The substantial increase of the atmospheric CO2 concentration over the last few decades has steered toward research on direct air capture (DAC). DAC is one of the mobile-source carbon capture and storage (CCS) processes that is variant from the conventional point-source CCS in terms of easing the CO2 concentration levels and promoting negative carbon emission. We propose here a sustainable material based on the quaternization of bamboo fiber and the implication in DAC. The quaternized bamboo cellulose could adsorb CO2 at a medium relative humidity (RH) of 60–80%, with the efficiency of quaternary ammonium groups over 0.65. The CO2 capacity at higher RH strikingly decreased, which indicates the moisture-swing characteristics, and that was further validated by a desorption ratio of 0.70 under humid gas purge. The unique phenomenon that a dry atmosphere (e.g., RH < 20%) is not favored by adsorption was revealed and could be attributed to the hydrophobic feature of the sorbent inherited from natural lignocellulose. The results suggest that quaternized cellulose with a low cost may open up new possibilities for the utilization of moisture-swing CO2 adsorbents in damp circumstances.

Quaternary ammonium-based polymeric ionic liquids (PILs) are novel CO2 sorbents as they have high capacity, high stability and high binding energy. Moreover, the binding energy of ionic pairs to CO2 is tunable by changing the hydration state so that the sorbent can be regenerated through humidity adjustment. In this study, theoretical calculations were conducted to reveal the mechanism of the humidity swing CO2 adsorption, based on model compounds of quaternary ammonium cation and carbonate anions. The electrostatic potential map demonstrates the anion, rather than the cation, is chemically preferential for CO2 adsorption. Further, the proton transfer process from water to carbonate at the sorbent interface is successfully depicted with an intermediate which has a higher energy state. By determining the CO2 adsorption energy and activation energy at different hydration states, it is discovered that water could promote CO2 adsorption by reducing the energy barrier of proton transfer. The adsorption/desorption equilibrium would shift to desorption by adding water, which constitutes the theoretical basis for humidity swing. By analyzing the hydrogen bonding and structure of the water molecules, it is interesting to find that the CO2 adsorption weakens the hydrophilicity of the sorbent and results in release of water. The requirement of latent heat for the phase change of water could significantly reduce the heat of adsorption. The special "self-cooling" effect during gas adsorption can lower the temperature of the sorbent and benefit the adsorption isotherms.

A polymeric ionic liquid (PIL), with quaternary ammonium ions attached to the polymer matrix, displays CO affinity controlled by moisture. This finding led to the development of moisture swing absorption (MSA) for direct air capture of CO. This work aims to elucidate the role of water in MSA. For some humidity range, CO absorption is an endothermic process associated with concurrent dehydration of the sorbent. The thermodynamic behavior of water indicates a decreased hydrophilicity of the PIL as the mobile anion transforms from CO to HCO during CO absorption. The decrease in hydrophilicity drives water out of the PIL, carrying heat away. The mechanism is elucidated by molecular modeling based on density functional theory. The finding of spontaneous cooling during absorption and its mechanism in the PIL opens new possibilities for designing an air capture sorbent with a strong CO affinity but low absorption heat.

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