Raman Analysis and Electrochemical Reduction of Silicate Ions in Molten NaCl–CaCl2

This study investigated the ionic species and electrochemical reduction of silicate ions at a solid graphite electrode in molten NaCl–CaCl2 eutectic melts with various concentrations of O2− ion at 1023 K. Silicate ion species in the melts with various O2−/SiO2 ratios ( r O 2 − / SiO 2 ) were determined by Raman spectroscopy. The dominant species was SiO3 2− for r O 2 − / SiO 2 = 1.0, and SiO4 4− for r O 2 − / SiO 2 = 1.5 and 2.0. From cyclic voltammetry, XRD, and SEM analyses, electrochemical reduction was indicated for SiO3 2− and SiO4 4− at more negative than 1.0 V and 0.80 V vs Na+/Na, respectively. Formation of CaSi2 was confirmed at 0.50 V in all molten salts with r O 2 − / SiO 2 = 1.0, 1.5, and 2.0. The potential ranges for pure Si deposition are almost the same in molten salts with r O 2 − / SiO 2 = 1.0 and 1.5.