a and b are given in Table 12.10
Table Constant a and b in correlation of resistivity as function of temperature for SOFC cell components
Substituting Eq into Eq yields
(12.21)
Where R” is the area specific resistance

42. Using the value of r given in Table 9. 7, 9
Using the value of r given in Table 9.7, 9.8, along with the values of b in Table
9.10, the ohmic overpotential change can be expressed as
The ohmic overpotential change is less than half of activation overpotential
change.
Substituting the above result into Eq , the cell potential change as a
function of temperature can be obtained.
(12.24)
The coefficient ~ Ω ·cm2/K is smaller than empirical value of
0.008 Ω·cm2/K (67 % H2, 22 % CO, 11 % H2O) .
The temperature dependence coefficient change significantly with
the composition of the fuel used.

43. The water-gas shift equilibrium reaction and the hydrogen concentration have a
significant contribution to the temperature dependence of the actual cell potential.
Effect of pressure
The pressure effect o the actual cell potential is
(12.25)
For the overall cell reaction in Eq. 12.3, the number of mole changes for
the reaction is ΔN=-1/2 → the coefficient for the pressure effect on the reversible
cell potential becomes at 1000 oC.
(12.26)
Based on empirical data, the pressure coefficient kP for the actual cell potential
would be around V.

44. For combined SOFC-gas turbine power systems,
Although higher pressure operation is beneficial for better SOFC performance, the
cost and effort involved in making stronger SOFC components, gas tight sealing,
and operational cost
For combined SOFC-gas turbine power systems,
higher pressure operation is worthwhile.
- Thermodynamic optimization is required between particular SOFC and gas turbine.
Effect of Reactant gas composition and utilization
Oxidant
Change potential in cathode in terms of the logarithmic dependence on the partial
pressure of oxygen in the cathode
(12.27)
Where the coefficient kP = RT/(2nF)=0.063 V at 1000 ℃ for the reversible cell potential

45. Fuel
The effect of the hydrogen concentration and utilization on the cell potential can be
expressed as
(12.28)
Where the coefficient ka = RT/(nF)=0.126 V at 1000 ℃ for the reversible cell potential
Limited experimental data implies a larger value of kP=0.172 for the actual cell
potential at the practical operating current densities when the cell operated on
reformed fuel.

46. Tolerance for NH3 upto 5000 ppm, HCl upto 1 ppm and H2S upto 0.1 ppm
Effect of Impurities
Tolerance for NH3 upto 5000 ppm, HCl upto 1 ppm and H2S upto 0.1 ppm
1 ppm of H2S would cause an immediate drop in the cell potential, followed by
a gradual decrease with a linear dependence over the time.
Effect of current density
For SOFC at 1000 ℃, ohmic loss is dominant; activation and concentration
overpotentials is relatively small
Under that condition, the cell potential will change linearly with the current density
(12.29)
Where the proportionality constant should equal the area specific resistance for the entire
cell assembly.

47. KJ value of 0. 33 Ω ·cm2/K for tubular SOFC, 0
KJ value of 0.33 Ω ·cm2/K for tubular SOFC, Ω ·cm2/K for planar SOFC,
respectively
Empirical value : KJ = 0.73 Ω ·cm2/K
12.6 Future R&D
High temp. SOFC(>1000℃) provides high oxygen ion conductivity, However
reduces the reliability and durability of cells.
A major thrust in the current SOFC development is the lower-temperature SOFC.
(500 ~ 800 ℃)
Lower operating temp. allows the use of cheaper materials and easy fabrication
process for the cell component.
Instead of YSZ, alternative electrolytes were investigated.
1. ceria based electrolyte - advantage: high oxygen ion conductivity in low
temperature
- disadvantage : electronic conduction in reduction
environment

48. 2. Lanthanum gallate (perovskite)
3. SrCeO3 : proton conductor electrolyte at high temp. in a hydrogen-containing
atmosphere.
4. BaCeO3, SrZrO3, and BaZrO3 : high proton conductor electrolyte at lower temp.
A planar cell structure is an attractive geometry for stack configuration
→ gas-tight sealing problem remain
- Innovative cell and stack configuration is needed for sealing purpose