Kinetics and Transport at AMTEC Electrodes: II . Temperature Dependence of the Interfacial Impedance of Na(g)/Porous Mo/Na‐Beta′ Alumina

Abstract

The exchange current, transfer coefficient, mass‐transport parameters, and electrode capacitance at the Na(g)/porous 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/m2 and 0.1–1.0 Ω‐cm2. If and 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 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.