A general descriptor for guiding the electrolysis of CO2 in molten carbonate

Molten carbonate is an excellent electrolyte for the electrochemical reduction of CO2 to carbonaceous materials. However, the electrolyte–electrode-reaction relationship has not been well understood. Herein, we propose a general descriptor, the CO2 activity, to reveal the electrolyte–electrode-reaction relationship by thermodynamic calculations and experimental studies. Experimental studies agree well with theoretical predictions that both cations (Li+, Ca2+, Sr2+ and Ba2+) and anions (BO2−, Ti5O148−, SiO32−) can modulate the CO2 activity to control both cathode and anode reactions in a typical molten carbonate electrolyzer in terms of tuning reaction products and overpotentials. In this regard, the reduction of CO32− can be interpreted as the direct reduction of CO2 generated from the dissociated CO32−, and the CO2 activity can be used as a general descriptor to predict the electrode reaction in molten carbonate. Overall, the CO2 activity descriptor unlocks the electrolyte–electrode-reaction relationship, thereby providing fundamental insights into guiding molten carbonate CO2 electrolysis.