Reaction Behavior of Stratified SiO 2 Granules during Electrochemical Reduction in Molten CaCl 2
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TRODUCTION
THE established energy economy, fueled by oil, coal, and natural gas, is being replaced by an economy powered by renewable energies such as solar, wind, and geothermal energy. Of the renewable energy sources, the growth of the solar power industry in the last decade can be described as explosive. According to the latest data released by the European Photovoltaic Industry Association (EPIA), the global cumulative installed solar cell capacity has increased from 2.2 GW in 2002 to 102 GW in 2012.[1] The annual installation of solar cells in the world has reached 28.6 GW in 2012.[2] Currently, crystalline silicon is the most prevalent raw material for solar cells on the market. Solar cells manufactured with silicon accounted for 88 pct of the global solar cell production in 2012.[2] It will take several decades before technologies based on other materials will become competitive from a business aspect. With the rapid growth of the solar power industry, the consumption of solar-grade silicon (SOG-Si; 6N purity) has grown dramatically, and an average growth of 10 to 20 pct per year is estimated for the next decade. The modified Siemens process, based on the hydrogen reduction and thermal decomposition of trichlorosilane (SiHCl3), is currently the dominating process for SOGSi production.[3] The productivity of this process is
reinforced by the smaller number of distillations of SiHCl3 and higher temperature of the reduction step compared with the conventional Siemens process. Even though the supply currently satisfies the market demand, the productivity limitation of this process will likely cause a shortage of the SOG-Si supply when the production of solar cells exceeds in a range from 50 to 60 GW per year.[4] Moreover, the modified Siemens process has other disadvantages such as low reaction efficiency and high energy consumption. Therefore, the development of an eco-friendly production process for SOG-Si with high yield and low cost is required. Various processes have been developed with the aim of displacing the current SOG-Si production process.[3,5–9] Electrochemical methods have been investigated as an alternative process for SOG-Si production.[10–13] Typically, silicon is electrodeposited on a cathode in molten salts, and a silicon compound is supplied as the raw material. In another method, silicon is produced on the cathode via electrorefining using a low-purity Si anode as the raw material. The authors have reported that solid silica (SiO2) can be electrochemically reduced to Si in CaCl2-based molten salts using the contacting electrode method.[14–21] In this method, SiO2 is reduced to Si by electrolysis at the three-phase zone of SiO2/CaCl2/electrode.[14,15] SiO2 ðsÞ þ 4e ! SiðsÞ þ 2O2
XIAO YANG, Project Research Associate, TOSHIYUKI NOHIRA, Associate Professor, and RIKA HAGIWARA, Professor, are with the Graduate School of Energy Science, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. Contact e-mail: [email protected] KOUJI YASUDA, Assistant Professor, is with the Gradua
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