Quaternized Engineered Hydrogels for Subsaturated Moisture‐Driven CO ₂ Capture

ABSTRACT 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 CO 2 , enabling cyclic operation without large thermal swings or vacuum. However, many reported systems require near‐saturated humidification for CO 2 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 CO 2 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 CO 2 , PDMH achieves a CO 2 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% CO 2 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.