Diffusion and Chemical Reaction in the Porous Structures of SOFCs

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Diffusion and Chemical Reaction in the Porous
Structures of SOFCs
By Ben Haberman and John Young
Hopkinson Laboratory, CUED, Cambridge, UK
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Overview

• The Rolls-Royce design
• Gas transport in porous materials
• Numerical method
• Results
• Conclusions
• Any questions?

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The Rolls Royce IP-SOFC Design
Fuel cells
Air flow
Ceramic module
Fuel flow
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Porous Flows

• Two types of flow

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Cylindrical Pore Interpolation Model
1
0
-1
Mass flux (mg m-2 s-1)
-2
Experiment CPIM Continuum flow only
-3
-30
-20
-10
0
10
20
30
Pressure difference (kPa)
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The Shift Reaction

• H2O CO H2 CO2

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Numerical Method

• Conservation of each species
• Gi from CPIM equations
• Conservation of energy

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Boundary Conditions
Fixed flux b/c

• Pressure - 7 bar
• Temperature - 1138 K
• Gas mixture - 34H2, 33H2O,
• 15CO2,
  18CO
• Current density - 3000 Am-2
• Heat production - 50 Wm-2

Electrolyte
Anode
z
Porous support
Fuel channel
Fixed property b/c

• Porosity - 20
• Thickness - 50mm (anode), 1.5mm (porous
  support)
• Pore size - 1mm, 2mm
• Conductivity - 5 Wm-1 K-1

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Results No Shift Reaction
H2
G
H2O
1.05
0.08
0.95
0.04
Ka/Kp
0.85
0.00
0.75
XH2 XH2O XCO2 XCO Ka/Kp
-0.04
0.65
0.0
0.4
0.8
1.2
1.6
z / mm
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Results Including Shift Reaction
H2
G
H2O
1.05
0.08
0.95
0.04
Ka/Kp
Kas/Kps
0.85
0.00
0.75
XH2 XH2O XCO2 XCO Ka/Kp
XH2 XH2O XCO2 XCO Kas/Kps
-0.04
0.65
0.0
0.4
0.8
1.2
1.6
z / mm
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Changing Fuel Channel Composition
1.04 1.00 0.96 0.92 0.88
Ka/Kp in anode
Increasing hydrogen production
0 1
2 3
Ka/Kp in fuel channel
Decreasing hydrogen
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Reduced Concentration Loss
1.05 1.00 0.95 0.90 0.85
Fuel channel
Anode
EN (V)
0 1
2 3
Ka/Kp in fuel channel
Decreasing hydrogen
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Improved Fuel Utilisation
1.05 1.00 0.95 0.90 0.85
R CO/H2
0 1
2 3
3
0
1
2
Ka/Kp in fuel channel
Decreasing hydrogen
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Conclusions and Further Work

• Fuel cell performance controlled by
• Mass transport in porous layers
• Shift reaction in anode
• Shift reaction cannot be at equilibrium
  everywhere
• Shift reaction improves performance
• Consumes CO in preference to H2
• Reduces concentration loss
• Further work required
• Assess effect of direct CO oxidation
• Analyse effect of porous support activity

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Thank You for Listening

• Any Questions ?