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

Abstract

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 H2OCO2 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.