Rationalisation and optimisation of solid state electro-reduction of SiO2 to Si in molten CaCl2 in accordance with dynamic three-phase interlines based voltammetry

The cyclic voltammograms of a silica sheathed tungsten disc (W-SiO2) electrode in molten CaCl2 at 900 °C exhibited an unusually increasing reduction current with decreasing the potential scan rate. When the cathodic limit was less negative than −1.00 V (vs. a quartz sealed Ag/AgCl reference electrode), the reduction current was also smaller in the forward (negative) potential scan than that in the reversed (positive) scan. However, at a given reduction charge, the reduction current increased with the scan rate, following approximately a logarithm law. These unique features have been elaborated according to the dynamic model of the conductor (silicon)/insulator (silica)/electrolyte (molten salt) three-phase interlines (3PIs). Combining the voltammetric observations with the composition analysis of the products from potentiostatic electrolysis of porous silica pellets, the optimal potential window was identified to be from −0.65 V to −0.95 V. In this potential range, silica was converted to pure silicon with the oxygen content being less than 0.5 wt.%. At potentials more negative than −0.95 V, the reduction of Ca2+ ions in the reduction-generated porous silicon layer led to the formation of various calcium silicides. These findings can help the development of an electrolytic process for clean, efficient and inexpensive production of high purity silicon.