Probing the Phase and Mechanism of Captured CO₂ in Supercapacitors by Pore Wetting and NMR Spectroscopy

Supercapacitive swing adsorption (SSA) is a promising electrochemical CO₂ capture technique, yet its development is limited by questions surrounding the mechanism of CO₂ uptake. The phase and speciation of dissolved inorganic carbon in these systems has not been resolved, and open questions remain as to whether CO₂ is captured as a gaseous or aqueous species, and if aqueous, whether the primary captured species is CO₂, HCO₃ ^-, or CO₃ ^2-. In this work, we investigate these questions in model SSA systems using solid-state nuclear magnetic resonance (NMR) spectroscopy and small-angle neutron scattering, and identify that the dominant species present are aqueous CO₂ and HCO₃ ^-. Examination of variably wetted activated carbon/aqueous electrolyte systems dosed with CO₂ indicates that a previously hypothesized uptake of gaseous CO₂ in dry pores does not occur in the studied SSA systems. Instead, quantitative NMR spectroscopy measurements reveal a large pool of aqueous CO₂ and HCO₃ ^- which adsorbs prior to charging and exceeds the observed uptake of supercapacitive CO₂ capture. Observation of fast CO₂:HCO₃ ^- exchange in this pool indicates that both species would be involved in any SSA process. These findings provide key insights into the mechanism of supercapacitive CO₂ capture and will guide its future technological development as a method for the mitigation of CO₂ emissions.