Direct and low energy electrolytic co-reduction of mixed oxides to zirconium-based multi-phase hydrogen storage alloys in molten salts

Abstract

Direct synthesis of Zr-based AB₂-type hydrogen storage alloys (HSAs) from mixed oxide precursors has been achieved by electrolysis in molten CaCl₂ at 900 °C and a cell voltage below 3.2 V. The process resembled direct oxide-to-metal conversion in solid state, and the target alloys, namely ZrCr₂, ZrCr_0.7Ni_1.3 and Zr_0.5Ti_0.5V_0.5Cr_0.2Ni_1.3, were formed in situ during electrolysis without going through any melting step. Electrolysis energy consumption could be as low as 9.59 kWh (kg-HSA)⁻¹ and the metal recovery yield was generally higher than 90%. The electrolytic products were readily obtained as powders with the designated compositions and crystal structures (e.g. the C14 and C15 Laves phases). More importantly, these Zr-based electrolytic HSA powders were composed of nodular micro-particles which are very desirable for fabrication of electrodes with micro-porosity to facilitate electrolyte ex- and ingression. Galvanostatic discharge-charge tests of the as-prepared electrolytic HSA powders resulted in similar or higher hydrogen storage capacities (up to 280 mAh g⁻¹) in comparison with the same HSAs prepared by e.g. arc-melting the individual metals as reported in literature. Particularly, the electrolytic Zr-based HSAs were unique for their high initial capacities without any pre-treatment for activation, and they also exhibited highly satisfactory discharge rate capability with less than 20% capacity loss when the discharge current increased from 50 to 600 mA g⁻¹.