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 Na₂SO_4(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.