Solar-Initiated Frontal Polymerization of Photothermic Hydrogels with High Swelling Properties for Efficient Water Evaporation

Hydrogel-based photothermal materials for enhanced solar water evaporation have drawn increasing attention due to their unique hierarchical nanostructure, reduced latent heat, and high evaporation rate. However, the laborious preparation process and high energy consumption restrict the on-site applicability and immediacy, seriously obstructing practical application of hydrogel evaporators. Herein, a low-cost hydrogel evaporator ($1.85 m⁻²) with high swelling ratio (≈2445%) prepared via solar-initiated frontal polymerization (FP) is demonstrated. Solar-initiated FP based on solar thermal technology not only realizes the field rapid hydrogel polymerization without extra energy input and complicated equipment, but also achieves the homogeneous interconnected macroporous structure to enhance water uptake (≈70 times evaporator weight) and light absorption (≈1% transmittance and ≈2% reflectance). Due to the lower vaporization enthalpy, the evaporator presented an evaporation rate of 2.42 kg m⁻² h⁻¹ with a light-to-vapor efficiency of ≈92.8% under 1 sun. More importantly, the high swelling ratio imparts the hydrogel evaporator with 2700% and 900% expansion in volume and surface area, resulting in a dramatically promoted solar evaporation rate compared with non-swelling photothermic materials. This feasible approach realizes the all-process solar thermal utilization, from material synthesis to final application, which would be significant for sustainable freshwater production in remote/off-grid areas.