Electrochemical behaviour of thorium(IV) in molten LiF–CaF2 medium on inert and reactive electrodes

The electrochemical behaviour of the Th(IV)/Th system was examined in molten LiF–CaF2 medium on inert (molybdenum), reactive (nickel) and liquid (bismuth) electrodes in the 810–920 °C temperature range by several electrochemical techniques. Experimental results showed that (i) thorium fluoride was reduced in a single step exchanging 4 electrons and limited by thorium ions diffusion in the solution, (ii) the oxide ions induce the precipitation of Th(IV) in the form of thorium oxide (ThO2), in a process involving as intermediate compound a soluble oxifluoride (ThOF2), (iii) the reduction of thorium ions on reactive (Ni and liquid Bi) electrodes yields compounds Ni–Th and Bi–Th with a potential shift of around 0.7 V (for Ni and Bi) more anodic than the reduction of Th(IV) on inert substrate.