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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

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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. Objectives

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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

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Mechanism Peroxide Mechanism

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

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

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

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

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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

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

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

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

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Boundary Conditions At x = 0 At x = 1

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Parameters

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Change in Overpotential Along the Thickness of the Electrode

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Effect of Exchange Current Density on Overpotential

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Effect of Electrolyte Conductivity on Overpotential

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Effect of CO 2 Gas Phase Diffusion Coefficient on Overpotential

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Effect of O 2 Gas Phase Diffusion Coefficient on Overpotential

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Effect of Gas Compositions on Overpotential

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Change in Concentration of CO 2 in the Liquid Phase Along the Thickness of the Electrode

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

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

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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

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Nyquist plot-Effect of Electrode Conductivity

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Nyquist Plot - Effect of Electrolyte Conductivity

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Nyquist Plot - Effect of Liquid Phase Diffusion Coefficient (O 2 )

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Nyquist Plot - Effect of Gas Compositions

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Bode Plot - Effect of CO 2 Gas Phase Diffusion Coefficient /Hz

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Bode Plot - Effect of O 2 Gas Phase Diffusion Coefficient /Hz

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Nyquist Plot - Effect of Liquid Phase Diffusion Coeffcient (CO 2 )

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Nyquist Plot - Effect of Exchange Current Density

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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. Conclusions

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Acknowledgements Financial sponsors – Department of Energy National Energy Technology Laboratory

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Nyquist Plot - Effect of Gas Phase Diffusion Coefficient (CO 2 )

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Nyquist Plot - Effect of Gas Phase Diffusion Coefficient (O 2 )

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Effect of CO 2 Liquid Phase Diffusion Coefficient on Overpotential

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Effect of O 2 Liquid Phase Diffusion Coefficient on Overpotential

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