Electrochemical Production of Si without Generation of CO₂ Based on the Use of a Dimensionally Stable Anode in Molten CaCl₂

Abstract

The current Si production process is based on the high-temperature (1700 8 8C) reduction of SiO 2 with carbon that produces large amounts of CO 2 .W ereport an alternative lowtemperature (850 8 8C) process based on the reduction of SiO 2 in molten CaCl 2 that does not produce CO 2 .I tu tilizes an anode material (Ti 4 O 7 )c apable of sustained oxygen evolution. Tw o types of this anode material, dense Ti 4 O 7 and porous Ti 4 O 7 , were tested. The dense anode showed ab etter performance. The anode stability is attributed to the formation of aprotective TiO 2 layer on its surface.I nsitu periodic current reversal and ex situ H 2 reduction could be used for extending the lifetime of the anodes.The findings showthat this material can be applied as ar ecyclable anode in molten CaCl 2 .S iw ires,f ilms,a nd particles were deposited with this anode under different cathodic current densities.T he prepared Si film exhibited % 30-40 %o ft he photocurrent response of ac ommercial p-type Si wafer,i ndicating potential use in photovoltaic cells.

Template‐Free Electrochemical Formation of Silicon Nanotubes from Silica

“…[43] Constant-voltage electrolysis is conducted, with the cell voltage between the solid cathodes and graphite anode being at 2.2-2.6 V to avoid the formation of Ca-based alloys. [31][32][33][34][35][36][37][38][39][40][41] The cathode potential (versus Ag/AgCl) was monitored in one case during constant-voltage electrolysis at a cell voltage of 2.2 V ( Figure S1, Supporting Information), which shows that the cathode potential (versus Ag/AgCl) during the two-electrode-electrolysis process is consistent with that in the CV curves in Figure 2a, further verifying the preferential reduction of AgCl and formation of Ag-Si.…”

“…[31][32][33][34][35][36][37][38][39][40][41] However, the solid-solid conversion from SiO 2 to Si makes the nucleation-growth process of Si hardly tunable, therefore construction of hollow Si nanostructure by molten salt electrolysis of silica is an insurmountable challenge. [31][32][33][34][35][36][37][38][39][40][41] As for coating a conductive buffer layer on the surface of Si nanostructures, carbon is commonly used as the candidate. [12][13][14][15][16] However, the unpleasant SiC is readily generated during molten salt electrolysis, therefore making the in situ coating strategy far from expectations.…”

“…The electrode kinetics for molten salt electrolysis of solid silica for Si extraction is governed by ohmic polarization and mass transfer of/through the solid cathode. [31][32][33][34][35][36][37][38][39][40][41] Electrolysis of SiO 2 -AgCl for Si formation ranks among the highest performance in terms of current efficiency (74%, Figure 4m) and energy consumption (12.1 kW h kg Si −1 , Figure 4m). This energyconsumption value is even lower than that of industrial production of metallurgical-grade silicon (>20 kW h kg Si −1 ), [41] promising future industrial production of Si nanotubes.…”

“…Of note, molten salt electrolysis, which is widely adopted for production of metals in industry, has been verified to be adaptable for large-scale production of silicon nanomaterials. [31][32][33][34][35][36][37][38][39][40][41] Compared with the conventional CVD method which uses toxic SiH 4 /SiCl 4 as raw materials and suffers from low production yield, [12,[17][18][19][20][21][22][23][24][25][26][27][28] the molten salt electrolysis strategy with a high production yield herein uses affordable SiO 2 as the starting material and shows a promise for practical production of Si nanotubes.…”

Reducing the Carbon Footprint: Primary Production of Aluminum and Silicon with Changing Energy Systems

“…A process based on condensation is, however, not likely to be efficient enough to be viable. Electrolysis of silicon oxides in a molten salt electrolyte, or direct electrolysis from molten oxides, has also been studied [41,42], but is at a relatively low technology readiness level at this time. According to Grjotheim et al [43], the decomposition potential for SiO 2 is 1.8 V, while for Al 2 O 3 it is 2.16 V. As the valency for Si ions is + 4, while it is + 3 for Al, a third more current would be required on molar basis to reduce Si than Al, while the molar mass is 28 g/mol for Si vs. 27 g/mol for Al.…”