Advances in Hydrogel-Based Photothermal Interfacial Solar Steam Generation: Classifications, Mechanisms, and Applications

The growing global demand for freshwater has driven considerable attention toward photothermal interfacial solar steam generation (PISSG), where hydrogels are widely used due to their excellent water absorption, efficient environmental friendliness, and favorable interface properties. However, recent review mostly focuses on photothermal materials' design and the mechanisms for reducing evaporation enthalpy, with limited discussion on diverse substrates, physicochemical properties of hydrogels, and thermodynamic analysis during the PISSG. This work aims to systematically review the current progress of hydrogel-based PISSG and the challenges in practical applications. It begins with the classification of hydrogels on different substrates, mainly including cellulose, polyvinyl alcohol, polyacrylamide, and sodium alginate. Subsequently, the properties of hydrogels, photothermal conversion mechanism, and thermodynamics of interfacial evaporation are highlighted to illustrate how hydrogels optimize PISSG. Furthermore, the practical applications, ranging from seawater desalination, wastewater purification, photocatalytic degradation, multiple energy harvesting and conversion, and sterilization are broadly presented. Finally, the future research directions are proposed. Through this review, a comprehensive understanding of hydrogel-based evaporators in PISSG is proposed to address potential issues in both fundamental research and practical applications, thereby contributing to the resolution of global water scarcity challenges.