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Objectives

Design and Optimization of Molten Carbonate Fuel Cell Cathodes Bala S. Haran, Nalini Subramanian, Anand Durairajan, Hector Colonmer, Prabhu Ganesan, Ralph White and Branko Popov Department of Chemical Engineering University of South Carolina Columbia, SC 29208. Objectives.

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Objectives

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  1. Design and Optimization of Molten Carbonate Fuel Cell CathodesBala S. Haran, Nalini Subramanian, Anand Durairajan, Hector Colonmer, Prabhu Ganesan, Ralph White and Branko PopovDepartment of Chemical EngineeringUniversity of South CarolinaColumbia, SC 29208

  2. Objectives • To develop a three phase homogeneous model using volume averaging technique to characterize the performance of the MCFC cathode. • To study the effect of various parameters on polarization Characteristics of MCFC • To understand the kinetics of the electrode reaction through modeling electrode impedance. • Use impedance analysis to understand to what extent ohmic, kinetic and mass transfer limitations affect the performance of molten carbonate fuel cell cathodes.

  3. Previous Work • Morita et al. - developed empirical equations for cathode resistances • Makkus et al. - studied the polarization under ohmic conditions • Prins-Jansen et al. - developed an impedance model for extracting kinetic data • Selman et al. - developed a steady state model assuming bimodal agglomerate

  4. Mechanism Peroxide Mechanism

  5. Schematic of Agglomerate Electrode Current Collector X=L Electrolyte Film Gas Flow Solid+Electrolyte Phase Gas dissolution and transport into film Electrolyte/Matrix d X=0 F1 F2

  6. n(ls) n(lg) Solid Phase V(s) n(gs) n(gl) Gas Phase V(g) Electrolyte Phase V(l) Electrolyte Phase Volume Averaging in Porous Electrode Gaseous Phase Solid Phase

  7. Derivation of Model Equations Material Balance on the liquid phase Material Balance on the gas phase Using jump balances and volume averaging

  8. Molar flux vector in the liquid phase Molar flux vector in the gas phase By volume averaging the molar flux vectors become

  9. Flux from the liquid phase into the gas phase Rate of production of species i at the liquid solid interface Rate of production of species i at the gas solid interface

  10. Ohm’s law in the liquid and solid phases By volume averaging the equations become Also

  11. Governing Equations Concentration in the liquid phase Potential in the liquid phase  Potential in the gas phase  Electroneutrality 

  12. Transfer of current to the liquid phase Concentration in the gas phase

  13. Boundary Conditions At x = 0 At x = 1

  14. Parameters

  15. Change in Overpotential Along the Thickness of the Electrode

  16. Effect of Exchange Current Density on Overpotential

  17. Effect of Electrolyte Conductivity on Overpotential

  18. Effect of CO2 Gas Phase Diffusion Coefficient on Overpotential

  19. Effect of O2 Gas Phase Diffusion Coefficient on Overpotential

  20. Effect of Gas Compositions on Overpotential

  21. Change in Concentration of CO2 in the Liquid Phase Along the Thickness of the Electrode

  22. Comparison of Model Predictions to Experimental Polarization Curve at 650o C

  23. Impedance Model Current for double layer charging Current for faradaic charging Net current flowing through the electrode Faradaic Impedance

  24. Impedance Analysis Step 1 - Linearize the equations Step 2 - Introduce deviation variables Step 3 - Convert into Laplace domain Step 4 - Express each laplace variable as a sum of imaginary and real part Step 5 - Solve the equations using BAND(j) for the real and imaginary parts of each variable

  25. Nyquist plot-Effect of Electrode Conductivity

  26. Nyquist Plot - Effect of Electrolyte Conductivity

  27. Nyquist Plot - Effect of Liquid Phase Diffusion Coefficient (O2)

  28. Nyquist Plot - Effect of Gas Compositions

  29. Bode Plot - Effect of CO2 Gas Phase Diffusion Coefficient /W /Hz

  30. Bode Plot - Effect of O2 Gas Phase Diffusion Coefficient /W /Hz

  31. Nyquist Plot - Effect of Liquid Phase Diffusion Coeffcient (CO2)

  32. Nyquist Plot - Effect of Exchange Current Density

  33. Conclusions • Developed a three phase homogeneous model using volume averaging technique to characterize the performance of the MCFC cathode. • Studied the the effect of various parameters on polarization characteristics of MCFC. • Exchange current density, electrolyte resistance and filling control the cathode performance. • Developed an impedance model for studying MCFC cathode behavior using the three-phase homogeneous approach. • Current efforts are focussed on extracting kinetic and transport parameters from the impedance model.

  34. Acknowledgements Financial sponsors – Department of Energy National Energy Technology Laboratory

  35. Nyquist Plot - Effect of Gas Phase Diffusion Coefficient (CO2)

  36. Nyquist Plot - Effect of Gas Phase Diffusion Coefficient (O2)

  37. Effect of CO2 Liquid Phase Diffusion Coefficient on Overpotential

  38. Effect of O2 Liquid Phase Diffusion Coefficient on Overpotential

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