Silicon Production by Electrolysis in NaF-LiF-Mg2Si3O8 Molten Salt

Silicate minerals on S-type asteroids are a potential resource for in situ production of silicon. We investigated the feasibility of synthesizing silicon from Mg2Si3O8 through electrolysis in NaF-LiF molten salt. Solubility of Mg2Si3O8 in NaF-LiF molten salt was determined using isothermal saturation. Dissolution equilibrium is reached after 120 min, and the solubility of Mg2Si3O8 in NaF-LiF molten salt at 750 °C is 28.9%. Electrochemical studies revealed that the reduction of Si(IV) in NaF-LiF molten salt is a two-step process. Si(IV) was reduced to Si(II) at −1.1 V (vs Pt), and Si(II) was further reduced to metallic silicon at −1.2 V (vs Pt). Electrolysis of Mg2Si3O8 in NaF-LiF molten salt was conducted at 750 °C using a high-purity graphite sheet as anode and a silver sheet as cathode. Electrolysis products were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). XRD indicated that the main phase of the electrolysis products was silicon when cell voltages were 2.0, 2.5, 3.0, and 3.5 V. Raman spectroscopy confirmed the presence of crystal silicon. SEM and EDS revealed that the electrolysis products exhibit granular morphology, and silicon concentration reached a maximum value of 98.14% when the cell voltage was 2.5 V.