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.