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The exchange current, transfer coefficient, mass-transport parameters, and electrode capacitance at the Nar Mo/BASE phase boundary have been evaluated from 740 to 1220 K. The transfer coefficient exhibits a value close to 0.5 and the exchange current is dominated by collision frequency, with no significant activation energy. Since the porous Mo electrode adopts a fairly regular microstructure on the beta" alumina solid electrolyte (BASE) surface, the magnitude of the exchange current of mature electrodes directly depends on the actual contact zone (the region where reaction occurs close to the porous metal/gas/electrolyte three-phase contact) of the porous metal film with the BASE ceramic, and decreases slightly as grain growth occurs. The exchange currents and the mass-transport parameters derived for very porous thin Mo electrodes indicate that the charge-transfer reaction occurs at a small fraction of the interface. The capacitance calculated from the admittance of mature, clean, thin, porous Mo electrodes shows significant frequency dependence, suggesting a contribution from a Warburg impedance associated with surface diffusion. High-frequency limiting capacitance and resistance values due to the interface show potential dependence and a value on the order of 1 F/m 2 and 0.1-1.0 ~-cm 2. If Na2MoO4 and Na2Mo306 are present, the interfacial capacitance is unstable with time as well as greatly increased, but after the electrodes have matured, with loss of these species, it exhibits very reproducible behavior with respect to potential, pressure, and frequency. Since the reaction area increases due to Na2MoO4/Na2Mo306 in the pores, when they are present the apparent exchange current is also increased. Grain growth in very thin, mature, porous Mo electrodes can have a small effect on the electrode's exchange current.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.0.65.67 Downloaded on 2015-03-25 to IP 9 J J I

Performance and impedance characteristics of porous molybdenum and tungsten electrodes, to which Na2MoO4 and Na2WO4, respectively, have been added, are significantly altered from the characteristics of similar but untreated electrodes. High initial power levels are more persistent in treated electrodes, and regeneration during discharge is observed during initial stages of degradation. Sheet resistances of treated electrodes at T > 1000 K are higher but decrease as performance degradation proceeds. AC impedance complex plane plots show an enhanced, high frequency, potentialindependent RC loop due to the ionic conducting molten salt and a large capacitance in the low frequency, potentialdependent loop due to a greatly increased interfacial area, as well as a reaction at the electrode/solid electrolyte and molten salt interface in the case of Na2MoOgMo electrodes. Corrosion of the porous electrode by Na2MoO4 or Na2WO4 to form Na2M0306 and WO2, respectively, and recrystallization of the molybdenum or tungsten as the salt evaporates, results in major morphological changes including loss of columnar structure and a significant increase in porosity. This effect is more pronounced in Na2MoO4/Mo electrodes due to the lower stability of Na2MoO4 relative to compounds with intermediate oxidation states of Mo. If there is not a very large accompanying increase in sheet resistance, the resulting electrode may retain significant improved performance after loss of the molten salt.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.11.21.2 Downloaded on 2014-11-01 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.11.21.2 Downloaded on 2014-11-01 to IP

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