Boosting Efficient Ammonium Rejection and Water Evaporation Rate by Solar‐Driven Hydrogel Evaporation

Solar‐driven evaporation using hydrogels and photothermal materials is a promising freshwater harvesting technology. However, due to the difficulty of removing volatile pollutants through evaporation and the fact that the evaporation efficiency has reached its limit, further improving efficiency becomes challenging. Therefore, a material simultaneously possessing low water evaporation enthalpy as well as the ability to inhibit volatile pollutants is desired. In this work, a dense inhibitory layer on the hydrogel framework of polyacrylamide introduced into polyvinyl alcohol is designed to enhance its suppression of volatile pollutants and regulate the water state. This not only reduces the energy demand for evaporation but also makes it more effective in treating wastewater containing volatile pollutant ammonia nitrogen. Finally, a high evaporation rate of 3.0 kg m−2 h−1 and an excellent ammonium rejection rate of 90% for high concentration ammonia wastewater are obtained under 1 kW m−2 illumination. This work opens up new avenues of application for the use of photothermal materials in the use of clean solar energy for the separation of freshwater and volatile small molecules. A dense positively charged auxiliary layer is uniquely designed to enhance the hydrogel's repellence toward ammonium and regulate water state. The resulting hydrogel has achieved an ammonium rejection rate of up to 90% facing high concentration ammonia nitrogen wastewater, and under 1 kW m−2 illumination, a high evaporation rate of up to 3.0 kg m−2 h−1 is achieved.