Electrochemical Behavior and Regulation of Eu(II)/Eu(III) Redox Couple in Molten Fluorides

Abstract In this study, the electrochemical behavior of Eu(II)/Eu(III) redox couple in LiF-NaF-KF (46.5-11.5-42 mol%) (FLiNaK) molten salt was studied by cyclic voltammetry (CV), square wave voltammetry (SWV), and chronopotentiometry (CP) on inert tungsten electrode at 923 K. The results showed that the Eu(II)/Eu(III) couple was a one-electron exchanged system, corresponding to the reduction of Eu(III) to Eu(II) and the oxidation of Eu(II) to Eu(III), and both processes were reversible and diffusion-controlled. Then, the concentration ratio of Eu(II)/Eu(III) was determined by combining SWV and X-ray photoelectron spectroscopy (XPS) techniques, and the results showed that Eu(III) was decomposed into Eu(II) during heat treatment and Eu(III) and Eu(II) would coexist in molten FLiNaK. Therefore, Eu(II)/Eu(III) was chosen as a redox indicator with its concentration ratio to assess the redox properties of molten salt. Then, the regulation of [Eu(II)]/[Eu(III)] ratio was studied by introducing Li and CeF₄. The addition of Li as a reducing agent increases the [Eu(II)]/[Eu(III)] ratio, while the addition of CeF₄ as an oxidizing agent decreases the [Eu(II)]/[Eu(III)] ratio. Moreover, the influence mechanism of O²⁻ on the [Eu(II)]/[Eu(III)] ratio was also investigated by Raman spectroscopy and X-ray diffraction (XRD). The results showed that the [Eu(II)]/[Eu(III)] ratio decreased with the increasing amount of O²⁻ added to the molten salt. This is related to the change of free F⁻ concentration and coordination environment in molten salt caused by the formation of EuOF(s), which promotes the transformation of Eu(II) into Eu(III) with a relatively strong coordination ability with F⁻.