Electrospun Nanofibrous Composite Membranes for Separations

Electrospinning is regarded as an efficient method for directly and continuously fabricating nanofibers. The electrospinning process is relatively simple and convenient to operate and can be used to prepare polymer nanofibers for almost all polymer solutions, melts, emulsions, and suspensions with sufficient viscosity. In addition, inorganic nanofibers can also be prepared via electrospinning by adding small amounts of polymers into the inorganic precursors, which are generally regarded as nonspinnable. The diameter of the electrospun nanofibers can be tuned from tens of nanometers to submicrons by changing the spinning parameters. The nonwoven fabric stacked with electrospun fibers is a porous material with interconnected submicron pores, providing a porosity above 80%. However, limited by the unstable rheological properties of the electrospinning fluid, it is difficult to obtain nanofibers stably and continuously with an average diameter of <100 nm, which narrows the separation applications of the electrospun nanofibrous membranes to only microfiltration, air filtration, or use as membrane substrates. Therefore, to fully take advantage of electrospun nanofibrous membranes in other separation applications, electrospun nanofibrous composite (ENC) membranes were developed to improve and optimize their selectivity, permeability, and other separation performances. The composite membranes not only have all the advantages of single-layered or single-component membranes, but also have more flexibility in the choice of functional components.In this account, we summarize the two combination strategies to design and fabricate ENC membranes. One is based on the component combination, in which functional components are homogeneously or heterogeneously mixed in the fiber matrix or modified on the nanofiber surface. The other one is termed as the interfacial combination, in which functional skin layers are fabricated on the top of the electrospun membranes via interfacial deposition or interfacial polymerization, to construct selective barriers. The specific preparation approaches in the two combination strategies are discussed systematically. Additionally, the structural characteristics and separation performances of ENC membranes fabricated via these approaches are also compared and analyzed to clarify their advantages and range of utilization. Subsequently, the six applications of ENC membranes we focus on are demonstrated, including adsorption, membrane distillation, oil/water emulsion separation, nanofiltration, hemodialysis, and pervaporation. To meet their different requirements for separations, our consideration about the choice of combination strategies, related preparation methods, and functional components are discussed based on typical research cases. In the end, we conclude this account with an overview of the challenges in industrial manufacturing, mechanical strength, and interfacial attachment of ENC membranes and prospect their future developments.