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

Abstract

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 (${r}_{{O}^{2-}/{text{SiO}}_{2}})$ were determined by Raman spectroscopy. The dominant species was SiO₃²⁻ for ${r}_{{O}^{2-}/{text{SiO}}_{2}}$ = 1.0, and SiO44− for ${r}_{{O}^{2-}/{text{SiO}}_{2}}$ = 1.5 and 2.0. From cyclic voltammetry, XRD, and SEM analyses, electrochemical reduction was indicated for SiO₃²⁻ and SiO₄⁴⁻ 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 ${r}_{{O}^{2-}/{text{SiO}}_{2}}$ = 1.0, 1.5, and 2.0. The potential ranges for pure Si deposition are almost the same in molten salts with ${r}_{{O}^{2-}/{text{SiO}}_{2}}$ = 1.0 and 1.5.