Crosslinked porous electrolytes derived from polybenzimidazole-metal organic framework for high-temperature polymer electrolyte membranes

In this study, a cross-linked porous electrolyte membrane (PMZ) is synthesized by using polybenzimidazole (PBI), zeolitic imidazolate framework-8 (ZIF-8), and methylene diphenyl diisocyanate as a crosslinker for high-temperature polymer electrolyte membranes. The isocyanate group of methylene diphenyl diisocyanate can form covalent bonding with imidazole groups of both PBI and ZIF-8, resulting in a cross-linked structure. The ZIF-8 nanoparticles that are uniformly distributed within the PBI matrix enhance pore structure and proton conductivity of PMZ, attributed to the removal of ZIF-8 during phosphoric acid doping. Despite the structural degradation of ZIF-8, the remaining imidazole groups increased the basicity and phosphoric acid uptake of PMZ. Notably, PMZ-10 that contains 10 wt% of ZIF-8 exhibited an optimal balance between mechanical properties and proton conductivity, demonstrating a tensile strength of 9.2 N mm⁻², elongation at break of 319 %, and proton conductivity of 8.7 × 10⁻² S cm⁻¹ at 150 ℃ under anhydrous condition. Moreover, the cross-linked structure of PMZ-10 significantly improved PA retention, with only 2.1 wt% phosphoric acid leaching, compared to 8.7 wt% for pristine PBI membrane. These results suggest that the PMZ membrane holds great potential for application in high-temperature polymer electrolyte membranes.