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

• Mechanism of ion transportation during moisture swing adsorption (MSA) of CO 2 is revealed by a transient model based on the diffusion-reaction of ions. • The homogeneous ionic transportation process under the effect of ion concentration, electrical field, and chemical equilibrium is derived and discretized using a finite difference approach in the model. • Moisture swing frequency response (MSFR) method is established for ascertaining the kinetic parameters of the ion transfer phenomenon in the model. • A concept of continuous capture of CO 2 from the air is demonstrated through the driven force of partial pressure difference of water vapor. Direct air capture of CO 2 (DAC) sufficiently mitigates atmospheric CO 2 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 H 2 O CO 2 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 CO 2 concentration, generating the spontaneous transportation of CO 2 of the MSA membrane from one side to another. Specifically, the diffusion coefficient of HCO 3 − drives the CO 2 adsorption process predominantly, where the diffusion coefficient of HCO 3 − increases about ten times, leading to a nearly 12 times enhanced CO 2 separation rate accordingly. Notably, CO 2 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 CO 2 mitigation using Moisture Swing Adsorption can be achieved for direct air capture of CO 2. [ABSTRACT FROM AUTHOR]