Synergistic optimization of the water state and transport pathway in hydrogels via pullulan and directional Freezing-Assisted salting out for efficient evaporation

Improving the evaporation efficiency of solar-driven hydrogel evaporators is crucial for addressing water scarcity. Traditional efforts have been focused on increasing the intermediate water content or shortening the evaporation route through hydrogel structure design. However, rare attempts have been made to combine these two approaches. Here, we report the simultaneous achievement of high intermediate water content and short water transport paths in a hydrogel for efficient solar-driven evaporation. By introducing pullulan and MXene nanosheets into a poly(vinyl alcohol) (PVA) system that is subjected to directional freezing-assisted salting-out, we achieve a considerably high intermediate water to free water ratio of 2.11. Meanwhile, low- tortuosity yet compact channels within the hydrogel formed during directional freezing-assisted salting-out accelerate water transport during evaporation. Consequently, the hydrogel evaporator achieves an energy conversion efficiency of 93 % under one sun illumination, with a high vapor generation rate of 3.92kg m −2 h −1 . Even in highly saline water, an evaporation rate of 2.93 kg m −2 h −1 is preserved, accompanied by voltage generation through water-driven ion migration. Moreover, the hydrogel evaporator can be recycled by heating using the remoulade properties of PVA without deteriorating the evaporation rate, holding great potential in sustainable and recyclable water collection.