Engineering Hydrogels for Efficient Solar Desalination and Water Purification

CONSPECTUS: Safe and adequate water is a global challenge due to the growing population and pollution in the wake of natural disasters. Access to clean water is a cornerstone for the further development of our modern society. Solar-powered desalination and water purification have attracted much research attention due to the high solar-to-thermal conversion efficiency and ease of implementation in multiple scales, from compact, stand-alone devices for individual households to large, centralized dimensions for communities. Nevertheless, the generation of water vapor at low temperatures below the boiling point is usually a slow process under natural sunlight. The production rate of clean water relies on the efficiencies of solar energy conversion and utilization, which can be enhanced in two directions: systems optimization and materials innovation. Starting from bottom heating and volumetric heating, interfacial evaporation has been adopted recently to systematically reduce energy loss by localizing heat near the evaporation surface using photothermal materials. Meanwhile, carbonaceous materials, narrow bandgap semiconductors, and polymers with different structures were designed to achieve thermal management and water transport. In this Account, we highlight the recent progress of hydrogels as a highly tunable material platform for solar-powered desalination and water purification. We begin by introducing how gelation chemistry endows hydrogels with desirable properties and multiscale tunability from molecular to macroscopic levels. Then, we review three major strategies to promote efficient solar-to-vapor conversion in terms of reducing water evaporation enthalpy, optimizing heat distribution, and tailoring the evaporation surface. First, by selecting polymeric backbones with hydratable functional groups, the water state in hydrogels can be regulated to facilitate intermediate water, which has a lower evaporation enthalpy compared to that of the bulk water. This polymer-water interaction can be further tuned during synthesis and post-treatment. Second, light-absorbing additives within hydrogels should not only convert solar energy to heat effectively but also confine the heat near the evaporative surfaces. Third, since water molecules are encapsulated in the polymer network of hydrogels, the surface of the hydrogel becomes the evaporation front of liquid water. Hence, tailoring the surface topography and wettability of hydrogels has important effects on the evaporation rate. Next, we discuss key functions that endow hydrogel evaporators with excellent desalination and purification performances, including anti-salt-fouling property for stable and long-term desalination, as well as easy integration of functional materials to remove other contaminants, such as heavy metal ions and organic dyes, for greater distillate quality. Last, existing challenges and future opportunities in both fundamental studies and practical implementation of the hydrogel-based solar water purification systems are discussed.