Hydrogels as promising platforms for solar-driven water evaporators

Water scarcity has forced people to develop advanced water purification techniques. Solar-driven water treatment has the potential to produce clean water through utilizing solar energy to alleviate the global water crisis. The development of advanced materials with new properties contribute to enhancing the photothermal conversion efficiency. Hydrogels have emerged as encouraging material platforms for solar-driven water purification because of good water absorption, water retention and water transport. The rational design of hydrogel structures can lower the heat loss during heat transfer process to improve the photothermal conversion efficiency. This current review summarizes the recent advance on hydrogel-based solar evaporators. Hydrogel materials are generally prepared by chemical or physical cross-linking. The strategies to enhance water evaporation efficiency is also highlighted, which mainly involve the reduction of evaporation enthalpy, photothermal management of interfaces, highly interconnected hole structures and salt resistance. Hydrogels are combined with photothermal materials (including carbon-based materials, polymer materials, semiconductor materials, plasmonic materials and hybrid materials) to prepare hydrogel-based solar evaporators. Finally, challenges of hydrogel-based solar evaporators for water purification are discussed. This review inspires future efforts to design hydrogel solar evaporators for mitigating water shortage.