Photothermal carbon black coatings enable efficient solar-driven membrane distillation

Membrane distillation (MD) is a promising thermally driven desalination technology; however, its practical implementation remains limited by low vapor flux and pronounced temperature polarization. In this study, a thin photothermal coating composed of polyvinyl alcohol (PVA) and nanostructured carbon black (CB) was spray-deposited onto a commercial porous PTFE membrane to enhance solar-driven MD performance. The resulting PVA-CB layer exhibited strong broadband light absorption and efficient photothermal conversion in the visible-near infrared (Vis-NIR) range. Structural and optical analyses confirmed a turbostratic graphitic structure with a crystallite size of 36.3 nm, a microstrain of 3.44 × 10^-4, and an effective direct optical band gap of 3.58 eV determined from Tauc analysis of the dominant π-π* transitions. Under simulated solar irradiation, the modified membrane showed a pronounced increase in surface temperature, enabling enhanced interfacial evaporation. Consequently, the PVA-CB membrane achieved a 45-60% increase in vapor flux compared with pristine PTFE, reaching up to 1.3 L m^-2 h^-1 at a temperature difference of 40 °C, while maintaining salt rejection above 99% and stable long-term operation. This improvement is attributed to the synergistic effects of CB-induced localized photothermal heating and PVA-enhanced surface hydrophilicity, which collectively reduce temperature polarization and facilitate efficient heat and mass transfer. These results highlight a scalable, cost-effective strategy for fabricating durable photothermal membranes, thereby advancing solar-driven desalination and sustainable water treatment.