Immobilizing imidazolium ionic liquid in flexible proton exchange membranes to modify microstructure fracture

Abstract

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.