Co-axially electrospun superhydrophobic nanofiber membranes with 3D-hierarchically structured surface for desalination by long-term membrane distillation

Electrospun nanofiber membranes (ENMs) have gained increasing interest in membrane distillation (MD) applications due to their high surface area, high hydrophobicity and porosity, and controllable pore size and membrane thickness. However, despite these advantages, ENMs still suffer from wetting issues in MD. Co-axial electrospinning is an attractive strategy for the one-step fabrication of non-woven membranes with core-sheath structures and improved wetting resistance for MD application. In the present study, we investigated poly (vinylidene fluoride- co -hexafluoropropylene) (PH) as the core and PH/silica aerogel (SiA) as the sheath to obtain superhydrophobic co-axial composite ENMs. The surface characterization results indicated that the active layer (i.e., PH) of the co-axial ENMs was rough, highly porous (>80%), and superhydrophobic (contact angle >160°). Further, the co-axial ENMs possessed small pore sizes (<0.39 μm) and a suitable liquid entry pressure (>1.72 bar). Upon the application in long-term (one month) direct contact MD testing using a 3.5 wt% NaCl solution as the feed, high water vapor flux and salt rejection of 14.5 L/m 2 h and 99.99% were achieved, respectively, when applying the optimal 4 wt% SiA solution loading at the sheath. The ENMs fabricated using the versatile co-axial electrospinning showed great potential for long-term applications in direct contact MD desalination. • 3D-hierarchically structured on nanofiber membranes are fabricated by co-axial electrospinning. • Superhydrophobic co-axial nanofiber membranes can prevent wetting issues during MD operation. • Co-axial nanofiber membranes show a stable flux performance due to its low thermal conductivity. • Co-axial nanofiber membranes can apply to MD for one month without any problems. • Co-axial electrospinning exhibits great potential for long-term MD desalination.