Electrodeposition of Si from Silicate Ions at Graphite and Liquid Zn Electrodes in Molten CaCl₂

Abstract

To establish a next-generation production process for SOG-Si, we proposed the electrochemical reduction process of SiO₂ using a liquid Zn cathode in molten CaCl₂. In this study, we focused on the dissolved SiO₂ to improve the productivity of the process. The ionic species and electrochemical reduction of silicate ions at solid graphite and liquid Zn electrodes in molten CaCl₂ with various O²⁻/SiO₂ ratios (rO²⁻/SiO₂) at 1123 K were investigated. Ionic species of silicates in the melts were determined by Raman spectroscopy. The dominant species were SiO₃²⁻ (rO²⁻/SiO₂ = 1.0) and SiO₄⁴⁻ (rO²⁻/SiO₂ = 1.5 and 2.0). Cyclic voltammetry, XRD, and SEM revealed that the electrochemical reduction of SiO₃²⁻, Si₂O₇⁶⁻, and SiO₄⁴⁻ occurs at the graphite electrode from 1.35 V, 1.12 V, and 0.75 V vs Ca²⁺/Ca, respectively. The electrochemical reduction potentials of silicate ions at the liquid Zn electrode shifted to more negative values in the order SiO₃²⁻ > Si₂O₇⁶⁻ > SiO₄⁴⁻. The reaction observed at 0.60 V was attributed to the simultaneous electrochemical reduction of silicate ions and formation of Ca-Zn alloy. The indirect reduction of silicate ions by Ca-Zn alloy was also confirmed.