Structural Limitations in
the Scale-Up of Solid
Oxide Fuel Cells (SOFCs)
Jan Thijssen
C. Fulton, R. Stringfellow, S. Sriramulu
Session #: Solid Oxide 2B
Acorn Park
Cambridge, MA 02140
Outline
Performance/structural simulations of SOFCs
Background
Objectives
Modeling Approach
Results
Summary
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Background: Scale-up of Planar SOFCs
Stresses may limit area scale-up of planar SOFC
stacks.
• Stationary & mobile applications (< 10 kW - > 250
kW)
• Current planar stacks < 2 kW
• Scale-up options for planar SOFCs:
– Multiple stacks or more cells
- Economy of scale, but manifolding and contact issues
– Larger area cells
- Reduced heat loss, but larger probability for stress buildup, difficult to manufacture, and seal
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Background: Motivation
Purely empirical assessment of limitations to stack
scale-up is difficult.
• Costly and time consuming to construct and test
stacks with different active areas
– Instrumentation
– Manufacturing yield
– Test equipment
• Complex phenomena
– Interpretation of results
– Reproducibility
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Background: Contributions to Stress
Multiple phenomena contribute to stress build-up.
Stress Distribution
Constrained
Deformation
Stack Geometry
Defects
Temperature
Gradients
Stack
Weight
Heat Loss
Conduction
Convection
Imposed
Loads
Compressive
Load
Heat Generation
Radiation
Chemical and
Electrochemical
Reactions
Joule
Heating
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Background: Objectives
Develop model to assess SOFC thermomechanical stresses during operation.
• How much do compressive load and temperature gradients
contribute to stress?
• How does stress distribution in the cell depend on design
parameters?
– Cell size (area)
• How does the stress distribution in the cell depend on the
operating conditions?
– Temperature and stoichiometry of the inlet cathode stream
– Cell voltage
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Approach: Overview
Numerical model for cell performance and stress
distribution.
Interconnect
Anode and cathode
reaction zones
Flow passages
Porous electrodes
Electrolyte
Conservation:
• Energy
• Mass
• Charge
Temperature
Gradients
Mechanical
Stress
Distribution
Constraints
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Approach: 1-d Model to Estimate Parameters
Detailed 1-d model was developed for estimating
unknown parameters.
Assumed TPB Microstructure*
10 µm electrolyte
h
(1-φ)w
w
d
Electrocatalyst
Ionic phase in the
electrode reaction zone
*J-W
Kim, A. Virkar, K-Z Fung, K. Mehta, S. Singhal, J. Electrochem. Soc., 146
(1999) 69-78,
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Approach: Results from 1-d Model
1-d Model was fit to data and then validated.
Comparison to Literature Data1
1.2
Experimental Data
Model
Cell Voltage / V
1
650 ° C
700 °C
750 ° C
800 ° C
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
Current Density / A cm
1
6
-2
• Literature values for
temperature dependence
of ion conductivity
• 800°C data was fit by
adjusting 6 parameters
– Comparisons to data
at other temperatures
were obtained
without further
parameter
adjustment
J-W Kim, A. Virkar, K-Z Fung, K. Mehta, S. Singhal, J. Electrochem. Soc., 146 (1999) 69-78.
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Results: 3-d Cell Configuration
Cross-flow anode-supported planar SOFC.
2 mm x 2
mm channels
6.4
cm
Air
650 0C
97 % H2, 3 % H2O
650 0C
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Results: Current Density Profile at 0.7 V
Maximum local current density is 20% higher than
the average.
Current density distribution, average = 1.3 A/cm2
A/cm2
1.6
1.5
1.4
1.3
1.2
1.1
1.0
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Results: Temperature Profile at 0.7 V
Cathode stoichiometry of ~ 5 ensures moderate
temperature gradients.
Temperature distribution
0C
800
775
750
725
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Results: Residual Stress at Room Temperature
Manufacturing steps are needed for estimating
residual stresses.
Total Displacement
= 120 µm
Cathode: 36 MPa
Sinter Ceramics
Cool to RT
Electrolyte:
-567 MPa
Anode: - 11 MPa to 3 MPa
Flatten Ceramics
Cathode: 39 MPa
Heat to Operating
Temperature
Electrolyte:
-556 MPa
Anode: - 3 MPa
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Results: Contact Pressure
Low contact pressure at the outer edges might
lead to high contact resistance.
Through thickness stress in the metallic interconnect
Potential for
high contact
resistance
Pa
1.1 x 105
- 1.1x 105
- 3.4x 105
- 5.7x 105
- 7.9 x 105
- 10 x 105
- 12 x 105
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Results: Stress at Operating Temperature at 0.7 V
Stresses are modest under operating conditions.
No slip between the electrodes and the interconnect
Cathode:
+9 to +22 MPa
Electrolyte:
-201 to -246 MPa
Anode:
-6 to +20 MPa
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Movie: Heat Up of Cell by Hot-Gases During Startup
10
H2 + H2 O
650 C
cm
Air
650 C
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Movie: Current Density Evolution During Startup
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Movie: Warping of MEA During Cool Down After Sintering
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Summary I: Baseline Stress Calculations
• High stress state occurs at room temperature
and not during cell operation.
• Significant residual stresses buildup in the
ceramic layers of the MEA during cool down
from the stress-free sintering temperature.
• At the steady state operating temperatures
(650–850°C), residual stresses are relieved to
some extent.
• The temperature gradients during steady-state
operation do not generate severe stresses.
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Summary II: Stress Dependence on Area
• Increasing cell size increases the lateral mismatch
strains but not stress state or overall flatness of the MEA.
– Increased potential for seal failure
• No-slip shear stresses between IC and MEA are not
sustainable without bonding.
– Slip may cause damage
• The average stresses in the MEA layers at room
temperature do not depend strongly on the cell area.
• If non-uniform confining pressure is applied, contact
resistance at cell edges might increase with area.
These conclusions depend on the assumed values of TEC
and elastic moduli and must be better characterized for
standard SOFC materials.
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Acknowledgements
• NETL (Contract Number 736222-30003), Mark
Williams, Wayne Surdoval, Rod Geisbrecht
• Anil Virkar
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