Investigation of electrochemical characteristics and nucleation mechanism of cerium influenced by F− in LiCl-KCl-CsCl melts

The present study investigated the influence of F− on the electrochemical characteristics and nucleation mechanism of cerium in LiCl-KCl-CsCl molten salts, a new electrolyte medium for pyroprocessing of spent nuclear fuel with lower melting point that reduces the temperature tolerance demand of operating equipment compared with LiCl-KCl melts. By employing different electrochemical techniques, the electrochemical reaction of Ce(III)/Ce in LiCl-KCl-CsCl melts containing F− was found to be a diffusion-controlled, electrochemical reversible and three electrons transfer redox reaction at 653 K (CCe(III) = 4.94 × 10−2 mol/kg). It was also discovered that exchange current density and the diffusion coefficient of Ce(III)/Ce dropped in the presence of F−, while shifting negatively the equilibrium potential and subsequently resulting in the change of thermodynamic characteristics. The electrodeposition mechanism of Ce was also studied, demonstrating that the nucleation mode of Ce successfully transformed into progressive nucleation when F− was added. The affected electrochemical characteristics and the mechanism of electrodeposition nucleation of Ce in LiCl-KCl-CsCl molten salts at lower temperatures were attributed to the stronger complex ion [CeCl5F]3− formed after the addition of F−, being illustrated by the mathematic analysis of the molten salt structure. The formation of complex ion [CeCl5F]3− provides a new solution to inhibit the dendrite growth issue in the pyroprocessing of spent nuclear fuel at lower temperatures.