Coral-like hierarchically nanostructured membrane with high free volume for salt-free solar-enabled water purification

Solar evaporation presents a sustainable means to alleviate global water shortages. Varying photothermal materials have been designed to enhance evaporation performance. However, the critical technology gap is their low efficiency due to the severe salt crystallization and organic pollution. Here, we proposed a coral-like polypyrrole (PPy)-based photothermal membrane by the interface modification of d-glucitol (DG) and tetrakis (hydroxymethyl)phosphonium chloride (THPC). The designed PPy@DG/THPC membrane exhibited a wide range of sunlight absorption capacity with an efficiency of ∼97.6% due to its hierarchical nanostructure. Moreover, this membrane displayed superhydrophilicity and rich charges, giving it a superior salt-resistance property. Furthermore, molecular dynamics simulations indicated that this membrane exhibited a high free volume to inhibit salt crystallization. Hence, the evaporation rate of the PPy@DG/THPC membrane reached 1.46 ± 0.02 kg/m2/h in simulated brine (3.5 wt% NaCl) under 1-sun illumination. Besides, it exhibited exceptional anti-fouling ability under the synergy of hydration and charge effects. Overall, this membrane can achieve effective and sustainable solar evaporation to produce freshwater water to meet drinking-water standards. This work will offer a new approach to developing efficient solar evaporation for water remediation.