Electrolytic Deposition Of UO₂ On Metal-oxide Modified Electrodes In Molten Salts

To obtain the fundamental data of electrochemical formation of UO₂ for the conceptual design and technology evaluation of spent oxide fuel reprocessing,it is necessary to investigate the effect of cathode materials on the formation of UO₂ by electro-reduction.In this work,metal-oxide modified electrodes ITO（tin-doped indium oxide）/W,MoO₂/Mo and WO₃/W nanoporous electrodes were firstly prepared.Then,the electrochemical behaviors of UO₂Cl₂ and deposition of UO₂ were studied on these prepared electrodes,and a comparative analysis of UO₂ deposited by molten salt electrolysis on different electrodes was carried out.The details are as follows:（1）The electrochemical behavior of UO₂Cl₂ on ITO/W electrode was studied by cyclic voltammetry,stripping voltammetry and linear polarization techniques.Therein,ITO/W was prepared by magnetron sputtering.It was found that as the temperature increased,the exchange current density of U（V）/UO₂couple increased,which is more conducive to the electrode reaction.However,the higher the temperature,the more unstable and more prone to disproportionation reaction,which is not conducive to the electrochemical generation of UO₂.Therefore,constant potential electrolysis was performed for 1 h at-1.0 V and 773 K.The deposit was characterized by SEM-EDS,which showed that UO₂ was successfully deposited on ITO/W electrode,but the compactness was not good.（2）The electrochemical reduction of UO₂Cl₂ in LiCl-KCl molten salt on MoO₂/Mo electrode was studied.Therein,the MoO₂/Mo modified electrode was directly prepared from metallic Mo by electrochemical anodic oxidation.The electrochemical reduction of UO₂²⁺was found to be a two-step single electron transfer process to form UO₂.The diffusion coefficient of UO₂²⁺was estimated at 773 K to be 2.39×10^-5 cm² s^-1.In addition,constant potential electrolysis was carried out Mo and MoO₂/Mo modified electrodes at-1.0V for 1 h.The deposits were characterized by SEM,XRD and TEM,which exhibited that UO₂was prepared on both electrodes,but the deposition morphology was different.The morphology of UO₂ on Mo electrode was dendritic,while UO₂ on MoO₂/Mo electrode was denser.（3）The electrochemical formation of UO₂ and nucleation mechanism on nanoporous WO₃/W electrode were studied in LiCl-KCl molten salts.Therein,the nanoporous WO₃/W modified electrode was directly prepared by electrochemical anodic oxidation.The electrochemical reduction of UO₂²⁺was found to be a two-step single electron transfer process:UO₂²⁺+e^-（?）UO₂⁺;UO₂⁺+e^-（?）UO₂（bulk）.The exchange current density of UO₂⁺/UO₂pair determined on nanoporous WO₃/W electrode is smaller than that on W electrode using Tafel technique,and the deposit morphology confirmed that the lower exchange current density could lead to the increase of critical radius.Meanwhile,the nucleation mechanism of UO₂ on WO₃/W eletrode was probed to be progressive nucleation with three-dimensional growth.Furthermore,to compare the difference of deposit morphology on W and nanoporous WO₃/W electrodes,constant potential electrolysis was conducted at-1.0 V for 1 h.The deposits were characterized employing SEM,XRD and TEM,which indicated that UO₂ was prepared on the both electrodes.However,the deposit morphologies are different.UO₂formed on WO₃/W electrode had regular octahedron,and preferential orientation along（111）direction;while the dendritic morphology UO₂ on W electrode.Compared with ITO/W and MoO₂/Mo electrodes,UO₂ could be better deposited on WO₃/W modified electrode.