Electrochemical Synthesis Electrochemical Preparation of Silicon and Its Alloys from Solid Oxides in Molten Calcium Chloride** Xianbo Jin, Pei Gao, Dihua Wang, Xiaohong Hu, and George Z. Chen* Silicon plays essential roles in the fabrication of solar cells, silicon chips, optical fibres, silicones, and is important as an element in lighter and stronger alloys, as well as hundreds of other advanced applications. [1,2] The industrial production of silicon is at present mainly through the carbothermic reduc- tion of SiO 2 at 1700 8C, in which the oxygen is removed by the generation of CO 2 . [3,4] This old-fashioned charcoal technology should be replaced by a more advanced process from the viewpoint of an environmentalist, in that the earth's climate is gradually becoming worse owing to the emission of green- house gases. [5] The world production of silicon in 2002 was about 4.110 6 tonnes, [6] equivalent to about 6.5  10 6 tonnes of CO 2 entering the atmosphere. A desired method of producing silicon may be the electrochemical reduction of SiO 2 in which the use of carbon reductant can be avoided and hence less environmental damage. In fact, electrolytic pro- duction of silicon began in 1854, [7] and silicon of 99.999% in purity was later claimed [8] upon electrolysis of fluorosilicates in molten fluorides. DeMattei et al. in 1982 suggested [9] that the ideal raw material for silicon production should be SiO 2 , and silicon metal with a purity of 99.97% was produced in the BaO/SiO 2 /BaF 2 system with the cell temperature being around 1450 8C. Recently, in molten CaCl 2 at about 900 8C, electrochemical deoxygenation of metals was investi- gated. [10–12] It was reported more recently [13–17] that when solid metal oxides are made into an electrode, regardless of their electron conductivities, they can be electroreduced (or electrodeoxidised) directly to the respective metals or alloys. This leads to a great possibility for the production of silicon directly from solid SiO 2 . However, until now, there is only one relevant report that concerns the electroreduction of solid SiO 2 , and only partial reduction of the SiO 2 electrode (quartz plate) was achieved. [18] Herein, we report the fast, complete, and low energy electroreduction of a porous electrode prepared from SiO 2 powder. We also report an experimental observation that might indicate the existence of an optimal thickness of an insulating solid oxide, for example, SiO 2 , through which the electroreduction can progress “quickly” to completion. Fur- thermore, two examples of silicon alloys have also been produced by the same electroreduction method. Recently, Nohira et al. succeeded in removing oxygen from the surfaces of solid SiO 2 plates (quartz) in a molten CaCl 2 electrolyte at 850 8C. [18] Similar results were also obtained in our laboratory by using a different electrode design. In the experiments, a tungsten wire of 300 mm in diameter was sealed in a quartz tube by using a gas flame. The end face of the tungsten wire was revealed by grinding. The SEM image showed a very intimate contact between the quartz and the W wire (Figure 1a). This W–SiO 2 electrode was then inserted into molten CaCl 2 at 850 8C and cyclic voltammetry was carried out by using a Pt wire and a graphite rod as the pseudoreference and counter electrodes, respec- tively. As shown in Figure 2a, the reduction of SiO 2 began at 0.85 V, which led to a sharp increase in the current that reached a peak of about 12.5 mA at 1.0 V. The current then went through a slightly inclined plateau on which a couple of small peaks can be seen (Figure 2a). These peaks are thought to correspond to the formation of the calcium and silicon compounds. [18,19] Upon reversing the potential sweep the current formed a typical stripping peak. When the sweep reached more positive potentials, a large poorly defined anodic peak occurred at about 0.7 V, thus suggesting that the reoxidation process may involve both pure and com- pounded forms of Si. A further study of the reaction mechanism is ongoing. Figure 1a–d shows the SEM images of the W-SiO 2 electrode before and after the first potential sweep cycle. After this sweep the central W disc was surrounded by a ring of porous product of about 200 mm in breadth (Figure 1b). [*] Dr. X. B. Jin, P. Gao, Dr. D. H. Wang, Dr. X. H. Hu College of Chemistry and Molecular Science Wuhan University, Wuhan, 430072 (P. R. China) Fax: (+ 86)27-87210319 E-mail: mel@chem.whu.edu.cn Dr. G. Z. Chen Specially Invited Professor College of Chemistry and Molecular Science Wuhan University, Wuhan, 430 072 (P.R. China) and School of Chemical, Environmental and Mining Engineering University of Nottingham University Park, Nottingham NG7 2RD (UK) Fax: (+ 44)115-9514171 E-mail: george.chen@nottingham.ac.uk [**] The authors are grateful to the Ministry of Education of China and the Natural Science Foundation of China for financial support. Angewandte Chemie 733 Angew. Chem. Int. Ed. 2004, 43, 733 –736 DOI: 10.1002/anie.200352786 # 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim