Quaternized Engineered Hydrogels for Subsaturated Moisture-Driven CO₂ Capture

Net zero emissions will require atmospheric carbon dioxide removal to counterbalance unavoidable emissions while sustaining human life. Moisture-swing direct air capture (DAC) uses humidity for regeneration of a material that absorbs CO₂, enabling cyclic operation without large thermal swings or vacuum. However, many reported systems require near-saturated humidification for CO₂ release, narrowing the passive operating window and increasing water management demands. Here, we present passive diurnal moisture-swing hydrogels (PDMHs) that are regenerated under subsaturated RH conditions, thereby broadening the climatic window for passive CO₂ capture. Specifically, we develop a quaternized poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) hydrogel that combines humidity-responsive quaternary ammonium sites with a phase-transition temperature (lower critical solution temperature) that governs hydration behavior. Using air containing 400 ppm CO₂, PDMH achieves a CO₂ uptake of 1.06 mmol g^-1 at 40°C and 30% relative humidity (RH). Humidification at 25°C enables regeneration efficiencies of 92% at 90% RH and 79% at 80% RH. Under a two-step, 24 h diurnal protocol, PDMH shows >90% CO₂ release during desorption and a stable working capacity of 0.97-1.05 mmol g^-1 over 40 cycles. Comparative energy analysis shows that PDMH outperforms the other sorbents evaluated in terms of energy efficiency.