Thermodynamic and experimental approach of electrochemical reduction of CO_2 in molten carbonates

摘要

This work is dedicated to the feasibility of CO_2 dissolution and transformation into CO by an electroreduction process. It is first based on a predictive thermodynamic study, through the establishment of potential-oxoacidity diagrams, mainly focused on competing reduction processes (CO_2/CO, CO_2/C, H_2O/H_2, M~+/M) relative to pure Li, Na and K carbonates, Li_2CO_3-K_2CO_3 (62-38 mol.%) and Li_2CO_3-Na_2CO_3 (52-48 mol.%) carbonate eutectics. Due to their lower melting points, only the Li-K and Li-Na eutectics are investigated experimentally by cyclic voltammetry at a gold flag or planar disk working electrode at temperatures from 575 ℃ to 650 ℃. CO_2 reduction wave gives evidence for both eutectics, showing that CO_2/CO system is relatively slow in the present conditions. Re-oxidation of CO formed at the Au electrode only produces a very low intensity signal, indicating a lower solubility of this species. Microelectrolysis at a potential corresponding to the reduction wave of CO_2 increases the amount of CO and an oxidation wave probably corresponding to adsorbed CO can be observed. The whole electrochemical process is complex, involving both soluble and adsorbed CO_2, mostly adsorbed CO and probably other intermediate species.