The urgency to address global climate change induced by greenhouse gas emissions is increasing. In particular, the rise in atmospheric CO2 levels is generating alarm. Technologies to remove CO2 from ambient air, or “direct air capture” (DAC), have recently demonstrated that they can contribute to “negative carbon emission.” Recent advances in surface chemistry and material synthesis have resulted in new generations of CO2 sorbents, which may drive the future of DAC and its large‐scale deployment. This Review describes major types of sorbents designed to capture CO2 from ambient air and they are categorized by the sorption mechanism: physisorption, chemisorption, and moisture‐swing sorption.

Developing inexpensive and highly efficient CO2 air capture technologies is an important solution for solving the greenhouse problem. In this work, we used the low-cost quaternized chitosan (QCS)/poly­(vinyl alcohol) (PVA) hybrid aerogels with quaternary ammonium groups and hydroxide ions to reversibly capture CO2 from ambient air by humidity swing. The CO2 capture capacity and adsorption rate of the aerogels were investigated over the temperature range 10–30 °C. The CO2 capture capacity of the aerogels was measured to be about 0.18 mmol/g, which is 38% higher than the state-of-the-art commercial membrane. In addition, we proposed a modified pseudo-first-order kinetic model considering both the CO2 adsorption and the H2O desorption, which describes the experimental results very well. For the first time, the moisture-swing CO2 adsorbent is built by low-cost biomass material, which opens up a new approach for the design of the moisture-swing CO2 adsorbent.

Reversible CO 2 capture from ambient air by a humidity swing has shown great potential in mitigating the greenhouse effect. In this work, we developed a new humidity-swing absorbent based on PO 4 3− /HPO 4 2− /H 2 PO 4 − ions exhibiting superior CO 2 absorption capacity and kinetics compared to that of CO 3 2− /HCO 3 − -based absorbent. After ion exchange with PO 4 3− ions, the ion-exchange resin (IER-PO 4 ) containing positive quaternary ammonium groups and movable PO 4 3− ions is able to reversibly capture CO 2 from the ambient air by a humidity swing, which triggers the transformation between the PO 4 3− ions and HPO 4 2− /H 2 PO 4 − ions in the resin. In a dry environment, PO 4 3− ions in IER-PO 4 are hydrolyzed into OH − ions and HPO 4 2− ions, which are further hydrolyzed into H 2 PO 4− ions and OH − ions. Both hydrolysis reactions produce OH − ions for CO 2 absorption, while the adsorbed CO 2 can be released in a humid environment. The results of quantum chemical calculation show that the hydrolysis of the ions is promoted by the reduction of water molecules in the nanoscale hydrated cluster. The adsorption capacity of IER-PO 4 during the moisture swing is 80% larger than that of IER-CO 3 , and the adsorption rate of the IER-PO 4 resin at a temperature range of 15−35 °C is much higher than that of the IER-CO 3 absorbent. A modified pseudo-first-order (MPFO) kinetic is developed, which can describe the experimental results well. The present study sheds light on the design of high performance moisture-swing absorbents with PO 4 3− ions.

The moisture-swing sorbent of amine-based anion exchange resins is one of the most promising materials for direct carbon capture from ambient air to alleviate excessive emissions of CO 2 . In this study, how the physical and chemical properties of amine-based anion exchange resins affect their carbon capture performances is studied systematically, through different kinds of resins. Contrary to previous understandings, the effect of the chemical functional groups on the overall capture performance is found to far outweigh that of the physical properties. It is found that the main property that determines the absorption capacity is the chemical one, i.e., the amine functional groups, while properties dominating the absorption kinetics are the physical ones, i.e., particle size and microporous structures. The absorption capacity of resins loaded with strong amine groups is strikingly higher than that with weak amine groups. The mechanism is explored by a combination of molecular dynamics and quantum chemical calculations. The most superior sorbent is the macro-porous strong base resin due to its strong amine groups and large inner pore sizes. The present study provides new insights into selecting suitable absorbents or preparing better ones for enhanced direct air capture of CO 2 .

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