Electrodeposition of Neodymium from NdCl3-Containing Eutectic LiCl–KCl Melts Investigated Using Voltammetry and Diffusion-Reaction Modeling

Electrodeposition of neodymium (Nd) metal from NdCl3-containing molten LiCl-KCl eutectic melts was investigated using voltammetry and diffusion-reaction modeling. Voltammetry studies confirmed that Nd electrodeposition is a two-step reduction process involving first a reversible one-electron transfer reduction of Nd3+ to Nd2+, followed by quasi-reversible reduction of Nd2+ to Nd metal. In the electrode potential range where Nd3+ is reduced to Nd2+, the peak current density measured in a voltammetry scan showed good agreement with the classical Randles-Sevcik model for reversible soluble-soluble redox transitions. However, in the potential range where Nd2+ is reduced to Nd metal, the experimentally measured peak currents in the voltammogram were substantially lower than those predicted by applying the Berzins-Delahay model for reversible soluble-insoluble redox transitions. In the present work, this discrepancy was addressed using transient diffusion-reaction modeling, which accounted for the multivalent (Nd2+ and Nd3+) species transport and their multi-step reduction to Nd metal. The diffusion-reaction model accurately predicts the voltammetric response during Nd electrodeposition in a broad range of operating conditions (species concentrations and voltammetry scan rates), while providing access to the kinetic parameters governing Nd electrodeposition from halide melts. The approach presented herein may also be applied to other electrodeposition systems which undergo multivalent redox transitions at the electrode-electrolyte interface. (C) The Author(s) 2017. Published by ECS. All rights reserved.