Solar-driven hydrogel-based interfacial evaporators: From principles to material manipulations

The increasing demand for energy in human society and pressure on freshwater sources place a challenging stain on the water-energy nexus. Solar-driven seawater desalination continues to be the core of the water portfolio to address the ever-increasing challenges of freshwater crisis worldwide. Solar-driven interfacial evaporation (SDIE) is a novel seawater desalination technology, emerging from 2014, with the advantages of low energy consumption, simple structure, and low cost. It is expected to alleviate water shortage issue all over the world. However, natural sunlight cannot meet the inherent energy requirements for the rapid vaporization of water, and the theoretical limit of solar input for vaporization rate is far from sufficient for practical applications. What’s more, the conventional SDIE materials place theoretical limits for the evaporation rate, round 1.5 kg/(m²·h). Hydrogels are gradually attracting the attention of researchers as a new material platform capable of meeting the demand for reduced evaporation latent heat. This review focuses on recent advancements in hydrogel solar evaporators, specifically discussing their working principles, photothermal conversion mechanisms, thermal management, water transport, salt resistance, and vapor condensation. It also explores the optimization of hydrogel-based solar evaporators from three perspectives: network structure design, evaporation surface manipulations, and functional applications. The paper concludes by analyzing the current challenges faced in the development of hydrogel-based solar evaporators and proposing future research directions. The aim of this review is to provide guidance for promoting the research and application of hydrogels in solar water desalination.