Electrochemical reduction of vanadium sesquioxide in low-temperature molten fluoride salts

Abstract

The low-temperature molten Na3AlF6–K3AlF6–AlF3 salts were evaluated in order to perform the electrochemical reduction of vanadium sesquioxide. By using cyclic voltammetry and constant potential electrolysis, the cathodic reduction reactions were determined and the promoting effect of oxygen concentration in the electrolyte on the process was proved. A rapid electro-reduction of vanadium sesquioxide was achieved under a constant voltage of 4 V at 850°C, the vanadium metal with an oxygen content of 0.3 mass% was obtained. The corresponding current efficiency was around 60%. The reaction kinetic pathway was systematically investigated by analyzing the phase transformations and microstructure evolutions of the products prepared at different electrolysis durations. The reduction mechanism was concluded to start by the direct electro-reduction of V2O3 accompanied with the implantation of aluminium ions to form AlV2O4 intermediate, which was then reduced to vanadium directly through aluminothermic reduction, with aluminium reductant being generated from the electrodeposition reaction. Additionally, by using the low-temperature fluoride salts as the electrolyte, the formed AlV2O4 showed a porous structure composed of small and uniform intermediate particles, which was obviously different from the observations in traditional molten Na3AlF6 salt. This structure could provide more active sites for the reduction reaction and shorten the diffusion path for O2− ion in the solid phase. The possible microstructure transformation mechanisms were also discussed.