Fundamental Study on Reduction Rate for Electrolytic Reduction of SiO2 Powder in Molten CaCl2

Direct electrolytic reduction of powdery SiO2 has been investigated in molten CaCl2 at 1123 K aiming at a low cost and mass production of solar-grade Si. SiO2 powder was set on a Si plate which was installed at the bottom of the crucible and connected to a graphite rod used as a current lead. SiO2 powder was reduced to Si by cathodic polarization of the Si plate. The reduction rate was evaluated by the magnitude of the reduction current, in which three parameters were varied: electrode potential, SiO2 layer thickness, and SiO2 particle size. As a result, the rate-determining step of the reduction is suggested to be the diffusion of O2- ions inside the reduced porous Si particles filled with the electrolyte. During the reduction process, however, other factors such as contact resistance between Si particles and porosity of SiO2 particles possibly influence the reduction rate.