Electrodeposition of Si from Silicate Ions at Graphite and Liquid Zn Electrodes in Molten CaCl 2

To establish a next-generation production process for SOG-Si, we proposed the electrochemical reduction process of SiO 2 using a liquid Zn cathode in molten CaCl 2 . In this study, we focused on the dissolved SiO 2 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 CaCl 2 with various O 2− /SiO 2 ratios ( r O 2− /SiO 2 ) at 1123 K were investigated. Ionic species of silicates in the melts were determined by Raman spectroscopy. The dominant species were SiO 3 2− ( r O 2− /SiO 2 = 1.0) and SiO 4 4− ( r O 2− /SiO 2 = 1.5 and 2.0). Cyclic voltammetry, XRD, and SEM revealed that the electrochemical reduction of SiO 3 2− , Si 2 O 7 6− , and SiO 4 4− occurs at the graphite electrode from 1.35 V, 1.12 V, and 0.75 V vs Ca 2+ /Ca, respectively. The electrochemical reduction potentials of silicate ions at the liquid Zn electrode shifted to more negative values in the order SiO 3 2− > Si 2 O 7 6− > SiO 4 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.