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 degrees and 120 degrees 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 similar to 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.