Insights into the effects of fluoride anions on the electrochemical behavior and solution structure of trivalent samarium in LiCl-KCl molten salt

Understanding the correlation between the electrochemical behavior and solution structure of lanthanides in molten salt is essential to improve the efficiency of electrorefining based actinide separation over lanthanides. Here we investigate the influence of fluoride ions on the electrochemical behaviors and coordination structure of Sm(III) in molten LiCl-KCl at 773 K. Cyclic voltammetry, square wave voltammetry, and chronopotentiometry reveal an electrochemically reversible, one-electron transferred, and diffusion-controlled Sm(III)/Sm(II) reaction in molten LiCl-KCl containing 0-10.8 equivalents of F− ions. Negative shifts of peak and half-wave potentials are observed with increasing F− concentration, suggesting that F− ions can interact with Sm(III) via substituting the surrounding Cl− ions. Moreover, the decrease in diffusivity of Sm(III) illustrates that F− ions with high concentrations also hinder the mass transfer of Sm(III) by increasing the melt viscosity. Subsequently, Raman spectroscopic study identifies the prevalence of [SmCl6]3− species in the LiCl-KCl-SmCl3 melt. As the F/Sm molar ratio increases, in situ high-temperature UV-Vis absorption spectroscopy demonstrates the weakening of the nephelauxetic effect stemming from the substitution of F− to Cl−. Based on the average coordination number of F−, it is confirmed that the weak Sm-F interactions lead to variations in the electrochemical behaviors of Sm(III).