Electrochemical behavior of dysprosium and lanthanum in molten LiF-NaF-KF (Flinak) salt

Abstract

Cyclic voltammetry (CV) analysis was performed to determine the behavior of Dysprosium (Dy) and Lanthanum (La) in molten FLiNaK salt (LiF-NaF-KF: 46.5–11.5–42 mol%) and to understand the mechanism driving the electrochemical reactions. The goals of these experiments are to understand the behavior of contaminants and nuclear fission products related to the Fluoride Salt-cooled High-Temperature Reactor (FHR) concept, and to generate a reliable source of electrochemical properties in fluoride salts. The tests were performed at 600–650–700 °C for Dy and 650–700–750 °C for La in a glassy carbon crucible contained within an inert argon atmosphere. A platinum wire quasi-reference electrode exhibited stable potential throughout experiments and allowed for thermodynamic reference between test species and known redox couple (F2/F−). The electrochemical reduction of Dy and La (assumed three-electron, single-step transfer) maintained quasi-reversibility at the solid tungsten electrode for scan rates of 160 mV/s and below during CV. For scan rates of 200 mV/s and above, significant deviation from reversibility was evidenced by increasing separation between anodic and cathodic peaks. For Dy3+, the diffusion coefficient was calculated as D [cm2/s] = 0.0538∗exp(−6193/T(K)) and the activation energy Ea = 51.5 kJ/mol. The diffusion coefficient of La3+ was calculated as D [cm2/s] = 14.252∗exp(−15285/T(K)) and the activation energy Ea = 127 kJ/mol. The apparent potential of the La3+/La0 redox reaction was determined as E0∗[V vs. K+/K] = −0.368 + 0.001119∗T[K] and for the Dy3+/Dy0 reaction as E0∗[V vs. K+/K] = 0.0626 + 0.00088∗T[K].