Ionic liquid functionalized ZIF-8 modified PVDF nanofiber-based composite proton exchange membranes towards direct methanol fuel cells applications

Abstract

In order to improve the proton conductivity, CH₃OH barrier performance and long-term durability of proton exchange membranes in direct methanol fuel cells (DMFCs), herein, a novel functional nanofiber substrate (IL-ZIF-8@PVDF) was prepared by in situ growing metal-organic framework (ZIF-8) on poly(vinylidene fluoride) (PVDF) nanofibers, followed by modification with ionic liquid (IL) containing sulfonic acid groups. PVDF nanofibers can provide flexible mechanical support for ZIF-8, thereby effectively solving the processing difficulties of ZIF-8 caused by its brittleness. Further IL modification of ZIF-8 can not only increase the compatibility between ZIF-8@PVDF and the filling polymer, but also utilize the continuous PVDF nanofiber network to establish long-distance continuous H⁺ transport pathways within the membrane. The as-prepared sulfonated poly (ether ether ketone) impregnated IL-ZIF-8@PVDF composite membrane showed a very high proton conductivity of 161.92 mS cm⁻¹ (80 °C). Meanwhile, the presence of nanofiber backbone and ZIF-8 inside the membrane effectively limited the swelling and methanol permeation. The peak power density (PPD) of the DMFC output equipped with the membrane is up to 114.86 mW cm⁻² (2 M methanol, 80 °C), 1.28 times that of the commercial Nafion 211. The durability test suggested that the composite membrane assembled DMFC possessed excellent open circuit voltage stability (only 6.3 % attenuation after 320 h of continuous testing). This work provides a new design idea for PEMs for DMFCs.