Unexpected Oversolubility of CO2 Measured at Electrode–Electrolyte Interfaces

Enhancements in gas solubility in pore-confined liquids─termed oversolubility─can drastically influence gas separation and catalytic efficiency in confined environments; however, they remain poorly understood in electrochemical CO2 capture and reduction systems. While previous investigations of oversolubility have emphasized the importance of mesoporosity and incomplete pore saturation by the solvent, in this work, we report an unprecedented 30-fold oversolubility effect for CO2 in solely microporous activated carbons saturated with 1 M Na2SO4(aq). The oversolubility effect occurs regardless of the activated carbon’s functional groups and level of disorder and is enhanced for smaller pore sizes. Oversolubility is quantified using solid-state 13C nuclear magnetic resonance spectroscopy (NMR), enabling differentiation between in-pore and ex-pore CO2 and HCO3–. Atomistic modeling of the system, based on a machine-learning model delivering first-principles accuracy, suggests that the effect is driven by an adsorption-like mechanism underpinned by favorable interactions between CO2 and the pore walls. Our findings demonstrate the unexpected importance of oversolubility for gas uptake in microporous, solvent-saturated carbon electrodes, an effect with direct relevance for improving electrochemical CO2 capture and conversion technologies.