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Different surfactants are introduced to study the diameter and morphology transformation characteristics of electrospun nanofiber. Surfactants increase the net charge density and instability motion of charged jet. The instability motion provides a good way to stretch the charged jets into finer ones, by which the beaded structures are also prevented. Ultrafine nanofiber with average diameter less than 65 nm can be fabricated. The nanofiber diameter decreases with the increase of surfactant concentration in polymer solution. The nanofibers with anionic surfactant sodium dodecyl sulfate (SDS) have the smallest diameter. The cationic surfactant hexadecyl trimethyl ammonium bromide (HTAB) plays the best role to prevent the formation of beaded structures in nanofibers, and helps to increase the uniformity of electrospun nanofiber. The effects of surfactants on the nanofiber diameter and morphology have been studied, which would promote the industrial application of ultrafine polymeric nanofibers.

The major risk of using carbon nanotubes (CNTs) to modify proton exchange membranes (PEMs) in fuel cells is possible short-circuiting due to the excellent electrical conductivity of CNTs. In this article, silica-coated CNTs (SiO 2 @CNTs) were successfully prepared by a simple sol-gel process and then used as a new additive in the preparation of sulfonated poly (ether ether ketone) (SPEEK)-based composite membranes. The insulated and hydrophilic silica coated on the surface of CNTs not only eliminated the risk of short-circuiting, but also enhanced the interfacial interaction between CNTs and SPEEK, and hence promoted the homogeneous dispersion of CNTs in the SPEEK matrix. Moreover, compared to the methanol permeability of the pure SPEEK membrane (3.42 × 10 À7 cm 2 s À1 ), the SPEEK/SiO 2 @CNT composite membrane with a SiO 2 @CNT loading of 5 wt% exhibits almost one order of magnitude decrease of methanol crossover, while the proton conductivity still remained above 10 À2 S cm À1 at room temperature. The obtained results expose the possibility of SPEEK/SiO 2 @CNT membranes to be served as highperformance PEMs in direct methanol fuel cells.