Highly charged hydrogel with enhanced donnan exclusion toward ammonium for efficient solar-driven water remediation

• Enhanced Donnan exclusion toward ammonium is designed for the hydrogel evaporator. • Ammonium rejection capability up to 95% is achieved by optimizing the charge density. • Evaportion rate is improved by activating the surface –OH group of hydrogel network. Developing solar-driven photothermal materials with superior rejection capability toward volatile ammonium (NH 4 + ) is a huge challenge but rather important for agricultural/municipal wastewater remediation. Herein, different from common hierarchical porosity designs, a positive charge-assisted Janus hydrogel evaporator with enhanced Donnan exclusion toward ammonium is uniquely achieved without sacrificing the water evaporation capability. The ammonium diffusion resistance of the so-fabricated Janus evaporator is significantly increased by optimizing the charge density and diffusion distance, yielding a high ammonium rejection rate up to 95%, which is never reported before. Meanwhile, the Janus evaporator also yields a water evaporation rate up to 3.3 kg m −2 h −1 for the natural wastewater via partial amorphization of cellulose Iα allomorph, which is ∼3 times faster than the previous photothermal materials for intercepting volatile phenol molecules. This work points out new avenues for the application of photothermal materials in separating freshwater from small volatile molecules using clean solar energy.