Moisture Swing Frequency Response Method for Characterization of Ion-Transport Kinetics of Co2 Adsorption

Abstract

Direct air capture of CO2 sufficiently mitigates atmospheric CO2. To overcome the challenges of the kinetics of sorbent under ultra-low partial pressure (40Pa), underlying mechanisms need to be constructed for better design, simulation, and development of adsorption/separation processes. 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 model with the experiment quantitatively, Fourier-transformed moisture-swing frequency response is applied to accurately measure H2O-CO2 concentration response, ensuring a systematic approach for unknown kinetic parameters for the model. Results show that the gradient of water vapor causes a counter gradient of CO2 concentration, resulting in the spontaneous transportation of CO2 from one side of MSA membrane to another side. Specifically, the diffusion coefficient of HCO3- drives mostly the CO2 adsorption process, where the diffusion coefficient of HCO3- increases about 10 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 sorbent. With the enhancement of adsorption kinetics and low capital cost, real progress in MSA can be achieved for direct air capture of CO2.Direct air capture of CO2 sufficiently mitigates atmospheric CO2. To overcome the challenges of the kinetics of sorbent under ultra-low partial pressure (40Pa), underlying mechanisms need to be constructed for better design, simulation, and development of adsorption/separation processes. 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 model with the experiment quantitatively, Fourier-transformed moisture-swing frequency response is applied to accurately measure H2O-CO2 concentration response, ensuring a systematic approach for unknown kinetic parameters for the model. Results show that the gradient of water vapor causes a counter gradient of CO2 concentration, resulting in the spontaneous transportation of CO2 from one side of MSA membrane to another side. Specifically, the diffusion coefficient of HCO3- drives mostly the CO2 adsorption process, where the diffusion coefficient of HCO3- increases about 10 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 sorbent. With the enhancement of adsorption kinetics and low capital cost, real progress in MSA can be achieved for direct air capture of CO2.

Keywords: CO2 Adsorption, Ion transport mechanism, ion exchange ploymer, moisture swing adsorption