A QUANTITATIVE MODEL FOR THE EXCHANGE CURRENT OF POROUS MOLYBDENUM ELECTRODES ON SODIUM BETA'-ALUMINA IN SODIUM VAPOR

摘要

This paper presents a model of the exchange current developed for porous molybdenum electrodes on sodium beta"-alumina ceramics in low pressure sodium vapor, but which has general applicability to gas/porous metal electrodes on solid electrolytes. The exchange current is first defined to be consistent with the quantity measured in dc electrochemical experiments; we make a distinction between the dc electrochemical exchange current and an ac electrochemical exchange current measured at about 103 to 104 Hz. We also distinguish the electrochemical exchange process from a chemical sodium exchange process between the solid electrolyte and the sodium vapor which does not involve the electrode. The model uses only physical parameters which have been or could be measured by other techniques. The parameters which control the rate and spatial extent of the electrochemical reaction at the three phase interphase region include the morphology of the electrode and solid electrolyte interface, the work function of the sodium coated porous metal surface, the adsorption/desorption rate of sodium atoms from the beta"-alumina surface, the potential energy function of sodium ions bound to the beta"-alumina surface, and the surface diffusion rate of sodium ions on the beta"-alumina spinel block surface.