Critical operating conditions for enhanced energy-efficient molten salt CO2 capture and electrolytic utilization as durable looping applications

CO2 electro-splitting is a promising approach for energy storage and conversion. In this paper, captured CO2 (in the form of carbonate ions) can be electrochemically converted into hollow carbon spheres and ultrathin carbon sheets via an energy-efficient molten salt electrolysis process in CaO-containing molten LiCl–KCl. During electrolysis, oxygen anions (O2−) acting as a CO2 absorbent are concurrently generated on the cathode to reproduce CO32− by carbonation with CO2, setting up a closed loop between sequential CO2 absorption and electrochemical conversion. The dissociated O2− in the melt exhibits more than 80% carbonation efficiency. However, CaO are easily solidified on the cathode when O2− is over-saturated in the melt. More importantly, the solidified CaO poorly reacts with CO2, leading to a cathodic deactivation effect and higher energy consumption for CO2 electro-splitting. To enhance the kinetics of CO2 conversion, it was found that applying forced convection by bubbling CO2 near the cathode, operating under lower current density, and higher temperature can effectively inhibit the cathodic deactivation effect, leading to a durable and more energy-efficient CO2 conversion process. The energy consumption for producing 1 kg of carbon from CO2 is much lower at higher operating temperature, i.e. 16.3 kW h/kg at 650 °C and 31.4 kW h/kg at 450 °C. The critical operating conditions for durable looping are calculated by finely coupling current density with O2− dissociation kinetics. The coupled current densities are 4.41, 9.60, and 27.3 mA/cm2 at 450, 550, and 650 °C, respectively.