Thermodynamic and experimental approach of electrochemical reduction of CO2 in molten carbonates

• Feasibility of CO2 electroreduction into CO investigated in molten carbonates.
• Predictive thermodynamic study in Li, K, Na, Li–K & Li–Na carbonates: 575–650 °C.
• CO2/CO system analyzed by cyclic voltammetry in Li–Na and Li–K eutectics.
• This system involving a multistep mechanism is relatively slow on Au electrode.
• Adsorbed CO re-oxidation evidenced after CO2 microelectrolysis.

This work is dedicated to the feasibility of CO2 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 (CO2/CO, CO2/C, H2O/H2, M+/M) relative to pure Li, Na and K carbonates, Li2CO3–K2CO3 (62–38 mol.%) and Li2CO3–Na2CO3 (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 °C to 650 °C. CO2 reduction wave gives evidence for both eutectics, showing that CO2/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 CO2 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 CO2, mostly adsorbed CO and probably other intermediate species.