Electrochemical studies on reduction of solid Nb2O5 in molten CaCl2-NaCl eutectic - I. Factors affecting electrodeoxidation of solid Nb2O5 to niobium

Electrodeoxidation of solid Nb2O5 to niobium was achieved by constant voltage electrolysis in molten CaCl2-NaCl eutectic at a controlled potential of 3.1 V. A number of important parameters affecting the extent and rate of electrodeoxidation have been investigated experimentally to better understand the electrodeoxidation process. The present results revealed that the structural features, which had been optimized by varying the Nb2O5 pellet fabrication conditions, were closely interrelated and changed during the course of electrolysis under the experimental conditions used. It was found that the porous cathode with a large surface area would facilitate the cathodic oxygen ionization at the cathode/electrolyte interface and that the porosity must be large enough to enable the O2- dissolved in the melt to diffuse readily out of the porous cathode. The optimal electrolysis duration and temperature were also determined experimentally. Furthermore, a conceptual model was proposed for explaining the electrodeoxidation mechanism. The model was based on two distinct processes occurring during the entire course of electrodeoxidation, (i) reduction of the oxide materials to Nb-O solid solutions and (ii) removal of the oxygen from the Nb-O solid solutions, which both relied on the oxygen ionization at the cathode. The proposed model was confirmed by the experimental results. (C) 2004 The Electrochemical Society.