Hydrogel-driven solar desalination: integrating material innovation with interfacial water transport

In recent years, new materials based on hydrogels have been identified as innovative platforms for solar desalination, offering novel possibilities for the integration of water transport, light harvesting and salt management in a single system. As hydrogels do not require a solid evaporator structure, they offer the inherent benefit of enabling the continuous delivery and adjustment of water, which can be finely controlled by controlling the transport, as opposed to the traditional rigid evaporator structure. The review covers the physical and chemical bases of the hydrogel-based desalination process in detail with emphasis on water, heat and ion transport mechanisms. In addition to the significance of polymer-water interactions and various forms of confined water in evaporation control, recent advances in the understanding of salt transport and evaporation entropy effects are discussed in detail. With a mechanistic perspective based on thermodynamics and transport principles, this review elucidates the principles that link the material structure and physicochemistry of hydrogels to their desalination properties. Hydrogel structures, ranging from layer-by-layer constructs to bioinspired constructs, are introduced, and the challenges associated with material performance in solar desalination applications are addressed. Moreover, discrepancies among the reported material performances and guidelines for performance evaluation are discussed. Overall, this review aims to provide guidance for the development of future solar desalination strategies based on hydrogel structures.