Two polymers, two pore sizes, and the race to pull CO2 from air using nothing but moisture

Materials Today Chemistry, 2026

What if pulling carbon dioxide from the atmosphere required no heat, no pressure, and no significant energy input? That is the promise of moisture-swing direct air capture, a technology that uses changes in humidity to drive CO2 absorption and release. The materials absorb carbon dioxide when dry and release it when wet, creating a low-energy cycle that could be powered by nothing more than natural fluctuations in humidity.

The catch is that the materials capable of this trick are poorly understood at the structural level. A team at Arizona State University, led by professor Petra Fromme, has now provided the first detailed structural characterization of two commercially available polymers used in moisture-swing direct air capture. Their findings, published in Materials Today Chemistry, reveal why one material outperforms the other and offer design principles for building better ones.

Two materials, one question

The study examined Fumasep FAA-3 and IRA-900, both anion exchange polymers capable of moisture-swing CO2 capture. Despite sharing the same basic mechanism, the two materials differ significantly in performance. The question was why.

The research team used an unusual breadth of characterization techniques. X-ray diffraction revealed molecular-scale ordering. Small-angle and wide-angle X-ray scattering (SAXS/WAXS) probed intermediate structures. Atomic force microscopy, focused ion beam scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM) provided images at various scales. These structural analyses were paired with functional experiments measuring how much CO2 and water each material absorbed and released under different humidity conditions.

Pore size makes the difference

The two polymers behaved almost identically when it came to absorbing and releasing water. This similarity suggests that water movement through these materials is controlled primarily by their molecular structure, specifically the charged sites along the polymer chains that interact with water molecules. Since both polymers have similar molecular architectures at this level, their hydration dynamics match.

CO2 capture was a different story. IRA-900, the material with larger macropores, captured more carbon dioxide and did so more quickly. The reason appears to be physical access: larger pores allow CO2 molecules to reach more of the material's active capture sites. In a material with smaller, more constricted pores, some sites are effectively blocked, reducing both the total capacity and the rate of uptake.

The imaging data confirmed this interpretation. FIB-SEM and TEM revealed differences in porosity, clustering of charged sites, and swelling behavior between the two materials. IRA-900's more open structure gave CO2 molecules easier access to the interior, while Fumasep FAA-3's more compact architecture created bottlenecks.

Designing the next generation

The practical implication is a design principle: for moisture-swing direct air capture, macropore architecture and charge site density matter more for CO2 performance than molecular-level structure alone. Materials designed for this application should prioritize open, accessible pore networks that allow rapid gas transport while maintaining sufficient density of active capture sites.

First author Gayathri Yogaganeshan, Fromme's doctoral student, emphasized that these insights provide a foundation for designing more energy-efficient materials for scalable carbon dioxide removal. The study does not propose a new material but establishes the structural-performance relationships that future material design should target.

Scaling challenges remain

Moisture-swing direct air capture has significant theoretical advantages over thermal approaches, which require heating materials to hundreds of degrees to release captured CO2. The energy cost of moisture-swing systems is potentially much lower. But the technology is still in early development, and several challenges stand between laboratory demonstration and climate-relevant deployment.

Current materials capture relatively small amounts of CO2 per cycle. Scaling to meaningful atmospheric impact would require enormous quantities of material and very large capture surfaces. The kinetics, while faster in IRA-900, are still slow compared to the rate at which humanity emits carbon dioxide.

The humidity dependence of the cycle also constrains where the technology can be deployed. Regions with consistent humidity fluctuations are most suitable, while arid or consistently humid environments pose challenges. Weather patterns and seasonal variations would affect performance.

The study does not address material durability, another critical factor for any technology that must operate continuously for years to be economically viable. How these polymers degrade over thousands of absorption-release cycles, and whether their pore structures change with age, remains to be investigated.