Comparative Study of the Electrochemical Behavior of Zr(IV) in LiF-BeF₂ and LiF-NaF-KF Molten Salts

This study examines the electrochemical behavior of Zr(IV) in two different molten salt systems: FLiBe (LiF-BeF 2 , 66.7–33.3 mol%) and FLiNaK (LiF-NaF-KF, 46.5-11.5-42.0 mol%). We found that the reduction of Zr(IV) occurs via a single 4-electron transfer step in both environments, with diffusion-controlled reversible reactions evidenced through scan rate dependency studies. The diffusion coefficients measured at 873 K were 3.21 × 10 −6 cm 2 s −1 in FLiBe and 3.85 × 10 −6 cm 2 s −1 in FLiNaK. The formal standard potentials for the Zr(IV)/Zr couple were estimated to be 0.445 V in FLiBe and −0.120 V in FLiNaK, indicating a significant polarization effect in FLiNaK. This polarization is attributed to the differing coordination spheres of Zr(IV) in the two melts, with Raman analysis suggesting that Zr(IV) exists as fully coordinated ZrF 8 4− in FLiNaK, contrasting with ZrF 5 − in FLiBe. The findings of Zr(IV) polarization effects are significant for the processes of lanthanide extraction by electrochemical techniques from molten salt fuels in molten salt reactors.