Cathodic behaviour of samarium(III) in LiF–CaF 2 media on molybdenum and nickel electrodes

Abstract

The electrochemical behaviour of SmF 3 is examined in molten LiF–CaF 2 medium on molybdenum and nickel electrodes. A previous thermodynamic analysis suggests that the reduction of SmF 3 into Sm proceeds according to a two-step mechanism: Sm III + e − = Sm II Sm II + 2e − = Sm The second step occurs at a potential lower than the reduction potential of Li + ions. Cyclic voltammetry, chronopotentiometry and square-wave voltammetry were used to confirm this mechanism and the results show that it was not possible to produce samarium metal in molten fluorides on an inert cathode (molybdenum) without discharging the solvent. The electrochemical reduction of SmF 3 is limited by the diffusion of SmF 3 in the solution. The diffusion coefficient was calculated at different temperatures and the values obtained obey Arrhenius’ law. For the extraction of the samarium from fluoride media, the use of a reactive cathode made of nickel leading to samarium–nickel alloys is shown to be a pertinent route. Cyclic voltammetry and open-circuit chronopotentiometry were used to identify and to characterise the formation of three alloys: liquid Sm 3Ni and a compact layer made of SmNi 3 and SmNi 2.