Moisture-Driven CO₂ Sorbents

An energy-saving system containing ion-exchange or nanoporous materials and carbonate ions is proposed, which is capable of capturing CO₂ from ambient air simply by controlling the amount of water (moisture) in contact with the sorbent. The system binds CO₂ from the air when the surrounding is dry, whereas it desorbs CO₂ when it is wet. A design of such CO₂ sorption and desorption systems is investigated using quantum mechanics simulations and is verified by experiments. Its working mechanism is revealed as the free energy change of the chemical reaction of the carbonate ions and water molecules; the free energy change decreases when the number of water molecules in the materials decreases. The influence of pore size, spacing of cations, and surface hydrophobicity of the sorbents on CO₂ capture efficiency are elucidated. The study sheds light on ways to optimize an efficient direct air capture system and therefore contributes to the development of “negative emission technologies.”