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 m -2 h -1 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. Here, we designed a coral-like polypyrrole (PPy)-based photothermal membrane by the interface modification of d-glucitol (DG) and tetrakis(hydroxymethyl)phosphonium chloride (THPC). The obtained PPy@DG/THPC membrane exhibited a wide range of sunlight absorption capacity with an efficiency of ∼97.6% because of its hierarchical nanostructure. Moreover, due to the introduction of THPC molecules, this membrane showed good hydrophilic ability and rich charges, giving it excellent salt-resistance property. Furthermore, molecular dynamics simulations indicated that this membrane exhibited high free volume to inhibit salt crystallization. Hence, the evaporation rate of this membrane reached 1.46 ± 0.02 kg m -2 h -1 in simulated brine (3.5 wt% NaCl) under 1-sun illumination. In addition, this membrane exhibited exceptional anti-fouling ability under the collaboration of hydration and charge effect. Therefore, this membrane can achieve effective and sustainable solar evaporation to produce freshwater water for meeting drinking-water standards. This work will offer a new approach to developing efficient solar evaporation for water remediation. 1. A coral-like PPy-based photothermal membrane was presented for solar evaporation. 2. It is endowed with high surface free energy due to the functionalization of THPC. 3. It showed ∼97.6% sunlight absorption ability due to its hierarchical structure. 4. It exhibited exceptional anti-fouling and salt-resistance properties. 5. It can achieve cyclic evaporation for simulated brine with stable performance.