Abstract
Molten Ag and Ag–Sb alloy have been examined as anodes in direct carbon solid oxide fuel cells (SOFC) at 1273K. For Ag, an open-circuit voltage (OCV) typical of that expected for carbon oxidation, 1.12V, was observed when charcoal was mixed with the molten Ag. However, the anode impedance was high, ∼100Ωcm2. The nature of the electrode losses was investigated by measuring the voltage–current characteristics of a cell with Ag but no carbon at the electrode, while ramping the voltage under fuel cell and electrolysis conditions. The results indicate that the cell potential is governed by the oxygen concentration in the Ag at the electrolyte interface. Using this and a model of carbon oxidation within the molten Ag, it is determined that the impedance of the electrode is limited by diffusion of oxygen in the Ag phase, due to the low solubility of oxygen in molten Ag. With Ag–Sb alloy with added charcoal, the OCV at 973K was 0.75V, the potential associated with equilibrium between Sb and Sb2O3, due to the low solubility of oxygen in the Sb phase. The implications of these results for using molten Ag electrodes for direct carbon fuel cells are discussed.
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