All-in-one photothermal/catalytic flexible membrane for highly efficient desalination and organic pollutant degradation

Interfacial solar vapor generation (ISVG) accompanied by photocatalytic degradation holds immense potential to mitigate water scarcity and pollution. Distinct from the two detached functional components (photothermal agent and photocatalyst) in a conventional evaporator, in this study, an all-in-one photothermal/catalytic agent, nitrogen-containing honeycomb carbon nanosheets (NHC), was engineered for synergistic high-efficiency steam generation and photocatalysis functions. It was demonstrated that the superoxide radical generated on the surface of NHC conferred its catalytic activity to the photodegradation of organic pollutants under full solar spectrum irradiation. A proof-of-concept multifunctional evaporator (called NHC@PEI/MCE), consisting of NHC grafted with polyethyleneimine (PEI) and a hydrophilic mixed cellulose ester membrane (MCE), was fabricated to achieve both solar-driven desalination and organic pollutant degradation. Owing to its excellent light absorption capability (∼96%), reduced evaporation enthalpy (1358 J g^-1) and minimized heat loss (8.8%), the bi-layered evaporator performed a rapid water evaporation rate of 1.66 kg m^-2 h^-1 under one standard sun illumination. Notably, the edge-preferential crystallization strategy enabled the bi-layered evaporator to maintain long-term stability for continuous water evaporation and salt harvesting over 80 h in a concentrated 3.5 wt% NaCl solution. The design of the all-in-one photothermal/catalytic agent NHC ensured the synchronous removal of organic pollutants. The removal rates of methylene blue and phenol were 99.82% and 79.6%, respectively. Additionally, the reduction rate of total organic carbon (TOC) in the actual coking wastewater was found to be 96.6%. The exceptional purification capabilities across diverse water systems surpassed those of membrane materials lacking NHC. The exploration of the multifunctional evaporator offers a novel approach to achieving high-efficiency utilization of solar energy for the conversion of both seawater and industrial wastewater into freshwater.