Scalable, bio-inspired and self-floating bi-layer polyurethane foam solar evaporator with excellent capillary hydrodynamic effect

Solar photothermal interfacial evaporation is one of the most promising strategies in seawater desalination and wastewater treatment, but its efficiency is limited by the trade-off between salt circulation and thermal positioning. Therefore, how to effectively control salt circulation and heat transfer to achieve high efficiency in a solar evaporator (SE) is a problem worthy of attention. Herein, inspired by water chestnut, a self-floating bi-layer foam consisting of a photo-thermal polypyrrole (PPy) coating and a lightweight polyurethane-carbon nanotube (PU/CNTs) foam matrix was constructed by simple liquid-phase in-situ polymerization and continuous in-situ polyurethane foaming technology. Based on the hydrodynamic effect of capillary wicking, the flow velocity control of aqueous solution in porous SE is realized by means of the bi-layer structure of macropores and micropores, and the heat transfer from the top photothermal zone to bulk water is slowed down without setting a separate thermal insulation layer. With the novel structure employed, an evaporation flux of 2.66 kg m−2 h−1 is achieved under 1 sun illumination. In addition, the SE with the length and width enlarged to 3 m and 1 m can be continuously evaporated in salt-saturated wastewater for 20 days without manual maintenance, and there is no obvious salt accumulation in the macroporous photothermal layer.