Study on the exchange current density of lanthanides in LiCl-KCl molten salt

Abstract

Pyroprocessing is regarded one of the methods for the sustainable development of nuclear power generation in aspects of economics, non-proliferation, and nuclear waste. For better simulation and performance assessment of the electrorefining and electrowinning processes in pyroprocessing, the precise and reliable electrochemical data of lanthanides and actinides are required. However, there are only few studies which have been performed to obtain the electrochemical data such as the exchange current density and the transfer coefficient. In this work, the exchange current density, the transfer coefficient, and the reaction activation energy of the Ln(III)/Ln(0) and Ln(III)/Ln(II) reactions in LiCl-KCl molten salt at $500^\circ C$ were obtained using the electrochemical techniques, especially Tafel measurement. With an exception of NdCl3 having a two-step reaction, namely Nd(III)/Nd(II) and Nd(II)/Nd(0), within the significantly narrowed potential range, all other lanthanides undergo the electrochemical reaction with a single redox couple in the entire measured potential window. Each redox couple has the distinctive exchange current density and transfer coefficient. The Ln(III)/Ln(0) reactions exhibited the exchange current densities ranging from 7.77 to 25.48 mA/$cm^2$, which are greater than those of the Ln(III)/Ln(II) reactions with 1.16 to 5.52 mA/$cm^2$ when taking the number of transferred electrons into account. In addition, the reaction activation energy was also determined using the temperature dependence of the exchange current density according to the Arrhenius’ law. The activation energy for the Ln(III)/Ln(0) reactions was mechanistically elucidated by considering the atomization energy of lanthanides. Furthermore, there are still controversial scientific arguments on the electrochemical effect of electrode materials. Therefore, in the present work, the effect of various electrode materials on the exchange current density was investigated for the soluble/soluble reactions or the soluble/insoluble reactions of lanthanide chlorides. For the electrochemical Ln(III)/Ln(II) reactions, namely soluble/soluble reactions, the exchange current density was found different with various electrode materials, such as glassy carbon, tungsten, nickel, and platinum, even for the same lanthanide chloride. It might be associated with different Fermi levels of the electrode material, and the exchange current density is proportional to the Fermi level of materials. On the other hand, Ln(III)/Ln(0) reactions showed the same exchange current density irrespective of the use of various electrode materials, since the occurrence of electrodeposition prior to the Tafel measurement excludes the participation of the electrode material in the electrochemical reaction. For further investigation, the electrochemical deposition behaviors in the La-Ce binary system were examined in high temperature molten chloride eutectic. First, the exchange current density of the mixed La-Ce binary system was measured at various concentration ratios of La and Ce. It was experimentally confirmed that the exchange current density obtained from the binary system could be described well by each individual exchange current densities for La and Ce obtained in the single element system. In addition, the electrodeposition results showed that the deposition amounts of La and Ce are straightforward proportional to the exchange current density of each elements after considering its concentration dependence.