The deoxidation speed of a solid oxide cathode in molten CaCl2 can be estimated by the PRS steady diffusion model of O2, which correlates the deoxidation speed with the precursor porosity, P, the metal-to-oxide molar volume ratio, R, and the cathode volume shrinkage S. The PRS model indicates that the porosity of the oxide cathode has important influence on the deoxidation speed, and provides a very simple equation for the prediction of the optimal cathode porosity. For the electrolysis of Ta205, the porosity of the cathode is better to be within 50%. The model and its predictions have been well verified by the electrolysis of solid Ta205 in molten CaCl2, suggesting the great significance of the PRS model for the high speed and high current efficiency electrolysis of solid compound cathode in molten salts
