Experimental investigation and modeling of CO₂-Water Co-Sorption isotherms on a Moisture-Swing sorbent for CO₂ Direct air capture

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.