Mechanism of Direct Electrolytic Reduction of Solid SiO[sub 2] to Si in Molten CaCl[sub 2]

Abstract

The mechanism of direct electrolytic reduction of SiO 2 in molten CaCl 2 was studied. Morphological and crystallographic investigations were conducted on Si prepared by potentiostatic electrolysis of SiO 2 contacting electrode at 1.10 V ͑vs. Ca 2+ /Ca͒ for 1 h at 1123 K. X-ray diffraction confirmed that amorphous SiO 2 was reduced to crystalline Si. From scanning electron microscopy ͑SEM͒ and energy-dispersive X-ray results, it was proved that Si columns were formed perpendicular to the reaction interface between Si and SiO 2 and that vacant spaces were formed between the columns. It was found from field emission SEM observation that the Si column had basically a hexagonal prismatic and stacking structure. Transmission electron microscopy and electron diffraction results revealed that the Si column was a single crystal having ͕111͖ twin planes perpendicular to the axis of the column. It is explained that amorphous Si is first formed by electrochemical reduction and then thermally transformed to crystalline Si. The rate-determining step of the reduction was found to be O 2− diffusion in the vacant space between the columns.Recently, electrolysis of metal oxides in molten chlorides such as CaCl 2 has come to attract attention as a substitute for the traditional metal production methods. Electrochemical reduction of metal oxide to metal in molten CaCl 2 was first reported by Okabe et al. 1 and recently also reported by Chen et al. 2,3 However, their works were restricted to metal oxides, which generally have nonstoichiometric oxide phases, and there was no report for oxides of nonmetallic elements.The present authors tried to electrochemically reduce SiO 2 to Si, where Si is classified as a nonmetal element, and found that direct electrolytic reduction of solid SiO 2 to Si was possible in molten CaCl 2 . 4 Despite the good insulating ability of SiO 2 , successful reduction was achieved by using a contacting electrode method in which a SiO 2 plate was wound by Mo wire. In the initial stage of the reduction, electrons are supplied directly to the SiO 2 through the Mo wire. The total reaction is written asThe reduction rate is extremely large in spite of it being a solid-state reaction, and the whole reduction of a SiO 2 plate ͑1 mm thick͒ has been achieved. 4 Recently, it was found that the reaction reaches a thickness of 200 m inside of the plate at 1.00 V ͑vs. Ca 2+ /Ca͒ for 1 h. 5 Applying this new electrochemical reaction, solar grade Si is expected to be produced by removing only oxygen from high-purity SiO 2 , called "solar grade SiO 2 " hereafter. This method can contribute to further widespread use of Si solar cells from the standpoints of both cost and productivity, because solar grade SiO 2 is not expensive and is easily purified. 6 With reference to our report, 4 Jin et al. recently reported that porous pellets of SiO 2 powder can be electrochemically reduced to Si in molten CaCl 2 . 7 Furthermore, by using this method, pinpoint reduction is possible at the neighborhood of the contacting point between the c...

Facile synthesis of freestanding Si nanowire arrays by one-step template-free electro-deoxidation of SiO2 in a molten salt

“…16,17 In this synthesis, it is found that the electrode structure plays an important role in the growth of Si NWAs. The nickel nets in the sandwich structure of the electrode (Fig.…”

Difference in the Mechanism of Electrochemical Deoxidation of Conducting and Non-conducting Solid Oxide Preforms: An Experimental Demonstration with TiO2 and SiO2 Pellet Electrodes in CaCl2 Melt

“…the 3PI mechanism explained above [17][18][19][20][21][22]27,28] . The initiation-expansion-shrinking-disappearance (IESD) process of the solid (conductive product)/solid (oxide)/liquid (electrolyte) three-phase reaction boundary (TPB), proposed to explain the electro-deoxidation of insulating compounds [29] , fits well with the deoxidation pattern of insulating SiO 2 preforms [23,[30][31][32][33] .…”

“…Therefore the released oxide ions are available in the vicinity of the reduction zone and react chemically with the adjacent unreduced SiO 2 particles in the presence of Ca 2+ ions to form CaSiO 3 (Ca 2+ + SiO 2 + O 2-→ CaSiO 3 ). Since Si is highly porous and would aid easy removal of the generated oxide ions away from the reaction site in the fully dipped condition, CaSiO 3 was not observed as an intermediate product during the electrochemical reduction of SiO 2 pellets [17,[19][20][21][23][24][25]27] .…”

“…[3,7,8] . Several studies have been reported on the electrochemical reduction of oxides such as TiO 2 [1,3,5,[8][9][10][11] , Nb 2 O 5 [7,12,13] , UO 2 [14][15][16] , SiO 2 [17][18][19][20][21][22][23][24][25][26] etc. The process is being pursued in view of production of solar grade silicon as well [27] .…”

Electrolytic Production of Silicon Using Liquid Zinc Alloy in Molten CaCl2

“…[10][11][12] In this method, electrochemical reduction of insulating SiO 2 is realized by using a SiO 2 contacting electrode, which provides a three-phase interface between the conductor, SiO 2 , and molten salt.…”