Engineering hygroscopic salt-impregnated zwitterionic and anionic hydrogels as advanced passive cooling materials.

Hydrogels present a promising, energy-free strategy for passive cooling due to their simple, low-cost fabrication and the high latent heat associated with water sorption and evaporation. However, conventional hydrogels often underperform in dry environments, suffer from mechanical weakness, and exhibit limited heat-transfer efficiency, which restricts their cooling capability. To overcome these challenges, this work develops two families of crosslinked hydrogels by copolymerizing acrylamide with the zwitterionic monomer SBMA and the anionic monomer AMPS. Optimized formulations with monomer to crosslinker volumes of (10:40:20) for SBMA and (40:20:10) for AMPS significantly improved mechanical strength, achieving compressive stresses of 1.67 MPa and 1.13 MPa, respectively, in their swollen states. Both systems also demonstrated enhanced water retention of ∼25% at 95% relative humidity. Incorporating low-cost CaCl₂ further increased water uptake, enabling ∼60-70% weight gain at 95% RH. Importantly, salt-loaded SBMA and AMPS hydrogels showed markedly improved heating rates of ∼0.47 °C s-1 and ∼0.51 °C s-1 over 90 s, compared to ∼0.26 °C s-1 for salt-free samples. The AMPS hydrogels outperformed SBMA due to the intrinsic anionic behaviour of AMPS that enables higher water uptake and facilitates more efficient heat conduction pathways while the positive-negative charge pairs in SBMA develop tightly bound hydration layers, restricting the mobility of salt ions leading to reduced heat transfer. These salt-enhanced hydrogels show strong potential for rapid, energy-free cooling in buildings and electronic systems.