Direct molten-salt electro-reduction of CO2 in porous electrodes

Abstract

Although the direct electro-reduction of CO2 is thermodynamically favorable, it is difficult to accomplish due to the limited solubility of CO2 in oxide-free molten salts. Thus, the electrochemical reduction of CO2 in molten carbonate should be assisted by the existing lithium and alkaline earth cations that alter the thermodynamic energy barrier of CO32−. In this study, we employ a porous metal foam electrode to realize the direct electro-reduction of CO2 in molten Na2CO3-K2CO3. The porous electrode increases the contact area between the solid electrode and CO2 gas, resulting in the direct electro-reduction of CO2 to CO and carbon through the thin electrolyte layer. The overall current efficiency reaches 90% for CO at 800 °C. The molten Na2CO3-K2CO3 makes the downstream product separation easy because Na2CO3 and K2CO3 are highly soluble in water. Overall, using the gas electrode achieves the direct electro-reduction of CO2 and the use of lithium- and alkaline-earth-metal-free molten carbonates, offering a more energy-efficient CO2 reduction path.