The influence of F− ion on the electrochemical behavior and coordination properties of uranium in LiCl-KCl molten salt

Abstract

• Combined electrochemical techniques, spectroscopy and molecular dynamics simulation. • Determined that F − reacts with [UCl6]2− and [UCl6]3− to form [UCl3F3]2− and [UCl5F]3−, and their coordination structures. • Determined the effects of [UCl6-iFi]2-/3− formation on the electrochemical behavior of U. • Compared with U(III), F − ion has a stronger effect on U(IV). The composition of electrolytes is one of the key factors that determine the electrochemical properties of elements in an electrochemical process. In this work, the influence of LiF introduction into LiCl-KCl molten salt on the electrochemical behavior and coordination properties of uranium was studied by combining electrochemical and spectroscopic techniques as well as ab initio molecular dynamic simulations. The results showed that, compared to coordination with U(III), F − ion readily coordinates with U(IV), which has a smaller ion radius and larger charge density. The involvement of F − ion in coordination makes U(IV) complex more stable, and the equilibrium potential of U(IV)/U(III) shifts to the negative direction, closer to that of U(III)/U(0). In the presence of a large amount of Cl − ions, F − ions cannot completely replace the role of Cl − ions, so U(IV) and U(III) prefer to form UCl 3 F 3 2− and UCl 5 F 3− complexes, respectively. Anyway, the involvement of F − ion in coordination may lead to smaller volume of uranium complexes in melt, resulting in slight increase of the diffusion coefficient of U(IV). In addition, the effect of substitution of Cl − by F − ions on the crystal field splitting energy of U(III) complex was observed. The in-depth understanding of the relationship between the coordination structure and electrochemical properties of uranium in melt will be of much significance for the design and control of more efficient electrochemical process for actinide separation.