Electrochemical behavior of molybdenum ions and their coordination structures in NaCl-KCl melt containing fluoride ions

Abstract

In the process of electrolytic extraction and refining of molybdenum metal, molybdenum ions exhibit diverse valence states and are involved in complex coordination reactions with various electrolyte ions, leading to intricate forms of existence and electrochemical behavior. In this study, the electrolyte composition was adjusted by precisely controlling the addition of fluoride ions, allowing for an in-depth investigation of their influence on the reduction kinetics of Mo(V) ions and the coordination environment within the molten salt. The introduction of fluoride ions was observed to streamline the reduction mechanism of high-valent molybdenum ions, simplifying their previously complex multi-step reduction pathway to a more efficient single-step process. The initial reaction of Mo(V) ions under varying concentrations of fluoride ions was identified as a diffusion-controlled reversible process, with the diffusion coefficient decreasing as fluoride ion concentration increased. Moreover, fluoride ions demonstrate greater polarizability than chloride ions, facilitating the formation of stable coordination compounds with molybdenum ions, as evidenced by XPS analysis and first-principles molecular dynamics methods. As fluoride ion concentration increases, the coordination structure of molybdenum progressively shifts from Mo–Cl to Mo–F bonds, thereby influencing the charge transfer and ion diffusion dynamics of high-valent molybdenum ions. This study provides valuable new insights for the selection of electrolytes in the molten salt electrolytic extraction and refining process of molybdenum metal.