Electrochemical Behavior And Electrodeposition Of La And U In Ionic Liquids

Reprocessing of spent fuel is the critical technology support for the sustainable development of nuclear energy. The main purpose of the reprocessing is to recycle uranium, plutonium and other actinides in the spent fuel. The dry process which is nonaqueous-based technology has several advantages, such as minimum waste generation, low criticality concern, good radiation stability. In the dry process, molten salt electrolysis technology is one of the most promising alternative to wet processing technology. But the inorganic molten salt is composed of an eutectic of alkali or alkaline earth chloride.Therefore, the processing temperature in this method is invariably above800K depending upon the composition of chosen eutectic. In view of this, the pyrochemical method offers a few challenges with requirement of inert atmosphere, high-temperature operation and corrosion problems, and so forth. Room temperature ionic liquid has many advantages, such as wide electrochemical window, low melting point, insignificant vapor pressure and high thermal stability, which is promising to replace high-temperature molten salt for spent fuel dry reprocessing. However, there is few research reports about electrochemical behavior of Lanthanides and Actinides in Ionic Liquids. Therefore, this work first use lanthanide ions to simulate the actinides and several studies are completed:Synthesis of La (Ⅲ)-containing ionic liquids, electrochemical behavior of La(Ⅲ) in MPPiNTf2and electrodeposition of La. Then,we use the same method to complete the electrochemical behavior and electrodeposition study of uranium. The main contents and results are as follows:1.The La(Ⅲ)-containing ionic liquids have been synthesized and the electrochemistry window and conductivity have been tested in our work.The synthesis of La(NTf2)3and U(NTf2)4was completed by reaction of La2O3or UO2with HNTf2.Then the organic salts was mixed with MPPiNTf2to prepare La (Ⅲ) and U(Ⅳ)-containing ionic liquids. Conductivity tests show that the conductivity of MPPiNTf2increase with increase of temperature. Cyclic voltammetry curves show that MPPiNTf2has an extended cathodic stability and wide electrochemical window,4.9V(vs.Pd).2.The electrochemical behavior of La (Ⅲ) in the MPPiNTf2was studied and lanthanum film was obtained after potentiostatic deposition. The La (Ⅲ) underwent a single step three-electron irreversible transfer to La (0) at platinum working electrode and the peak potential is-2.8V(vs.Pd). La (Ⅲ) has a poor dispersal ability in the MPPiNTf2, the diffusion coefficient increases with the increase of temperature from4.8×10-8cm2/s at308K to2.88×10-7cm2/s at323K. It is observed that the influence of temperature on diffusion coefficient was insignificant and the energy of activation was found to be99.4kJ/mol. Electrolysis at-2.9、-3.0、-3.1V (vs. Pd), results in a deposition of lanthanum in metallic form.The grain size of the lanthanum decrease as the increasing of the deposition potential.3.The electrochemical behavior of U(Ⅳ) in the MPPiNTf2was studied and uranium film was obtained after potentiostatic deposition. It is observed that the MPPiNTf2can be used for the spent fuel non-aqueous reprocessing application. The U(Ⅳ) underwent a two step four-electron transfer to U(0) at glassy carbon electrode and the peak potential is-2.7V(vs.Pd). Electrolysis at-2.8V(vs.Pd), results in a deposition of uranium in metallic form.