Investigation on electrochemical reduction process of Nb 2O 5 powder in molten CaCl 2 with metallic cavity electrode

Abstract

The direct electrochemical reduction process of Nb 2O 5 powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb 2O 6 was formed upon addition of solid Nb 2O 5 into molten CaCl 2 when CaO was present. During the electrolysis solid Nb 2O 5 was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V ( vs. Ag/AgCl), which was more positive than the reduction potential of Ca 2+. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V ( vs. Ag/AgCl). Nb 2O 5 pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb 2O 5 powder in a molten system.