Kinetics process of Tb(III)/Tb couple at liquid Zn electrode and thermodynamic properties of Tb-Zn alloys formation

Abstract

Electrochemical properties of rare-earth elements in the LiCl-KCl eutectic are important for the pyrometallurgical recycling process of spent nuclear fuels. In this work, the electrochemical properties of Tb (III)/Tb (0) couple were studied by the cyclic voltammetry (CV) at a liquid Zn pool electrode. The results showed that this electrochemical reaction is quasi-reversible with mixed reversible diffusion control and the charge transfer control. The diffusion coefficient of Tb (III) was determined to be in the order of ~10⁻⁵ cm² s⁻¹. Moreover, kinetic parameters, such as the standard rate constants (k _s) and charge transfer coefficient (α) for the electroreduction of Tb (III) to Tb (0) at the liquid Zn electrode, were calculated by the Nicholson method at 873 K. Additionally, it was found that Tb-Zn intermetallic compounds were easily to be formed in the measurements. Hence, the reduction process of Tb (III)/Tb (0) couple on the Zn-coated Mo electrode was also studied to obtain more information of the Tb-Zn intermetallic compounds. Electrochemical signals stemming from various intermetallic compounds associated with TbZn₁₂, Tb₂Zn₁₇, Tb₁₃Zn₅₈, Tb₃Zn₁₁, TbZn₃, TbZn₂ and TbZn, were observed. The thermodynamic data were thereafter estimated by applying the emf method at 823–923 K. The standard formation Gibbs energies and the standard equilibrium constant of each Tb-Zn intermetallic compounds were also calculated. Finally, enthalpies and entropies of formation and the apparent standard potentials of various Tb-Zn intermetallic compounds were also obtained.