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Porous Nafion nanofiber composite membrane with vertical pathways for efficient through-plane proton conduction

Wang, Jingtao, Li, Ping, Zhang, Yafang, Liu, Yarong, Wu, Wenjia, Liu, Jindun
Journal of membrane science 2019 v.585 pp. 157-165
anisotropy, asymmetric membranes, chitosan, fuel cells, humidity, ionic liquids, nanofibers
Nafion has been the benchmark of proton exchange membrane for decades due to the excellent proton conduction and physicochemical properties. While the strong dependence of transfer channel continuity on humidity always causes serious conductivity drops and hampers the wide application. Herein, a combination of electrospinning and soft template (ionic liquid) methods is proposed to fabricate porous Nafion nanofiber. The resultant nanofiber mat is then impregnated with chitosan (CS) to prepare porous nanofiber composite membrane (PNFCM). This is different from traditional nanofiber composite membrane (NFCM), of which through-plane conductivity is much lower than in-plane one (i.e., strong transfer anisotropy). The abundant pores inside porous nanofibers provide numerous vertical transfer channels at interfaces between CS and pore walls. Meanwhile, the –NH/–NH2 groups on CS form acid-base pairs with –SO3H groups along pore walls. These stable vertical pathways significantly facilitate the through-plane proton conduction at both hydrated and anhydrous conditions. Particularly, PNFCM attains perpendicular conductivities of 307 mS cm−1 at 90 °C and 100% RH, and 150 mS cm−1 at 120 °C and 0% RH, which are, respectively, 3.2 and 2.7 times of that of NFCM. Consequently, the transfer anisotropy coefficient decreases and fuel cell performances enhance obviously.