Engineering anti-scaling superhydrophobic membranes for photothermal membrane distillation

The emerging photothermal membrane distillation (PMD), which combines solar harvesting and heat localization, has the potential to address the water-energy nexus. However, PMD membrane scaling and its underlying mechanisms need in-depth examination. Herein, we describe a microsphere structured composite membrane (#PCNT-0.5) with simultaneous superior light-to-heat conversion and superhydrophobic properties, produced by electrospraying a polyvinylidene fluoride (PVDF)/polydimethylsiloxane (PDMS)/multi-walled carbon nanotubes (MWCNTs) hybrid solution on an electrospun PVDF nanofibrous substrate (#PVDF). The carbon-based material and a micro-rough structure endowed the membrane with high light absorptivity, heat recovery and vapor production. Meanwhile, it showed excellent wetting resistance with contact angles beyond 150° and 120° for deionized (DI) water and 40% v/v ethanol aqueous solutions. The robustness of the surface multifunctional coating of #PCNT-0.5 was confirmed by chemical erosions and physical treatments. The localized heating alleviated the temperature polarization, increased the permeation flux of the #PCNT-0.5 by ∼15% and reduced its specific thermal energy consumption. In addition, the modified #PCNT-0.5 exhibited better anti-scaling properties compared to #PVDF. The illumination slowed down the flux decline of #PCNT-0.5 and inhibited salt crystallization. The excellent anti-scaling properties of the illuminated #PCNT-0.5 are attributed to the inhibited bulk and heterogeneous crystallization, permission of slippage and accelerated dissolution-diffusion. This study demonstrates that PMD is a feasible and promising method for hypersaline wastewater desalination.