Electrochemical behaviour of LiCl-KCl eutectic melts containing moisture as impurity. Part II: Uranium electrode

Abstract

In Part I, LiCl-KCl eutectic melt purified by chlorine and LiCl-KCl + xwt.% H2O prepared by thermal equilibration of eutectic mixture with H2O at 500 °C were analysed for moisture content using acid-base titration. They were further investigated by electrochemical impedance spectroscopy (EIS) at inert tungsten electrode. It was established that cathodic reduction of H2O led to formation of OH- that adsorbed on tungsten electrode in potential range −0.80 to −1.50 V. The OH- film contributed to capacitive response as well as it offered a resistance to any other faradaic process occurring in that potential range. In this work, same LiCl-KCl eutectic melts were used to explore the anodic dissolution of uranium electrode in potential range −1.40 to −0.90 V by EIS. Complex impedance data in all melts were fitted to different equivalent circuit models. Presence of OH- film, which influenced the dissolution of uranium in LiCl-KCl + H2O melts, was modelled using additional resistor and constant phase elements in the equivalent circuit. Impedance data were further validated by Kramers-Kronig transforms where pseudo-χ2 values varied from 8.5×10−6 to 1.2×10−4 confirming a good compliance with spread of fit residuals across logω limited to less than ±3%. After EIS measurements, melts were again analysed by cyclic voltammetry and additional voltammetric features complicating the redox behaviour of U3+|U couple were analyzed.