Electroreduction Mechanism of UO2Cl2 and Nucleation and Morphology of Uranium Dioxide at Different Conditions

To investigate the electroreduction mechanism of UO2Cl2 and the nucleation mode and morphology of uranium dioxide, the electrochemical behavior of UO22+ on the tungsten (W) electrode was studied in LiCl-KCl molten salt by cyclic voltammetry. The results demonstrated that UO22+ underwent two sequential single-electron transfers to form UO2. Prior to its reduction to UO2+, UO22+ was initially converted into a low-coordination complex ion. Besides the reduction peak of bulk UO2, a peak associated with the strong adsorption of UO2 on the electrode surface was also observed. The nucleation mode of UO2 was examined by chronoamperometry, revealing that the nucleation mechanism was related to UO2Cl2 concentration. The influence of current densities on nucleation morphology was studied using an atomic force microscope and found the nuclei was smaller and denser at higher current density. The deposits obtained by galvanostatic electrolysis at different current densities were examined by scanning electron microscopy-energy-dispersive X-ray spectroscopy and X-ray diffraction, showing that a regular octahedral UO2 was formed at low current density, while a dendritic UO2 was formed at high current density. A two-step electrolysis with an initial high current density followed by a low current density was conducted, and a dense UO2 with a lychee-like morphology was produced.