Electrochemical behaviour of erbium in the eutectic LiCl–KCl at W and Al electrodes

Abstract

The electrochemical behaviour of Er(III) was studied in the molten LiCl–KCl eutectic for temperatures ranging from 653 to 823 K. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species at a tungsten electrode. The results showed that in the eutectic LiCl–KCl, the reduction of Er(III) occurs in a step with a global exchange of three electrons, and that electrocrystallization plays an important role in the electrodeposition process. Chronoamperometric studies indicated instantaneous nucleation of erbium with three-dimensional growth of the nuclei whatever the applied overpotential. Mass transport towards the electrode is a simple diffusion process, and the temperature dependence of the diffusion coefficient of the electroactive specie Er(III) was written in the following equation: log D Er ( III ) = − 2.77 ( ± 0.175 ) − 1695 ( ± 121 ) T The electrode reactions of the LiCl–KCl–ErCl 3 solutions at an Al wire were also investigated by cyclic voltammetry and open circuit chronopotentiometry in the temperature range of 673–823 K. The redox potential of the Er(III)/Er couple at the Al electrode was observed at more positive potentials values than those at the inert electrode. This potential shift was thermodynamically analyzed by a lowering of activity of Er in the metal phase due to the formation of intermetallic compounds. The activities and relative partial molar Gibbs energies of Er were obtained from the open circuit chronopotentiometry measurements. The standard Gibbs energies of formation for the ErAl 3 intermetallic compound were estimated and the relative partial molar entropies and enthalpies of Er were also calculated.