Abstract:
The silicon dioxide electroreduction process in molten salts is of interest both for elemental silicon obtaining and for the electrochemical synthesis of silicides and silicon-based alloys. The most detailed investigation of silica electroreduction processes in chloride-fluoride melts was done in [1, 2], in fluoride melts – in [3], in cryolite melts – in [4]. Chronopotentiometric studies in the Na3AlF6-SiO2 melt have shown that electroreduction process at Ni-cathode surface includes a "weak" pre-dissociation of the complex into the electroactive state followed by 4-electron process of Si (IV) discharge.
This work aim is to find an indifferent electrode material for such systems, to obtain new data on the electrochemical behavior of Si (IV) in molten NaCl-Na3AlF6, and to determine conditions of electrodeposition of silicon coatings.
As the main research method, voltammetry with single and cyclic potential sweep was selected. In addition, potentiostatic electrolysis followed by metallographic and X-ray diffraction analysis of deposits was used.
The silica solubility in cryolite melt is caused by the silicon oxifluoride complexes formation. of Current-voltage curves analysis in semilog coordinate system shows that the experimental values of current-voltage dependencies slope are higher than those for reversible four-electron process. The polarization curves slope for the silicon electrode in coordinates lg i - η (n = 2.5-3.2) also exceeds the values for the four-electron reversible process. These experimental facts, as well as presence of two peaks at the anodic dissolution curves, suggesting the possibility of multistage character of the overall process of oxifluoride complex reduction in molten NaCl-Na3AlF6 system according to the following scheme:
Si(IV) +2e --> Si(II)+2e --> Si(0)
Coatings were deposited from the NaCl-Na3AlF6-0.5-1.0%(wt.)SiO2 melt at 900ºC at current densities (1-8)∙10-2 A/cm2. Deposition rate within this range of current densities was 10-40 μm/h, and the current yield as a silicon coating was up to 70-80%. Silicon coatings were obtained on samples of glassy carbon, graphite, nickel, copper, molybdenum, tungsten, and steel 3.
Thus, platinum as the electrode material is not indifferent in the systems under investigation. At the current-voltage curves obtained at the glassy-carbon electrode, there is Si (IV) reduction wave stretched along the axis of the potential which potential is close to the potential calculated from thermodynamic quantities. It was shown that the electroreduction of Si (IV) proceeds irreversibly in two stages. Silicon coating on different materials surface were obtained.
References:[1] S.V. Devyatkin, J. Min. Metall. Sect. B-Metall. 39 (1-2) B (2003) 303-307.
[2] Z. Cai, Y. Li, X. He, J. Liang, Metallurgical and Materials Transactions B 41 (2010) 1033-1037.
[3] G.M. Haarberg, L. Famiyeh, A. Martinez, K. Osen, Electrochimica Acta 100 (2018) 226-228.
[4] S. Sokhanvaran, M. Barati, Journal of the Electrochemical Society 161 (2014) E6-E11.
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