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 (r O2−/SiO2 ) at 1123 K were investigated. Ionic species of silicates in the melts were determined by Raman spectroscopy. The dominant species were SiO3 2− (r O2−/SiO2 = 1.0) and SiO4 4− (r O2−/SiO2 = 1.5 and 2.0). Cyclic voltammetry, XRD, and SEM revealed that the electrochemical reduction of SiO3 2−, Si2O7 6−, and SiO4 4− 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 SiO3 2− > Si2O7 6− > SiO4 4−. 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.