A direct electrochemical route from oxide precursors to the terbium–nickel intermetallic compound TbNi 5

Abstract

Successful direct electrochemical reduction of mixed powders of terbium oxide (Tb 4O 7) and nickel oxide (NiO) to the intermetallic compound, TbNi 5, is demonstrated in molten CaCl 2 at 850 °C by constant voltage (2.4–3.2 V) electrolysis. The reduction mechanism was investigated by cyclic voltammetry using a molybdenum cavity electrode in conjunction with characterisations of the products from both constant voltage and potentiostatic electrolysis under different conditions by XRD, SEM and EDX. It was found that the reduction started from NiO to Ni, followed by that of Tb 2O 3 (resulting from Tb 4O 7 decomposition) on the pre-formed Ni to form the intermetallic compound. The reduction speed increased with increasing the cell voltage, but the speed gain was counterbalanced by decreased current efficiency and increased electric energy consumption. At 2.4 V, the current efficiency reached 63.2%, and the energy consumption by electrolysis was as low as 3.2 kWh/kg TbNi 5 when the oxide phase was converted fully to the metal phase (XRD) in 4 h. The oxygen level in the produced TbNi 5 could readily reach 1800 ppm by electrolysis at 3.2 V for 12 h with the energy consumption being 18.9 kWh/kg TbNi 5.