Polyphenol-mediated hierarchical porous hydrogel evaporators for accelerated water transport and reduced evaporation enthalpy

Hydrogel-based evaporators have shown great potential in interfacial solar steam generation due to their unique water activation effect. However, the slow water transport still remains a significant challenge limiting the continuous and rapid evaporation. Herein, we introduced polyphenol-mediated pore engineering as a new design strategy to construct hierarchically porous hydrogel evaporators, which exhibited an exceptional water transport rate over two orders of magnitude higher than that of ordinary hydrogels. By incorporating gallic acid (GA), a natural polyphenol, into a polyvinyl alcohol (PVA) matrix and applying a directional freezing process followed by ferric ion (Fe 3+ ) coordination, we achieved a dual-scale porous architecture composed of vertically aligned micron channels and GA-induced micropores. This hierarchical structure skillfully balanced the conflicting demands of water transport and water activation. Simultaneously, GA further regulated the water state by increasing the content of intermediate water, thereby significantly reducing the evaporation enthalpy, which finally achieved an admirable evaporation rate of 3.82 kg m −2 h −1 under one sun irradiation. More importantly, the long-term stability and exceptionally low production cost of this natural molecule-based evaporator enabled an important step towards the possibility of large-scale practical applications. It is believed that this polyphenol-mediated hierarchical porous structured hydrogel provides a new paradigm to accelerate water transport and reduce evaporation enthalpy for efficient and stable solar evaporation.