Electrochemical and Spectroscopic Study on Electrochemical Processing of Silicon in Fluoride Melt

Abstract

For sustainable space utilization on the Moon and the Mars, it is essential to establish in-situ resources and energy utilization systems in space. Because of the large amount of silica in the regolith which composed of metallic oxides on the Moon and the Mars, the technology development for the covering the silica to silicon and oxygen is indispensable. The silicon can be used as a key material for silicon photovoltaics. Against this background, the authors have studied the electrochemical reduction of silica to silicon in fluoride melt. We propose a one-step process to obtain a silicon film on the cathode by electrolysis using molten salts containing silica. To determine the feasibility of the proposed process, it is necessary to understand the effects of the physicochemical properties such as the coordination structure of Si ions in the fluoride melt since the difference in the melt structure affects the nucleation and growth mechanisms of the silicon electrodeposition. However, there are few reports on the dissolved state of silicon ions in fluoride melt. The author thus clarified the relationship of the electrochemical behavior of coordination structure of Si ions in fluoride melts composed of LiF, KF, and NaF.The coordination structure of Si ions in the fluoride melt was investigated by high-temperature Raman spectroscopy and quantum chemical calculations based on the density functional theory (DFT). The vibrational modes of the Si ions were detected through high-temperature Raman spectroscopy and the Raman bands were simulated by DFT calculations. We clarified the interaction between Si ions and cation and anion species such as Li+, Na+, K+, O2−, and F−. In addition, the electrochemical behavior of Si ions was investigated by electrochemical measurements. The effect of O2− ions on the dissolution of SiO2 and the electrodeposition process of Si in fluoride molten salts revealed a significant increase in the reduction current of Si ions as well as in the film thickness and current efficiency of deposited Si. The molten fluoride system, which can fabricate Si films from SiO2 at low temperatures, has the potential to develop environmentally friendly and sustainable electrolytic systems.