Constructing Anhydrous Proton Conductive Aramid Membranes through Grafting Kevlar Micro-fibrils with Phosphoric Acid

Abstract

Polyparaphenylene terephthalamide (PPTA) is a para-aramid polymer and it has been widely used as the ultra-light functional material in the fields of energy storage and transformation, military and aviation owing to some merits of strength, stability and lightweight, etc. The aim of this work is to propose a facile strategy of constructing anhydrous proton conductive aramid membranes through grafting Kevlar micro-fibrils with phosphoric acid (PA). In the prepared Kevlar-PA structure, PA molecules functioned as bridges to link the neighboring molecular chains through the formed intermolecular hydrogen bonds. More PA molecules were doped while Kevlar-PA membranes were immersed into (100−50) wt.% PA solutions with the formation of (Kevlar-PA)/(100%−50%)PA membranes. Successful construction of Kevlar-PA structure could guarantee fine performance on proton conduction in (Kevlar-PA)/(100%−50%)PA membranes. Specifically, (Kevlar-PA)/100%PA membranes showed the anhydrous proton conductivity of 2.68×10−1 S/cm at 160 °C and the stable value of 2.25×10−1 S/cm at 120 °C in a 300-hour non-stop test. The similar proton conduction resistance could be revealed from the invariable activation energy (Ea) values. Higher proton conductivity thus signified more proton conduction carriers participating the proton conduction processes. The introduction of PA molecules in Kevlar micro-fibrils that was substantially more effective at constructing conductive aramid membranes comparing to the strategies of treating the surface of Kevlar fibers. This research is expected to provide a facile and promising strategy to construct anhydrous proton conductive aramid membranes through grafting micro-fibrils of aramid nanofibers (ANFs), showing a great promise in the preparation of membrane electrolytes from ANFs.