Effects of F– on the Electrochemical Behavior of Zr(IV) and the Nucleation Mechanism of Zr in the LiCl–KCl–K2ZrF6 System

To determine the effect of F- on the electrochemical formation of Zr, the reduction mechanism, kinetic properties, and nucleation mechanism of Zr(IV) were compared in the LiCl-KCl-K2ZrF6 system before and after the addition of F- at different concentration ratios of F-/Zr(IV). As indicated by the results, when the ratio of F-/Zr(IV) ranged from 7 to 10, the intermediate state Zr(III) was detected, and the reduction mechanism of Zr(IV) was converted into Zr(IV) → Zr(III) → Zr. The diffusion coefficients of Zr(IV), Zr(III), and Zr(II) decreased with an increase in the value of F-/Zr(IV). The exchange current density (j0) of Zr(II)/Zr exceeded that of Zr(III)/Zr, and the j0 and α values of Zr(III)/Zr decreased with the increase of F-/Zr(IV). The nucleation mechanism at different ratios of F-/Zr(IV) was investigated through chronoamperometry. The result suggested that the nucleation mechanism of Zr varied with the overpotential at F-/Zr(IV) = 6. The addition amount of F- led to the variation of the nucleation mechanism of Zr, i.e., progressive nucleation when F-/Zr(IV) = 7 and instantaneous nucleation when F-/Zr(IV) = 10. Zr was prepared through constant current electrolysis at different concentrations of F- and then analyzed through X-ray diffraction (XRD) and scanning electron microscopy (SEM), suggesting that the concentration of F- can exert a certain effect on the surface morphology of products.