Suppressing Carbon Deposition by Introducing SiO₃ ²⁻ in Molten CaCl₂ for Efficient Electro-Deoxidation

Electrolysis of solid oxides in molten CaCl2 has been proven a straightforward process to extract various metals thanks to the wide electrochemical window of CaCl2 and CaO which is soluble in molten CaCl2. However, the CaO produced during the cathodic reduction of oxides not only shuttles O2−, but also captures CO2 to generate CO3 2− that can be reduced to carbon at the cathode, resulting in decreasing current efficiency and contaminating the cathodic products by carbon. To solve this problem, we employed CaSiO3 to replace CaO to suppress/avoid the formation of CO3 2− and thereafter prevent the carbon deposition. The suppression of CO3 2− formation is because SiO3 2− is thermodynamically more stable than CO3 2−, i.e., the CO2 generated on the graphite anode can not be captured by SiO3 2−. In molten CaCl2−8.0 wt% CaSiO3, Cr2O3 and SiO2 are reduced to Cr and Si with much less carbon deposition than in molten CaCl2. As a result, the current efficiency increases from 21.4% to 51.9% for the reduction of Cr2O3 and 37.1% to 72.2% for the reduction of SiO2. Although introducing SiO3 2− will slightly decrease the electrochemical window of CaCl2, it will be a promising method to avoid carbon contamination and increase current efficiency especially for Si production.