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

Molten carbonate is an excellent electrolyte for the electrochemical reduction of CO 2 to carbonaceous materials. However, the electrolyte–electrode-reaction relationship has not been well understood. Herein, we propose a general descriptor, the CO 2 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 + , Ca 2+ , Sr 2+ and Ba 2+ ) and anions (BO 2 − , Ti 5 O 14 8 − , Si O 3 2 − ) can modulate the CO 2 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 C O 3 2 − can be interpreted as the direct reduction of CO 2 generated from the dissociated C O 3 2 − , and the CO 2 activity can be used as a general descriptor to predict the electrode reaction in molten carbonate. Overall, the CO 2 activity descriptor unlocks the electrolyte–electrode-reaction relationship, thereby providing fundamental insights into guiding molten carbonate CO 2 electrolysis.