Direct air capture of CO2 (DAC) is one of the promising technologies to mitigating of anthropogenic CO2 increases. However, significant challenges of cost and kinetics under the ultra-low partial pressure of 40 Pa hinder the large-scale application in industry. Moisture swing adsorption (MSA), by featuring a low energy consumption in DAC, is considered an alternative technology to low-cost DAC. This work demonstrates an unsteady dynamic model based on MSA cycles driven through multi-ions transport coupled diffusion-reaction process of quaternary ammonium ion exchange resin (QMPRs). QMPRs can be driven by the H2O evaporation energy without external heat input, which adsorbs atmospheric CO2 in a low humidity atmosphere and releases it when the humidity rises. The measured adsorption kinetics of QMPRs separating CO2 from air ranges from different resin backbones, porous structures, humidity differences, and the efficiencies of grafting of functional groups to influence the overall CO2 adsorption. The kinetics of QMPRs is also simulated by a novel unsteady dynamic model that reveals the mechanism of its homogeneous ionic transportation, combing the influence of concentration field, electrical field, and chemical equilibria. Unknow key parameters from the model are also fitted by experiments using the moisture-swing frequency response (MSFR) method, which creates periodic humidity signals and measures the responses of concentration of CO2 under variable atmospheres. The results reveal that the gradient of humidity causes a counter gradient of CO¬2 concentration of sorbents, of which with uniform cylindrical mesoporous structure could tune the MSA ability through capillary condensation of water inside its mesopores. We also introduced hydrophilic groups - fluorinated blocks - in quaternary ammonium-based sorbents, which allowing automatically transporting CO2 from the membrane and improving the kinetics by controlling the conductivity of ions (influenced mainly by carbonate and bicarbonate). With enhancing the adsorption kinetics and decreasing the capital cost, the progress of moisture swing adsorption would be made for the implications of moisture swing adsorption in the industrial field of DAC.

Ions transportation kinetics in mesoporous resin sorbent for moisture swing reversible of direct air capture of CO2

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