Silicon dioxide (SiO(2)) is conventionally reduced to silicon by carbothermal reduction, in which the oxygen is removed by a heterogeneous-homogeneous reaction sequence at approximately 1,700 degrees C. Here we report pinpoint and bulk electrochemical methods for removing oxygen from solid SiO(2) in a molten CaCl(2) electrolyte at 850 degrees C. This approach involves a 'contacting electrode', in which a metal wire supplies electrons to a selected region of the insulating SiO(2). Bulk reduction of SiO(2) is possible by increasing the number of contacting points. The same method was also demonstrated with molten LiCl-KCl-CaCl(2) at 500 degrees C. The novelty and relative simplicity of this method might lead to new processes in silicon semiconductor technology, as well as in high-purity silicon production. The methodology may be applicable to electrochemical processing of a wide variety of insulating materials, provided that the electrolyte dissolves the appropriate constituent ion(s) of the material.

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Understanding ion transport in electrolytes is crucial for fabricating high-performance batteries. Although several ionic liquids have been explored for use as electrolytes in Na secondary batteries, little is known about the transport properties of Na + ions. In this study, the thermal and transport properties of Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA − : bis(fluorosulfonyl)amide and C 3 C 1 pyrr + : N-methyl-Npropylpyrrolidinium) ionic liquids were investigated in order to determine their suitability for use as electrolytes in Na secondary batteries. In the x(Na[FSA]) range of 0.0-0.5 (x(Na[FSA]) = molar fraction of Na[FSA]), a wide liquid-phase temperature range was observed at close to room temperature. The viscosity and ionic conductivity of this system, which obey the Vogel-Tamman-Fulcher equation, increases and decreases, respectively, with an increase in x(Na[FSA]). Further, its viscosity and molar ionic conductivity satisfy the fractional Walden rule. The apparent transport number of Na + in the investigated ionic liquids, as determined by the potential step method at 353 K, increases monotonously with an increase in x(Na[FSA]), going from 0.08 for x(Na[FSA]) = 0.1 to 0.59 for x(Na[FSA]) = 0.7. The Na + ion conductivity, determined by multiplying the ionic conductivity with the apparent transport number, is an indicator of Na + ion transport in Na secondary batteries and is high when x(Na[FSA]) is in the 0.2-0.4 range.

Phase diagrams of binary mixtures of alkali bis(trifluoromethylsulfonyl)amides have been constructed and their eutectic compositions and temperatures have been determined. It has been revealed that the molten salt electrolytes having the melting points in the intermediate temperature range (373 to 473) K are easily formed by simple mixing of two kinds of single alkali bis(trifluoromethylsulfonyl)amides salts. The 1:1 or 3:1 double salt is occasionally formed for some binary systems.

We evaluated the charge−discharge performance of a Sn 4 P 3 negative electrode in an ionic liquid electrolyte comprised of Nmethyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) and NaFSA. We also conducted cyclic voltammetry and transmission electron microscopy for the Sn 4 P 3 electrode to reveal the reaction mechanism. It was suggested that Na 15 Sn 4 and Na 3 P are formed via phase separation in the first sodiation and that elemental Sn and elemental P formed by following a desodiation reaction with Na ions in the subsequent cycles. The Sn 4 P 3 electrode exhibited a high Coulombic efficiency of 99.1% at the fourth cycle and an excellent cycling performance with a high reversible capacity of 750 mA h g −1 even at the 200th cycle. We demonstrated that there are two important factors to improve the performance: (i) higher volume fraction of Sn than P and (ii) uniform dispersion of Sn nanoparticles in a P matrix. The ionic liquid electrolyte showed good applicability to the Sn 4 P 3 negative electrode due to its superior electrochemical stability.

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...

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Ammonia was successfully synthesized by using a new electrochemical reaction with high current efficiency at atmospheric pressure and at lower temperatures than the Haber-Bosch process. In this method, nitride ion (N3-), which is produced by the reduction from nitrogen gas at the cathode, is anodically oxidized and reacts with hydrogen to produce ammonia at the anode.