Direction-limited water transport and inhibited heat convection loss of gradient-structured hydrogels for highly efficient interfacial evaporation

Solar interfacial evaporation is a promising way to generate clean water from seawater using clean, abundant, and sustainable solar energy. However, non-directionally moving water with high thermal conductivity acts inevitably as a medium for heat transfer from the surface to the bottom through heat convection, resulting in low energy efficiency and a low evaporation rate. Herein, a gradient structured composite hydrogel combining a desired functional differentiation is reported. The network sizes in the hydrogel continuously change, demonstrating an enhanced capillary effect and a direction-limited water supply capability, inhibiting the heat convection loss. In addition, the reported gradient hydrogel shows broadband light absorption, satisfactory mechanical properties under a saturated swelling state, an unusually higher water evaporation rate for brine compared with that for pure water, and an inherent ability to evaporate under both 1 sun illumination and dark environment. The proposed gradient channels to restrict the flow direction of liquid, and consequently the heat convection loss, will have an significant impact on the design of other efficient photothermal evaporation systems and even the field of mass and heat transfer.