A hydrogel foam with a gradient pore structure for highly efficient solar-driven seawater desalination

Solar interface evaporation is a sustainable and clean water production approach. Polymer hydrogels, as some of the core materials of solar-driven interface evaporation systems, have received extensive attention. However, traditional hydrogel foams have randomly distributed open cells and closed cells, which make it difficult to simultaneously achieve rapid water transport and efficient steam escape, and show excellent thermal insulation performance. In this study, a polyvinyl alcohol/acetylene black hydrogel foam (PAPHF) evaporator with a gradient pore structure was developed through a simple and scalable "foaming-resting-crosslinking-freezing" strategy. The top layer of this evaporator has a large-sized open-cell structure, which enhances light absorption, water transport and steam escape, while the bottom layer of the evaporator has a small-sized closed-cell structure, which can effectively reduce heat conduction loss and improve surface heat localization ability. The optimized PAPHF evaporator achieved an evaporation rate of 3.13 kg m^-2 h^-1 and an energy conversion efficiency of 88.9% under 1 sun radiation (1 kW m^-2), exceeding the performance of many of the reported evaporators of the same type. In addition, the PAPHF evaporator can effectively perform seawater desalination and wastewater purification. This study provides a new idea for the structural design of high-performance solar evaporators.