Moisture swing frequency response method for characterization of ion-transport kinetics of CO₂ adsorption

Direct air capture of CO₂ (DAC) sufficiently mitigates atmospheric CO₂ to prevent the impacts of global warming of 1.5 °C above preindustrial levels. To overcome the challenges of the kinetics of sorbent under the ultra-low partial pressure (40 Pa) of DAC, the underlying mechanisms need to be constructed for better designs, simulations, and developments of the separation processes of carbon dioxide. Herein, a transient model based on the diffusion-reaction of ions at the molecular scale is developed for the moisture swing adsorption (MSA) process, disclosing the mechanism of mass transfer of multi-ions of sorbents. To compare the model with the experiment quantitatively, Fourier-transformed moisture-swing frequency response is applied to accurately measure the H₂O–CO₂ concentration response, ensuring a systematic approach for unknown kinetic parameters for the model. The results reveal that the gradient of water vapor causes a counter gradient of CO₂ concentration, generating the spontaneous transportation of CO₂ of the MSA membrane from one side to another. Specifically, the diffusion coefficient of HCO₃⁻ drives the CO₂ adsorption process predominantly, where the diffusion coefficient of HCO₃⁻ increases about ten times, leading to a nearly 12 times enhanced CO₂ separation rate accordingly. Notably, CO₂ adsorption kinetics can be stimulated by controlling specific ion conductivity in the moisture swing sorbent. With the enhancement of adsorption kinetics and low capital cost, the progress of CO₂ mitigation using Moisture Swing Adsorption can be achieved for direct air capture of CO₂.