Comprehensive structural characterization of charged polymers involved in moisture-driven direct air capture

The rise in atmospheric carbon dioxide (CO 2 ) levels has led to urgent calls for effective carbon management, with direct air capture (DAC) emerging as a promising carbon removal solution. This study examines commercially available alkaline anion-exchange polymers, Fumasep FAA-3 and IRA-900, for use in low-energy, moisture-driven DAC applications, with an emphasis on linking structure to sorption behavior. A combination of X-ray diffraction, small and wide-angle X-ray scattering (SAXS/WAXS), atomic force microscopy (AFM), focused ion beam-scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM) was employed to explore the structural features of these materials. In parallel, moisture-swing sorption experiments enabled simultaneous quantification of CO 2 and H 2 O uptake and release under controlled relative humidity, providing direct insight into capacity and kinetic differences between the two materials. X-ray scattering revealed molecular ordering and large-scale structural organization in both materials, while humidity-induced changes highlighted the impact of moisture on subtle structural changes. Despite substantial differences in macroscopic morphology and sorption capacity, both materials exhibit similar water (de)sorption kinetics, indicating that hydration dynamics are governed primarily by molecular-scale structure. In contrast, CO 2 sorption kinetics and capacity are strongly influenced by macropore architecture and charge site density, with the macroporous IRA-900 exhibiting enhanced uptake and faster initial kinetics. AFM surface analysis further indicated the presence of clustering, porosity, and swelling, which were corroborated by FIB-SEM and TEM imaging. These structural insights offer a deeper understanding of the behavior of anion exchanging DAC materials during CO 2 capture and release, emphasizing the role of moisture in these processes. This work lays the foundation for the development of more energy-efficient MS-DAC polymers, paving the way for improved CO 2 capture technologies.