Estimates for Polarization Losses in Molten Carbonate Fuel Cell Cathodes

Abstract

This paper compares the polarization losses associated with the various diffusion-reaction-conduction processes in molten carbonate cathodes. The comparisons are made by estimating each type of loss in terms of component electrochemical potentials in joules/mole; this allows diffusive, charge-transfer, and ohmic losses to all be put on equal footing. For characteristic parameter values, diffusion in both the gas and electrolyte phases and conduction in the electrolyte account for similar polarization losses; charge-transfer and conduction in the solid electrode account for significantly smaller losses. These results tend to support and unify the previous work of numerous investigators. Also molecular-channel interactions are found not to contribute significantly to the polarization loss.In recent years, a number of studies have attempted to determine which portion of the overall cathodic process accounts for the majority of polarization losses in molten carbonate fuel cell ͑MCFC͒ cathodes. These studies have resulted in a variety of conclusions, some of which may at first glance appear contradictory. Through ac impedance measurements, Yuh and Selman 1 found that chargetransfer accounts for the majority of the overall polarization loss at lower temperatures ͑600°C͒, while mass transport ͑diffusion͒ and/or slow recombination are more important at higher temperatures ͑700°C͒. Their work does not distinguish the roles of gas-phase and electrolyte-phase transport. Prins et al. 2-4 also made ac impedance measurements; they concluded that for relatively rich inlet gas concentrations, the polarization losses are dominated by ohmic loss ͑electrolyte conduction͒. For lower inlet gas concentrations, electrolyte diffusive transport became more significant. 4 Fehribach et al. 5 directly computed the current production in a small 100 ϫ 100 m cathode cross section, and found that on this scale, the diffusive transport in the electrolyte is the greatest contributor to the polarization loss. Kunz and Murphy, 6 on the other hand, suggest that gas-phase diffusional resistance increases with decreasing inlet gas concentrations and accounts for a significant portion of the polarization loss. Recent work by Peelen et al. 7 found that in particular a carbon dioxide partial pressure below 0.05 atm was detrimental to a MCFC cathode current density of 150 mA/cm 2 .The present work is a brief, but we hope useful, theoretical estimate of the polarization losses in MCFC cathodes; it supports and reinforces many of the conclusions discussed above, and to some extent, unifies them. The estimates are made using the component electrochemical potentials introduced by Fehribach et al. 5,8,9 This formulation is particularly useful here because it allows the various cathodic processes ͑diffusion, conduction, electrochemical reactions͒ to be compared on the same scale ͑i.e., equal footing͒. This work differs from our earlier work 5 in that here we are considering an entire laboratory cathode ͑800 m thick͒ rather than a small cross section, and that ...

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Three-dimensional modeling of polarization characteristics in molten carbonate fuel cells using peroxide and superoxide mechanisms

“…Then, the reaction rates for the slow reactions are assumed to be proportional to the differences in these component potentials. Fehribach et al [23] employed this model for the peroxide mechanism describing the electrochemistry of an MCFC cathode. Fehribach and Hemmes [24] compared the polarization losses associated with the various diffusion-reactionconduction processes in MCFC cathodes.…”

Comment on “Analysis of Molten Carbonate Fuel Cell Performance Using a Three-Phase Homogeneous Model” [J. Electrochem. Soc., 150, A46 (2003)]

“…Lastly, the electrolyte conductivity of 2.0 S/m used in this study seems rather low as compared to values that other groups have been using. [4][5][6] The latter are almost 2 orders of magnitude larger ͑140 S/m͒.…”