Influence of Fluoride Ions on Electrochemical Behavior of Lanthanum and Neodymium Ions in Molten LiCl

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The influence of fluoride ions on electrochemical behavior of lanthanum and neodymium ions in molten LiCl at 1073 K was experimentally investigated. Cyclic voltammetry and square wave voltammetry were conducted using a molybdenum working electrode. Galvanostatic electrolysis using a molybdenum electrode as cathode in molten LiCl-5 mol%LaCl₃, LiCl-5 mol%NdCl₃, and LiCl-30 mol%LiF-5 mol%NdCl₃ were conducted to investigate the influence of fluoride ions. The result indicates that the cathodic process for La³⁺ in molten LiCl is a one-step reaction, i.e., La³⁺ + 3e = La. The cathodic process of Nd³⁺ in molten LiCl is a two-step reaction, including Nd³⁺ + e = Nd²⁺ and Nd²⁺ + 2e = Nd. With the introduction of fluoride ions, however, the current of the first cathodic reaction decreases and when the molar ratio of F⁻ to Nd³⁺ reaches to 6, the cathodic process of Nd³⁺ turns to a one-step process: Nd³⁺ + 3e = Nd. The results indicate that the addition of fluoride ions makes Nd³⁺ stabler and thus inhibits the reaction to Nd²⁺ in molten chloride. The deposited neodymium metal was dispersed in the electrolyte in pure chloride melt due to the shuttle of disproportionation and proportionation reactions of neodymium ions. With the addition of enough LiF to melt, the deposited neodymium metal adhered on the cathode. The current efficiency of neodymium electrolysis was remarkably improved and reached the similar value as that for lanthanum electrolysis.