Surface charge modulated hydrogel for faster atmospheric water harvesting

Atmospheric water harvesting (AWH) promotes equitable access to clean water. However, slow sorption-desorption kinetics of water sorbents lead to a low water production rate, especially for hydrogel sorbents with a hydrogen bond-mediated sorption-desorption process. The faster AWH calls for a deep understanding of the underpinning water-polymer interactions that govern the water sorption process. Here, we first investigate the overlooked water approach process to the surface before water capture at sorption sites, elucidating the mechanism of the dipole moments-induced reorientation of water molecules during the water approach process. An interesting phenomenon was found that the near-neutrally charged surface allows more water molecules to get closer to the polymer surface (≤ 10 Å) and better stabilize them (system energy decreased by 42.7%), facilitating subsequent sorption through water-polymer hydrogen bonds. Benefiting from the facilitated approach of water molecules to the near-neutrally charged surface, the sorption kinetics of the developed surface-charge modulated polyelectrolyte complex water harvesting hydrogel (WHG) are elevated by 44%. The intrinsically hygroscopic WHG can quickly harvest 0.52-1.18 g g^-1 water within 2 h under a relative humidity of 60-95%. The deep insights into the water-polymer interactions hold the potential for future surface chemistry design to realize ultrafast AWH and other sorption-based applications.