Novel Janus membrane with unprecedented osmosis transport performance

Low energy consumption in water desalination is highly demanded. Herein, we show that the Janus porous membrane, which comprises a superhydrophobic SiO2 coating on a superhydrophilic polyvinylidene fluoride (PVDF) membrane, exhibits unprecedented osmotic water permeability (2.88 L m−2 h−1 bar−1) and near-complete reverse salt rejection (Js/Jw = 0.007 g L−1) far beyond the-state-of-art thin-film composite (TFC) membranes. The appropriate SiO2 seeding allows the hydrophilic PVDF fibrils to stretch out of the top tip of the microspheres to form a one-way valve, serving as nano-structured water channels while the superhydrophobic side, by trapping air, functions as the active layer to repel reverse salt diffusion. The hydrophilic microsphere partially covered by F-SiO2 is regarded as a micro-osmosis unit. Both external and internal concentration polarization are well diminished, with the superhydrophobic side facing towards the draw solution. This intriguing result deepens our understanding of unidirectional mass transfer through the Janus nano-channels in fluid osmosis.