Eco-Friendly Bilayer Solar Evaporator Based on Sodium Alginate Hydrogel and Polypyrrole for Water Purification

Solar-driven interfacial evaporation is regarded as a sustainable and practical strategy for water purification; however, complex fabrication procedures, limited evaporation efficiency, and poor salt tolerance still restrict its practical application. In this work, a dual-layer biomass-based solar evaporator is developed through rational material selection and structural design. The evaporator consists of a polypyrrole-coated nickel foam photothermal layer fabricated by electrochemical deposition, coupled with a sodium alginate hydrogel layer featuring interconnected microchannels for continuous water transport and effective salt rejection. The hydrogel layer is prepared via a two-step ionic cross-linking process, which suppresses salt accumulation while ensuring stable water supply during evaporation. Owing to the synergistic integration of efficient light absorption and rapid water transport, the bilayer evaporator exhibits a high evaporation rate of 3.22 kg m^-2 h^-1 with a photothermal conversion efficiency of 92% under 1 sun irradiation. Notably, stable evaporation without observable salt accumulation is maintained during long-term operation in high-salinity brine, demonstrating excellent salt resistance. In addition, effective removal of representative organic dyes is achieved during wastewater treatment, with negligible contaminants detected in the collected condensate. These results highlight the strong potential of the polypyrrole/sodium alginate hydrogel (PSH) evaporator for practical solar-driven desalination and wastewater purification.