Structure–Property Relationships for Moisture-Swing Direct Air Capture

Efficient, low-cost atmospheric CO₂ capture is essential for scaling negative-emission technologies. Moisture-swing carbon capture─which adsorbs CO₂ from dry air and releases it under humid conditions─offers a low-energy alternative, yet the structure-property relationships governing its performance remain underexplored. Here, we systematically investigate humidity-driven capture on strong-base ion-exchange resins (IERs), varying polymer backbones (acrylic vs styrenic), ammonium functionality (Type I vs Type II), pore architecture (gel-type vs macroporous), and counteranion (dibasic phosphate vs carbonate) across 10 commercial resins. Thermodynamic and kinetic behaviors were assessed via closed-loop cycling with ambient CO₂ at 20-70% RH. Morphological and chemical properties were characterized by SEM/EDS, N₂ sorption, NMR cryoporometry, and solid-state NMR and FTIR spectroscopies. Macroporous IERs outperformed gel-type analogues in capacity and kinetics, but only when possessing intermediate-sized, well-connected pores, with the phase lag scaling inversely with the pore size. Ion identity and ammonium functionality acted jointly: Type I and Type II IERs exhibited higher swing capacities with phosphate and carbonate loadings, respectively. Anion choice also governed the kinetics, with phosphate slowing adsorption and carbonate slowing desorption. Acrylic backbones drove greater water uptake than styrenic ones. Solid-state NMR revealed humidity-driven protonation, consistent with the proposed swing mechanism. Together, these findings provide practical design rules for improving direct air capture via moisture-swing sorbents.