Electrodeposition of Tb on Mo and Al electrodes: Thermodynamic properties of TbCl3 and TbAl2 in the LiCl-KCl eutectic melts

Abstract

The electrochemical behavior of Tb(III) in the LiCl-KCl eutectic melts was studied on Mo and Al electrodes by cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry in the temperature range of 773–873K. On a Mo electrode, the reduction of Tb(III) was one-step electrochemical process. The diffusion coefficient of Tb(III) was determined by applying the Berzins and Delahay equation. The activation energy for diffusion of Tb(III) ions was found to be 30.5kJmol−1. The equilibrium potential of Tb(III)/Tb(0) redox couple was measured by open circuit chronopotentiometry, with subsequent calculation of the apparent standard potential, ETb(III)/Tb(0)*0, and the apparent Gibbs free energy of formation, ΔGf*0(TbCl3). The activity coefficients for Tb(III), γTb(III) was also determined from the difference of apparent and standard Gibbs free energies of formation, ΔGf*0(TbCl3)−ΔGf0(TbCl3,SC). On an Al electrode, the reduction potential of Tb(III)/Tb was observed at more positive potential values than that on Mo electrode, due to the formation of Al-Tb intermetallic compound when Tb(III) ions react with the Al substrate. The TbAl2 intermetallic compound characterized by XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy and energy dispersive spectrometer), was obtained in the LiCl-KCl melts containing Tb(III) by potentiostatic electrolysis at −1.8V and −1.9V (vs Ag/Ag+), respectively. The activity of Tb, in the Al phase, as well as the standard Gibbs free energy, enthalpy and entropy of formation for TbAl2 were estimated from the open circuit chronopotentiometric measurements.