Electrochemical Reduction of Silicate Ions at Liquid Zn Electrode in Molten NaCl–CaCl₂

To establish a next-generation production process for solar-grade Si, the electrochemical reduction of silicate ions was investigated at a liquid Zn electrode in molten NaCl–CaCl₂ with various O²⁻/SiO₂ ratios (\({{r}}_{{\text{O}}^{{2}-}\text{/}{\text{SiO}}_{2}}\) = 1.0, 1.5, and 2.0) at 1023 K and 1123 K. The electrochemical reduction of SiO₃²⁻ in the molten salt with \({{r}}_{{\text{O}}^{{2}-}\text{/}{\text{SiO}}_{2}}\)= 1.0 was observed from 1.1 V vs Na⁺/Na using cyclic voltammetry at 1023 K. In the molten salt with \({{r}}_{{\text{O}}^{{2}-}\text{/}{\text{SiO}}_{2}}\)= 2.0, electrochemical reduction of SiO₄⁴⁻ was observed from 0.95 V. When the temperature of the molten salt was raised to 1123 K, the electrochemical reduction potentials of silicate ions shifted to more positive values. Samples obtained by potentiostatic electrolysis were analyzed using SEM and ICP-AES. These results indicated that the formation of the Si–Zn and Ca–Zn alloys by the electrochemical reduction of silicates and Ca²⁺ ions, respectively, proceeded simultaneously at 0.60 V. Moreover, the indirect reduction of silicate ions by the Ca–Zn alloy was confirmed during the slow-cooling period for Si precipitation. In summary, the electrolysis at 0.60 V at 1123 K, where the simultaneous formations of Si–Zn and Ca–Zn alloys proceed, is preferable because of its high current density.