New insights into the role of carbon nanotubes spray-coated on both sides of the PTFE membrane in suppressing temperature polarization and enhancing water flux in direct contact membrane distillation

Wetting and temperature polarization (TP) are two critical issues that are closely associated with the lifespan and production efficiency in membrane distillation (MD). An ideal MD membrane should be wetting resistant with low TP. Although great achievements were made to date in MD design, maintaining wetting resistance and low TP is still challenging. This study developed a facile and effective strategy to construct a high-performance hierarchical MD membrane by spraying carbon nanotubes (CNTs) on both sides of a pristine PTFE membrane (PTFE-dCNTs membrane, d stands for double layers). Spray coating the pristine PTFE membrane by less than 800 nm thick CNTs double-layers enhanced the water flux by 1.6 fold (52.3 LMH) and increased the wetting resistance time by 2.2-fold (51.1 h) with a temperature difference of 40 degrees C (feed of 60 degrees C and permeate of 20 degrees C). With optimized CNTs coating density, the PTFE-dCNT (2) membrane displays a water flux as high as 126 L m- 2 h-1 with a salt rejection rate of 99.9% under the temperature difference of 60 degrees C in direct contact MD, surpassing most of the reported MD membranes. Experimental and simulation results demonstrate that a unique sandwich structure of MD membrane having thermal conductive and wetting resistant layers on both sides of membranes is beneficial to enhance the water flux and operation stability by suppressing temperature polarization and membrane wetting on both sides in direct contact MD. The strategy proposed here can be extended to a wide range of materials, holding a great potential to fabricate high-efficiency MD membranes.