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

Abstract

In this work, we explored the electrochemical behavior and reduction process of uranyl ions UO 2 2 + in molten LiCl-KCl eutectic at 773 K. Cyclic voltammetry (CV) and square wave voltammetry (SWV) results showed that the reduction of UO 2 2 + ions on the inert W electrode was a three-step process: (1) U O 2 2 + + e − ↔ UO 2 + , (2) UO 2 + + e − ↔ UO 2 and (3) UO 2 + 4 e − ↔ U . Electrolysis experiments further confirmed this reduction mechanism that UO 2 2 + 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, UO 2 2 + 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 UO 2 2 + ions in molten LiCl-KCl, providing a guiding ideology for the pyrochemical reprocessing of oxide spent fuels.