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

Abstract

In this study, the exchange current density, transfer coefficient, and reaction activation energy of Ln(III)/Ln(0) and Ln(III)/Ln(II) reactions in LiCl-KCl molten salt at 500 °C were precisely obtained using electrochemical techniques, especially Tafel measurement. The number of redox couples and the redox potential of lanthanides in the LiCl-KCl redox range were investigated through cyclic voltammetry (CV). With the exception of NdCl3, which has a two-step reaction (Nd(III)/Nd(II) and Nd(II)/Nd(0)) within a significantly narrow potential range, all lanthanides have a single redox couple in the measured potential window. Each redox couple has distinctive exchange current densities and transfer coefficients. Ln(III)/Ln(0) reactions exhibited exchange current densities ranging from 7.77 to 25.48 mA/cm2, which are greater values compared to the exchange current densities of Ln(III)/Ln(II) reactions that range from 1.16 to 5.52 mA/cm2 when taking the number of transferred electrons into account. The number of transferred electrons of Sm, Eu, Tm, and Yb were determined to be 1.02, 1.01, 0.98, and 1.01, respectively, through square wave voltammetry (SWV). In addition, the reaction activation energy was determined using the temperature dependency of exchange current density according to Arrhenius’ law. The activation energy tendency of the Ln(III)/Ln(0) reactions was mechanistically elucidated by considering the atomization energy of lanthanides.