Sulfur-doped hierarchically porous open cellular polymer/acid complex electrolyte membranes for efficient water-free proton transport

Fuel cells are cleaner alternatives of sustainable green energy sources. Their proton exchange membranes continue to be a key component; however, there are several challenges associated with their application. To overcome these challenges, we designed and fabricated open cellular anhydrous polybenzimidazole (PBI) electrolyte nanohybrid membranes with high proton conductivity and long-term stability by facile non-solvent-induced phase separation. When the membranes were heated to 300 °C, the Friedel-Crafts reaction between the electron-rich phenyl ring and carboxylic acid groups covalently cross-linked the PBI chains. A hierarchical porous structure was effectively tailored from fingerlike cavities to the sponge pores simply by tuning the S-doped-reduced graphene oxide (S-RGO) concentration. The direct incorporation of S-RGO into the micropore channels of membrane yielded exceptional phosphoric acid (H3PO4) doping level of 1158.1% and high proton conductivity of 0.23 S cm-1 at 160 °C and 0% humidity, thereby showing a synergistic effect by proton transfer bridge, continuous proton transport channels, and H3PO4 reservoirs. Furthermore, the current density at 0.6 V and maximum power density of the S-RGO-reinforced PBI (S-PBI) nanohybrid membranes were increased to 716 mA cm-2 and 850 mW cm-2, respectively. The interconnected spongelike porous S-PBI nanohybrid membranes with significantly reduced mass transfer resistances exhibited excellent performance in high-temperature water-free hydrogen fuel cells.