Electrochemical behavior of Sm(III)/Sm(II) and extraction of Sm on reactive electrode from molten LiF-BeF2

Abstract

The electrochemical behavior of Sm(III)/Sm(II) in 2LiF-BeF2 (FLiBe) molten salt is studied by cyclic voltammetry (CV), square wave voltammetry (SWV), chrono amperometry (CA) and Tafel method in the temperature range of 773 to 973 K. The coexistence of Sm(III) and Sm(II) and the concentration ratio of Sm(III)/Sm(II) are determined by SWV test. The redox reaction of Sm(III)/Sm(II) couple on an inert tungsten electrode is demonstrated to be a quasi-reversible process with diffusion controlled mass transfer. The diffusion coefficients of Sm(III) and Sm(II) determined by chrono amperometry are in the range of 1.17 × 10-6 to 1.09 × 10-5 cm2 s−1 for Sm(III) and 2.49 × 10-6 to 3.90 × 10-5 cm2 s−1 for Sm(II) at 773 K to 973 K and the diffusion activation energy is 8.63 and 10.1 kJ mol−1 for Sm(III) and Sm(II) respectively. The coordination and transport characteristics of the Sm(III)-F and Sm(II)-F are also studied by the first principles molecular dynamics simulations. The Gibbs free energy of the SmF2 is calculated to be −1004.4 kJ mol−1 at 873 K by experimental data, and the standard reduction potential of E0SmF2/Sm is about −0.583 V vs E0BeF2/Be at 873 K. Thermodynamic evaluation and experimental results indicates that the metallic Sm could be extracted as M−rich SmxMy intermetallics (M = Cu, Ni, Al) from FLiBe based molten salts.