Electrochemical Behavior and Reduction of UO₂²⁺ in LiCl-KCl Molten Salt

In this work, we explored the electrochemical behavior and reduction process of uranyl ions UO22+ in molten LiCl-KCl eutectic at 773 K. Cyclic voltammetry (CV) and square wave voltammetry (SWV) results showed that the reduction of UO22+ ions on the inert W electrode was a three-step process: (1) UO22++e(- ) (UO2+)-U- , (2) UO2++ e(- )UO2 and (3) UO2+4e(-) U. Electrolysis experiments further confirmed this reduction mechanism that UO22+ ions were reduced to UO2 on the molybdenum electrode by applying a constant potential of -1.00 V vs Ag/AgCl and subsequently to uranium metal at a more negative potential of -2.35 V vs Ag/AgCl. In addition, UO22+ ions could be thoroughly reduced to uranium metal through a 4-h constant current electrolysis at -18 mA cm(-2). Electronic absorption spectroscopy (EAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) respectively illustrated that the oxidation state of uranium was unchanged and uranium concentration gradually decreased during the electrolysis. Finally, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase composition and microstructure of deposited products. Nano-sized UO2 and U metal particles were successfully obtained by constant potential and current electrolysis. The results of this work further reveal the electrochemical behavior and reduction mechanism of UO22+ ions in molten LiCl-KCl, providing a guiding ideology for the pyrochemical reprocessing of oxide spent fuels.