Electrochemistry of SmCl3 and SmF3 in Molten NaCl-KCl

Abstract

Lanthanide elements, which are present in spent fuel from fast nuclear reactors, can be converted into molten salts by anodic dissolution. Electrolysis of molten salts is promising for separation of lanthanides and actinides. To solve this problem, it is necessary to have experimental data on the electrochemical behavior of various lanthanides in molten salts. The aim of this study is determine the influence of the first coordination sphere composition of samarium complexes on its electrochemistry in molten equimolar mixture NaCl-KCl. Electrochemical investigations were carried out in a temperature range of 973-1173 K by cyclic voltammetry using an AUTOLAB PGSTAT 20 potentiostat with a package of application programs GPES (version 4.4.). The melt container was a glassy carbon crucible of SU-2000 brand, which also served as an auxiliary electrode. Tungsten wire was used as working electrode and Pt wire was utilized as a quasi-reference electrode. In the NaCl-KCl-SmCl3 melt it was found that peak current of the recharge process of Sm(III) to Sm(II) is directly proportional to the square root of the polarization rate, while the peak potential does not depend on the polarization rate up to v=0.1 V s-1. According to the theory of linear sweep voltammetry, up to the polarization rate 0.1 V s-1, the electrode process is reversible. However, in the NaCl-KCl-SmF3 molten system the recharge process was reversible up to the scan rate 1.0 V s-1. Diffusion coefficients of Sm(III) in chloride and chloride-fluoride melts were determined by cyclic voltammetry using the Randels–Sevchik equation. The quasi-reversible process for the Sm(III)/Sm(II) redox couple was determined in NaCl-KCl-SmCl3 melt at a sweep rate 0.1<v<0.3 V s-1. This mechanism is clearly evidenced by the deviation of the experimental points from linearity in the IC p vs. v 1/2 plot, by the dependence EC p on v and by the magnitude of the difference between EA p and EC p, which is larger than is required for a reversible process. A transition from a reversible to a quasi–reversible process was found at v > 1.0 V s-1 in the NaCl-KCl-SmF3 melt. The standard rate constants of charge transfer (k s) of the Sm(III)/Sm(II) redox couple were determined by cyclic voltammetry both in NaCl-KCl-SmCl3 and NaCl-KCl-SmF3 molten salts using the Nicholson’s equation, which is valid for quasi-reversible processes. The higher values of k s were obtained in chloride melt than in chloride-fluoride molten system.