Electrochemistry of Hf(IV) in NaCl–KCl–NaF–K₂HfF₆ molten salts

The cathodic reduction mechanism of Hf(IV) ions in a fused NaCl–KCl–NaF–K₂HfF₆ salt system was studied in various NaF concentrations at 1073 K to obtain a purified dendritic Hf metal. The results of cyclic voltammetry and square wave voltammetry indicated that the reduction process comprised two steps of Hf(IV) → Hf(II) and Hf(II) → Hf at low NaF concentrations (0 < molar ratio of F⁻/Hf ⁴⁺ ≤ 17.39) and one step of Hf(IV) → Hf at high NaF concentrations (17.39 < molar ratio of F⁻/Hf ⁴⁺ < 23.27). The structure and morphology of the deposits obtained in potentiostatic electrolysis in the one-step reduction process were analyzed and verified by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectrometry. In the one-step reduction process, the disproportionation reaction between the Hf metal and Hf complex ions was inhibited, and a large dendrite Hf metal was achieved in molten salt electrorefining.