The electrochemical behavior of Th(IV) ion on molybdenum (Mo) electrode was studied by cyclic voltammetry (CV) and square wave voltammetry (SWV) technologies at 773 K in ThF4-LiCl-KCl melt. The reduction of Th(IV) to metal Th at −1.67 V (vs. Ag/AgCl) is a four-electron exchange process, which is quasi-reversible and diffusion-controlled. The diffusion coefficient (D) and activation energy of diffusion process for Th(IV) were determined to be 3.77 × 10−5 cm2 s−1 and 59.2 kJ mol−1. The pulse potential electrolysis of ThF4-LiCl-KCl melt revealed that 86.8% of Th(IV) can be separated from the melt based on the inductively coupled plasma atomic emission spectrometer (ICP-AES) results.

The transformation of Eu(III) to Eu(II) was confirmed in a fluoride eutectic, LiF-NaF-KF (46.5-11.5-42.0 mol%, FLiNaK) molten salt during a treatment of high temperature as high as 1023 K. The coexistence of Eu(III)-Eu(II) was characterized by X-ray photoelectron spectroscopy (XPS) and voltammetry method, and their concentrations were measured. The electrochemical behavior of Eu(III) and Eu(II) in the fluoride salt was investigated. The mechanism of the electrode reaction was determined using cyclic voltammetry (CV) and square wave voltammetry (SWV). The results indicated a one-electron exchange process, corresponding to the reduction of Eu(III) to Eu(II) and the oxidation of Eu(II) to Eu(III). This process is reversible and diffusion-controlled. The diffusion coefficients (D) of Eu(III) and Eu(II) were determined using the conventional CV by changing the scanning rate and a modified method by changing the area of the working electrode successively. The values obtained by these two different methods were consistent. The temperature dependence of diffusion coefficient was investigated, and the activation energies of diffusion process were calculated to be 38.9 ± 4.6 kJ mol-1 for Eu(III) and 34.7 ± 1.6 kJ mol-1 for Eu(II), respectively.