Production of Silicon by Solid Oxide Membrane-Based Electrolysis Process
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Production of Silicon by Solid Oxide Membrane-Based Electrolysis Process Yihong Jiang1, JiaPeng Xu1, Xiaofei Guan1, Uday B. Pal1, 2, Soumendra N. Basu1, 2 1 Division of Materials Science and Engineering, Boston University, Brookline, MA 02446, USA 2 Department of Mechanical Engineering, Boston University, Boston, MA 02215, USA ABSTRACT The solid oxide membrane (SOM) electrolysis process has been successfully tested on a laboratory scale to produce silicon directly from silica in a cost-effective and eco-friendly way. A one-end-closed yttria-stabilized zirconia (YSZ) tube was employed to separate a molten salt containing dissolved silica from a liquid metal anode placed inside the YSZ tube. When an applied electric potential between a liquid tin cathode in the molten salt and the anode exceeds the dissociation potential of silica, oxygen ions are transported out of the molten salt through the YSZ membrane and oxidized at the anode while the silicon cations in the flux are reduced to silicon on the surface of the liquid tin cathode. A potentiodynamic scan (PDS) was performed to determine the dissociation potential of silica in the molten salt system. Electrolysis was performed at 1.05 V for 8 hours. The presence of high-purity silicon crystals on the surface of liquid tin cathode was confirmed by scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDS). INTRODUCTION To reduce dependence on non-renewable fossil energy sources, the investment and need of alternative energy sources, such as solar-photovoltaics, are growing rapidly [1]. The expensive, traditional manufacturing process of solar-grade silicon (SoG-Si) still remains a major barrier to lowering the cost of photovoltaic devices. As silicon alone typically costs $50-$60 per kg [2], it is critical that advanced methods of silicon production with lower cost should be developed to ensure the widely use of photovoltaic devices and solar power. Currently the Siemens process, an energy intensive process utilizing carbothermic reduction of silica, is the primary method of silicon production [3]. The feasibility of using a solid oxide membrane (SOM) process, developed by Pal et al. [4], to produce silicon from silica in a costeffective and eco-friendly way for solar cell applications has been studied. The production of high purity magnesium, copper, titanium, and tantalum directly from their oxides by the SOM process has been successfully demonstrated. By employing an yttria-stabilized zirconia (YSZ) SOM tube, metal oxide can be directly reduced at the cathode in a molten flux with an applied electric potential [4]. In addition, the byproducts from the anode are environmentally friendly and contain fewer greenhouse gases (GHG), compared with other methods of primary metals production. This paper reports on the success of depositing silicon from silica on a tin cathode using SOM technology. The future development of the SOM process for producing high purity silicon will reduce both the cost of solar power utilization and the emissions of GHG.
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