Exchange current density of Gd(III)/Gd reaction in LiCl-KCl eutectic and analysis of errors caused by various methods

Reliable measurement of exchange current density i0 is important for systems using molten salts as electrolyte or/and coolant. The values of i0 of Gd(III)/Gd reaction in LiCl-KCl eutectic at different GdCl3 concentrations (1-9 wt%) and temperatures (723-823 K) are reported. The values are determined by optimization fitting method using the non-simplified electrode kinetics equation, and other three commonly used methods including Tafel method, linear polarization (LP) method, and electrochemical impedance spectroscopy (EIS) method. Results show that significant errors can be introduced by using the three conventional methods in the mass-transfer involved case as they are based on the simplified electrode kinetics equations with an assumption of no mass-transfer effect. The optimization fitting method shows more reliable results but has limitations at 1 wt% Gd concentration. The standard rate constant k0 values determined by the optimization fitting are 4.93-9.13 × 10−5 cm/s in the studied temperature range and the activation energy is 30.5 kJ/mol.