Electroreduction of La³⁺ Ions on a Nickel-Based Cathode in a Fluoride System

The regeneration and utilization of La from lanthanum thermal reduction of rare earth slag are essential for the high-value recovery of secondary rare earth resources. Reduction of lanthanum in fluoride molten salt systems by using active electrodes is a promising technique. In this study, electroreduction of La³⁺ ions on the surface of an active Ni cathode in a fluoride system was analyzed using cyclic voltammetry, square-wave voltammetry, and chronoamperometry. Scanning electrochemical microscopy (SECM) was used to analyze the current response and change in La³⁺ ions on the surface of the Ni electrode substrate. The products formed on the Ni cathode surface, upon electrolysis at a constant potential difference of 4.1 V, were analyzed and characterized using an electron probe technique. The results showed that electrochemical reduction of La³⁺ ions at the Ni cathode in the (LiF-LaF₃)_eut.-La₂O₃ molten system at 1223 K proceeded through two steps, i.e., La³⁺ + e → La²⁺ and La²⁺ + 2e → La. The reduction of La³⁺ ions at the cathode was transiently nucleated and diffusion controlled with a diffusion coefficient of approximately 1.33 × 10^-5 cm²·s^-1. With a negative shift in the Ni electrode potential, the banded two-dimensional nucleation region on the cathode surface continued to expand. Further, the three-dimensional (3D) nucleation active points also increased and became uniformly distributed. The La³⁺ ions in the system were fully activated when the Ni electrode potential was in the range of -0.95 to 0.95 V (vs Ag/AgCl). This protocol yielded an alloy product mainly constituting the LaNi₃ and LaNi₅ phases.