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This study investigates the H2O and CO2 sorption behavior of two chemically distinct polystyrene-divinylbenzene-based ion exchange sorbents: a primary amine and a permanently charged strong base quaternary ammonium (QA+) group with (bi)carbonate counter anions. We compare their distinct interactions with H2O and CO2 through simultaneous thermal gravimetric, calorimetric, gas analysis, and molecular modeling approaches to evaluate their performance for dilute CO2 separations like direct air capture. Thermal and hybrid (heat + low-temperature hydration) desorption experiments demonstrate that the QA+-based sorbent binds both water and CO2 more strongly than the amine counterparts but undergoes degradation at moderate temperatures, limiting its compatibility with thermal swing regeneration. However, a low-temperature moisture-driven regeneration pathway is uniquely effective for the QA+-based sorbent. To inform the energetics of a moisture-based CO2 separation (i.e., a moisture swing), we compare calorimetric water sorption enthalpies to Clausius–Clapeyron-derived total isosteric enthalpies. To our knowledge, this includes the first direct calorimetric measurement of water sorption enthalpy in a QA+-based sorbent. Both methods reveal monolayer-multilayer sorption behavior for both sorbents, with the QA+-based material having slightly higher water sorption enthalpies at the initially occupied strongest sorption sites. Molecular modeling supports this observation, showing higher water sorption energies and denser charge distributions in the QA+-based sorbent at λH2O = 1 mmol/mmolsite. Mixed gas experiments in the QA+-based sorbent show that not only does water influence CO2 binding, but CO2 influences water uptake through counterion-dependent hydration states, and that moisture swing responsiveness in this material causes hydration-induced CO2 release and drying-induced CO2 uptake, an important feature for low-energy CO2 separation under ambient conditions. Overall, the two classes of sorbents offer distinct pathways for the CO2 separation. 

Anion exchange materials (AEMs) containing quaternary ammonium groups with charge balancing alkaline anions have shown promise for CO2 direct air capture (DAC), particularly under low-humidity conditions. These materials can be regenerated by increasing water activity, leveraging the moisture swing (MS) effect. The regeneration step releases heat due to water sorption, providing an opportunity to develop an autothermal Vacuum Moisture Swing (aVMS) process that utilizes both a change in CO2 affinity due to moisture and the heat of water sorption for efficient atmospheric CO2 capture. In this work, the moisture-driven CO2 sorption was studied for the first time using dynamic column breakthrough (DCB) experiments and subsequent modeling of the obtained sorption isotherms. The results confirm that humidity significantly affects the shape and capacity of the CO2 isotherms. CO2 uptake increased sharply at lower relative humidity (RH), while temperature had a less pronounced effect, especially at higher RH. At 15 % RH, the CO2 loading saturates at 200 ppm, with maximum loads of 0.82 mmol/g at 25 °C and 0.64 mmol/g at 45 °C. However, at 80 % RH, the CO2 partial pressure required for saturation increases significantly, reaching 60,000 ppm, and the maximum loading drops below 0.4 mmol/g. Interestingly, under certain conditions, partial water desorption was observed during CO2 sorption, suggesting a complex interplay between the two molecules and the MS sorbent. In addition, the influence of sorbent form factor, flow rate and column geometry on the separation performance was investigated. These findings not only advance the understanding of the complex interaction between CO2 and water during moisture swing processes but also provide a basis for the engineering of a cost-effective aVMS process for CO2 DAC.