Hydrophobic-hydrophilic Janus anodic aluminum oxide membrane via physical deposition for enhanced interfacial water evaporation

Solar-driven interfacial evaporation of water has attracted an increasing attention for clean water production due to its energy efficiency and environmental friendliness. However, it still suffers from the relatively low water evaporation rate, generally due to the limited water transport capacity of conventional evaporators and challenges in fabricating the precisely controlled air-water interface. In this study, we developed a simple and scalable physical deposition method to fabricate two types of hydrophobic-hydrophilic Janus anodic aluminum oxide (AAO) membrane evaporators. The hydrophobically modified TiO2 or carbon nanotube (CNT) were coated onto one surface of hydrophilic AAO membrane by using a polyvinylidene fluoride (PVDF) binder to achieve a hydrophobic membrane surface, while the other AAO membrane surface was still hydrophilic. Under heating-driven conditions, the TiO2-coated Janus membrane with asymmetric water contact angles of 117.38 ± 5.83°/55.42 ± 4.35°achieved a high water evaporation rate of 1.31 kg/m2·h at 45 °C (bulk water temperature), which was 56.32 % higher than the natural evaporation. This increased water evaporate rate was mainly attributed to a system-level synergy between a rapid capillary-driven water transport through the nanochannels and an enhanced heat and mass transfer at the Janus interface, as evidenced by a reduction in intrinsic phase transition barrier driven by the Janus interface, resulting in a low apparent evaporation enthalpy of water with 1635 kJ/kg. Moreover, driven by solar energy, the CNT-coated Janus membrane (3.18 mg-CNT/cm2 loading) exhibited an excellent water evaporation rate of 3.13 kg/m2·h under 1 sun illumination (1 kW/m2), which was 42.27 % higher than TiO2-coated Janus membrane, demonstrating a superior photothermal conversion efficiency. Furthermore, the CNT-coated Janus AAO membrane evaporator was also successfully adopted to concentrate the dark biogas slurry featured with a strong solar absorption performance, a ∼4 % improvement in water evaporation efficiency and a high ammonia nitrogen rejection rate of ∼91 % were achieved when compared to the natural evaporation, confirming its practical potential for wastewater concentration and nutrient recovery.