Superwetting reduced graphene oxide/alginate hydrogel sponge with low evaporation enthalpy for highly efficient solar-driven water purification

Solar-driven interface evaporators have been widely used and studied in water purification due to their low energy consumption. However, it is still a challenge to improve the efficiency of the interface evaporator to break through the theoretical water vapor output value by designing the material structure and chemical properties. Herein, rGO/SA hydrogel sponge (RSHS) evaporator was prepared by in-situ deposition of reduced graphene oxide (rGO)/sodium alginate (SA) hydrogel on melamine sponge (MS) skeletons through a simple and scalable dipping-baking-crosslinking technology. The increased number of conjugated pi-bond of tannin acid (TA) reduced GO and the multiple reflections of light traps make the RSHS evaporator with excellent solar absorption of the whole solar spectrum. The porous structure of MS provided a platform for water pumping and steam overflowing. The salt concentration in RSHS is always kept at a low level due to the superhydrophilicity and interconnected pore structure of RSHS, thus avoiding salt crystallization on the evaporator surface. More importantly, the evaporation enthalpy of water in RSHS was reduced to 1094.85 J g- 1 due to the existence of plentiful hydrophilic groups in the rGO/SA network, thus effectively accelerating the vapor generation. Under simulated solar irra-diation (1000 W/m2), RSHS has a high evaporation rate of 3.13 kg m-2h-1 and an energy conversion efficiency of 91.54 %. This study puts forward a low-cost, scalable, simple, and effective strategy for fabricating high-performance hydrogel evaporators that will contribute to global goals for addressing freshwater and energy issues.