Electrochemical behavior of silicon species in high-temperature molten CaCl2

Abstract

Silicon (Si) is a pivotal element in devices that leverage renewable energy. Ongoing research has focused on electrochemical Si production in high-temperature molten salts to replace conventional energy-intensive and CO2-emissive Si production processes. Although Si species such as silicate and SiO2 have been used for electrochemical Si production, their electrochemical behavior is unclear. Here, we systematically analyzed the electrochemical behavior of Si, CaSiO3, and SiO2 in molten CaCl2 at 850 degrees C using cyclic voltammetry at glassy carbon and Si wafer electrodes. The aim of this study was to establish the electrochemical characteristics of Si species in molten CaCl2. Whereas Si undergoes electrochemical oxidation, leading to the formation of a passivation layer on its surface, silicate undergoes electrochemical reduction, resulting in the deposition of Si on the electrode. This reduction reaction was not kinetically facile and is associated with the diffusion of silicate ions in the molten salt. Moreover, the presence of silicate increased the oxidation current of Si. This likely originates from the involvement of silicate in the oxidation reaction, which subsequently enhances the solubility of the oxidation products. The SiO2 nanoparticles exhibited electrochemical characteristics similar to those of silicates; however, the electrochemical reduction of SiO2 occurred predominantly for surface-bound particles rather than for diffusing particles, and the increase in the oxidation current of Si was less significant than for silicates. Our investigation clarified the electrochemical behavior of Si species in high-temperature molten salts, which can help the development of electrochemical Si production.