Supercapacitor-Based CO2 Capture Enhanced by Electrolyte pH Control

Electrochemical CO2 capture by supercapacitors is emerging as an energy-efficient alternative to traditional thermally-driven technologies. Despite recent advances, the underlying mechanism of capture remains unresolved, with two competing hypotheses currently debated: a molecular mechanism, involving the uptake of CO2 molecules, and an ionic mechanism, driven by the uptake of bicarbonate (HCO3‾), which is in equilibrium with the gaseous CO2. In this study, combined electrochemical CO2 sorption measurements and solid-state NMR spectroscopy experiments reveal that CO2 capture occurs in both highly basic and highly acidic electrolytes, where the formation of aqueous CO2 or bicarbonate is suppressed, respectively. In basic electrolytes where bicarbonate is the dominant species, we observe 30% higher CO2 adsorption capacities and capture rates compared to neutral electrolytes, supporting the dominance of the ionic mechanism. In contrast, in an acidic electrolyte, where CO2 is the only species present, we observe a 60% decrease in CO2 adsorption capacities, despite having higher electrochemical capacitances. This indicates that bicarbonate is the primary species responsible for high electrochemical CO2 capture rates in supercapacitors that utilize aqueous electrolytes and porous carbon electrodes. Overall, this work provides mechanistic insight into the mechanism of electrochemical CO2 capture using aqueous supercapacitors and highlights the importance of CO2 speciation and electrolyte pH in optimizing performance.