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The imidazolium ionic liquid of (Kevlar/bmimCl/SEBS) 5 membrane was immobilized in flexible proton exchange membranes (PEMs) with the spin coating technology. In the prepared (Kevlar/bmimCl/SEBS) 5 membrane, the imidazolium ionic liquid of 1-butyl-3-methylimidazolium chloride (bmimCl) functioned as glue to modify the microstructure fracture from the stretching operation through occupying the cracks. The proton conduction resistance was reduced with the well-ordered distribution of components in the multilayered microstructure. Although the doped phosphoric acid (PA) molecules charged the proton conductivity, the imidazolium cations of bmim+ could participate in the proton conduction through the formation of continuous proton conduction channels. In this research, the folding and stretching operations exerted the negligible effect on microstructure and property of the prepared PEMs. Besides the modification of bmimCl, the deformation of polystyrene- block -poly(ethylene- ran -butylene)- block -polystyrene (SBES) molecular chains and Kevlar nanofibers in the stretching operation contributed to maintain the stable microstructure. The stretching and folding operations led to slight variation on the membrane property. Specifically, the proton conductivities were respectively 3.21 × 10−2 S/cm and 4.16 × 10−2 S/cm at 160 °C, which were even superior to 2.77 × 10−2 S/cm of the pristine membranes. Furthermore, the tensile stress values of the folding and stretching membranes can reach (18.9 ± 1.19) MPa and (32.6 ± 2.63) MPa. [Display omitted] • Flexible proton exchange membranes were constructed. • Ionic liquid modified microstructure fracture of stretching membranes. • High and stable proton conductivity was obtained. [ABSTRACT FROM AUTHOR]

The well-ordered microstructure has been demonstrated to accelerate the proton conduction process through reducing proton conduction resistance. In this research, the proton exchange membranes (PEMs) with sandwich-like microstructure have been constructed based on Kevlar nanofibers and bifunctional nanofibers of chitosan (CS) with polyvinyl alcohol (PVA). The CS/PVA bifunctional nanofibers (CPNF) layer has been prepared with electrospinning process, which is stably adhered to the Kevlar nanofibers layer owing to compatible interfacial property. The fine structure stability without layer separation is achieved even with phosphoric acid (PA) molecules doping in the prepared PA doped composite membranes. The fibrous microstructure accelerats proton conduction through regulating proton conduction pathways. The objective of accelerating proton conduction at subzero temperature has been realized, reflecting in high and stable proton conductivities. For instance, the (Kevlar/CPNF/Kevlar)/PA membrane exhibits proton conductivities of 9.75 × 10−3 S/cm at −30 °C and 8.22 × 10−2 S/cm at 30 °C. Most importantly, the fine proton conductivity stability is identified by the results of the cycle test and long-term test. Specifically, the proton conductivities are respectively 8.62 × 10−3 S/cm at −30 °C and 8.07 × 10−2 S/cm at 30 °C after a five heating/cooling cycle process. The proton conductivities remain 2.97 × 10−2 S/cm at −25 °C and 7.19 × 10−2 S/cm at 30 °C after a 1032 h non-stop test. Additionally, the single proton exchange membrane fuel cell with the (Kevlar/CPNF/Kevlar)/PA membrane exhibits the peak power densities of 279.5 mW/cm2 at 100 °C and 352.7 mW/cm2 at 130 °C. [Display omitted] • Bifunctional nanofibers were used to construct proton exchange membranes. • Accelerating proton conduction at subzero temperature was achieved. • Conductivity reached 2.97 × 10−2 S/cm at −30 °C after a 1015 h non-stop test. [ABSTRACT FROM AUTHOR]

[Display omitted] • Multilayered membranes were prepared with vacuum assisted flocculation. • Proton conductivity reaches 2.80 × 10-2 S/cm at −30 °C. • Low temperature proton conduction mechanism was discussed. The application of proton exchange membranes in low temperature proton exchange membrane fuel cells requires the quick and stable proton conduction at subzero temperature. In this research, we constructed the multilayered membranes through the alternate deposition of poly(vinyl alcohol) (PVA), Kevlar nanofibers and carbon nanotube oxides (OCNTs) through the vacuum assisted flocculation (VAF) technique. The intermolecular hydrogen bonds resulted in the preparation of (PVA/Kevlar/OCNTs) 3 membranes with layered structure. Phosphoric acid (PA) molecules entered the gaps between the neighboring layers accompanying with the phase separation. The prepared membranes exhibited the high and stable proton conductivity at subzero temperature, such as 2.04 × 10-2 S/cm at −30 °C and 1.52 × 10-1 S/cm at 30 °C. The proton conductivity gradually changed to 5.52 × 10-2 S/cm and 1.84 × 10-1 S/cm even after the ten-cycles process of −30 °C–30 °C. In addition, the value can retain 1.42 × 10-2 S/cm at −30 °C in a 1900 h non-stop test. The evaporation and liquefaction of water molecules could affect proton conductivity besides PA dominating the proton conduction behavior during heating/cooling processes. Furthermore, the balance on low temperature proton conduction and mechanical strength makes the multilayered (PVA/Kevlar/OCNTs) 3 /PA membranes as a promising PEM candidate at subzero temperature. [ABSTRACT FROM AUTHOR]