Electrochemical studies on reduction of solid Nb2O5 in molten CaCl2-NaCl eutectic

Cathodic processes during the electrodeoxidation of solid Nb2O5 in thermally dried and pre-electrolyzed molten CaCl2-NaCl eutectic were studied at temperatures 850-950 degrees C using cyclic voltammetry and chronoamperometry, respectively. In cyclic voltammograms at 850- 950 C, several reduction peaks were clearly observed at the potentials at least 0.8 V vs (Na+, Ca2+)/(Na-Ca) less cathodic than those of the metals deposition from the eutectic melt, confirming the concept of cathodic oxygen ionization. The chronoamperometric studies showed that the cathodic processes at the potentials of 0.7-0.2 V vs (Na+,Ca2+)/(Na-Ca) always started with the oxygen ionization of the CaNb2O6 film on the surface of the Nb2O5 electrode in the molten CaCl2- NaCl eutectic electrolyte, which was governed by the oxygen diffusion within the film toward the CaNb2O6-electrolyte interface. The subsequent oxygen ionization of the oxides at the surface took place consecutively and/or simultaneously. Surface characterizations of the Nb2O5 electrodes before and after the chronoamperometric measurements revealed that there existed a marked dependence of the morphologies, compositions, and phases on the imposed potential steps and that the calcium cations were intercalated electrochemically into the Nb2O5 structure at the surface at relatively less cathodic potentials than that of the sodium deposition from the molten CaCl2-NaCl eutectic. (c) 2005 The Electrochemical Society.