Energy consumption and performance optimization of moisture swing sorbents for direct air capture of CO2 [湿法再生CO2空气捕集材料的能耗与性能优化]

The reduction of the number of water molecules in a nano-system actively promotes the hydrolysis of CO32- to HCO3- and OH-, leading to a moisture-swing nano-structured CO2 sorbent that spontaneously binds CO2 in ambient air when dry, while releasing it when wet. As it trades the input of heat in a thermal swing or the mechanical energy in a pressure swing of the traditional sorbents, against the consumption of water, the energy input for CO2 capture is very low. This brings about an inexpensive and efficient direct air capture technology for mitigating the greenhouse problem. The sorbents are usually heterogeneous sheets composed of amine-based anion exchange resins and a matrix polymer. In order to obtain a hierarchical microporous structure to allow sufficient access of the air to the ion-exchange resins buried inside the matrix, previously, hydrothermal pretreatment of the resin sheets was employed before put into operation. In addition, the desorption ratio (desorption quantity/adsorption quantity) of the material is only ~30% when desorption is activated by exposing to bulk water. The present work aims to reduce the energy consumption for pretreatment and improve the desorption performance. The hydrothermal pretreatment time and temperature are studied systematically. It was found that the room-temperature soaking pretreatment instead of hydrothermal pretreatment of the resin sheets, can also lead to excellent microporous structures and carbon capture performances, concerning both the capture capacity and kinetics. More importantly, based on the infiltration mechanism of gas and liquid into nanopores, we found that the micron-sized water particles produced by ultrasonic atomization could greatly promote the desorption ratio, from ~30% to ~60%, compared to that by exposing the sorbent to bulk water. The mechanism is analyzed from the perspectives of the water quantity, water particle size and its diffusion/infiltration ability. The optimization of these pretreatment energy consumption and desorption performance provides favorable conditions for the engineering implementation of large-scale air capture. © 2021, Editorial Board of CIESC Journal. All right reserved.