Electrode Reactions of Nd3+/Nd Couple in LiCl-KCl-NdCl3 Solutions at Solid W and Liquid Cd Electrodes

Abstract

The behavior of Nd(III)/Nd(0) redox reaction at a solid tungsten (W) and liquid cadmium (Cd) electrodes in the LiCl-KCl-NdCl3 system at 773 K was analyzed by using a cyclic voltammetry. The Nd(III)/Nd(0) reaction in the solid W electrode was found to occur in two steps through the process of Nd(II) ion generation. As the analytic results of the maximum current value according to scan rates in the region of 0.05 V/s to 0.3 V/s, the Nd(III)/Nd(II) reaction showed reversible reaction characteristics over the whole range, while the Nd (II)/Nd(0) reaction showed the reversible reaction in the region of 0.05 V/s to 0.2 V/s, as well as the quasi-reversible in the range of 0.2 V/s or higher. Furthermore, the diffusion coefficients of Nd(III) ion and Nd(II) ion were calculated by using the maximum current value, which were considerably consistent with the previous results of other researchers. The reduction potential of the Nd (III)/Nd(0) reaction in the liquid Cd electrode was measured as approximately 0.54 V higher than that of the solid W electrode because the reduced Nd formed an Nd-Cd intermetallic compound with a liquid Cd electrode, lowering the activity of Nd. To confirm this mechanism, experiments using Cd-coated W electrodes were conducted by using cyclic voltammetry and chronopotentiometry. The experimental results showed that 6 Nd-Cd intermetallic compounds were present. When the ratio of Cd is the highest, NdCd11 is formed, and the potential in this case was very similar to the potential at the liquid Cd electrode. Furthermore, the Gibbs free energy and activity coefficient of Nd-Cd intermetallic compounds were calculated by analyzing the results of chronopotentiometry. The results could thermodynamically explain the potential difference of the Nd(III)/Nd(0) reactions occurring between the solid W electrode and the liquid Cd electrode.