Superhydrophobic\nCarbon Nanotube Network Membranes\nfor Membrane Distillation: High-Throughput Performance and Transport\nMechanism

Abstract

Despite\nincreasing sustainable water purification, current desalination\nmembranes still suffer from insufficient permeability and treatment\nefficiency, greatly hindering extensive practical applications. In\nthis work, we provide a new membrane design protocol and molecule-level\nmechanistic understanding of vapor transport for the treatment of\nhypersaline waters via a membrane distillation process by rationally\nfabricating more robust metal-based carbon nanotube (CNT) network\nmembranes, featuring a superhydrophobic superporous surface (80.0\n± 2.3% surface porosity). With highly permeable ductile metal\nhollow fibers as substrates, the construction of a superhydrophobic\n(water contact angle ∼170°) CNT network layer endows the\nmembranes with not only almost perfect salt rejection (over 99.9%)\nbut a promising water flux (43.6 L·m^–2·h^–1), which outperforms most existing inorganic distillation\nmembranes. Both experimental and molecular dynamics simulation results\nindicate that such an enhanced water flux can be ascribed to an ultra-low\nliquid–solid contact interface (∼3.23%), allowing water\nvapor to rapidly transport across the membrane structure via a combined\nmechanism of Knudsen diffusion (more dominant) and viscous flow while\nefficiently repelling high-salinity feed via forming a Cassie–Baxter\nstate. A more hydrophobic surface is more in favor of not only water\ndesorption from the CNT outer surface but superfast and frictionless\nwater vapor transport. By constructing a new superhydrophobic triple-phase\ninterface, the conceptional design strategy proposed in this work\ncan be expected to be extended to other membrane material systems\nas well as more water treatment applications.