A study on the electrochemical deposition behavior of uranium ion in a LiCl–KCl molten salt on solid and liquid electrode

Electrochemical behaviors of uranium ion (U3+) from a molten salt electrolyte to metal solid and liquid cathodes were studied for an electrorefinning recovery in pyrochemical separation. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out to investigate the interfacial processes related with cathodic reduction of uranium ion. Electrodeposition at solid Mo and liquid Cd electrodes are discussed as well as mass transfer in liquid Cd metal and LiCl–KCl molten salt phases. The reduction of U3+/U at both of the electrodes preceded a (quasi-)reversible reaction with exchange of three electrons. The difference of the standard formal potential at both solid and liquid electrodes was found which may be attributable to lowering activity of U in liquid Cd phase. The impedance data measured at different potentials were fitted to the equivalent circuits for each electrode. In case of solid electrode, the impedance spectrum was related with surface diffusion followed by lattice formation as well as charge transfer at low cathodic potential. At high cathodic potential, the effect of surface relaxation induced by U deposition on the surface of Mo electrode was found. In case of liquid electrode, the atomic diffusion of reduced U metal in liquid Cd phase appeared before the saturation of Cd by U. Over the solubility limit, the formation of U dendrite on the Cd surface made the impedance magnitude decreased by increasing the electrode surface area.