Electrochemical behaviour of terbium in the eutectic LiCl-KCl in Cd liquid electrodes.- Evaluation of the thermochemical properties of the TbCdx intermetallic compounds

Abstract

The electrode reaction of Tb(III)/Tb couple in the eutectic LiCl-KCl, at Cd liquid electrodes (i.e a Cd pool and a Cd coated W electrodes) was investigated in the temperature range of 673–823K. In both electrodes, the electrochemical reduction of Tb(III) was observed at less cathodic potential values than at the surface of an inert W electrode, due to the decrease of Tb activity in the metal phase.Cyclic voltammetry, using a Cd bulk electrode, suggest a quasi-reversible behaviour of the system Tb(III)/Tb(dissolvedinliquid Cd), and the values of the kinetic parameters, k0 and α, as well as the reversible half wave potential, Er1/2, have been obtained. The differences between the equilibrium potential adopted by a Tb electrode and the Er1/2 observed with the same Tb(III) solution at the Cd pool electrode, were used to calculate approximate values of the excess Gibbs energy change of Tb in liquid metal, and hence the activity coefficient of Tb in Cd.The formation of intermetallic compounds was also studied. Electromotive force, emf, measurements for five intermetallic compounds in two-phase coexisting states were carried out using a Cd coated tungsten electrode. The activities and relative partial molar Gibbs energies of Tb were obtained for TbCd6, TbCd45/11, TbCd3, TbCd2 and TbCd. The formation energy of each intermetallic compound, and the global formation constants were also calculated. The linear dependence of the Gibbs free energies with temperature yields to the enthalpies and entropies of formation of the five intemetallic compounds.Analysis of the samples after electrolysis runs by scanning electron microscopy (SEM) with energy dispersive X-ray allowed the identification of TbCd6, TbCd3, TbCd2 and TbCd.