Phosphoric acid doped crosslinked polybenzimidazole (PBI-OO) blend membranes for high temperature polymer electrolyte fuel cells

Ionically crosslinked acid/base blend membranes of PBI-OO and a sulfonated polysulfone can be covalently crosslinked through aromatic sulfone groups, which form in a thermally induced Friedel-Crafts reaction. Here we systematically compare a series of blend membranes before and after curing. Even though the cured membranes have a lower phosphoric acid uptake even at increased doping time and temperature, they have an improved conductivity and therefore fuel cell performance than the ionically crosslinked membranes. For example, a covalently crosslinked blend membrane containing 5% of the acid component (c-BM 1) reached a conductivity of 260 mS/cm at 160 °C and a relative humidity of 5%, even though the PA uptake was just 266 wt%. In the fuel cell (H₂, air, 160 °C), this membrane yielded a peak power density of 452 mW cm⁻², about 100 mW cm⁻² above that of the commercial meta-PBI membrane. In a long term stability test, the ionically crosslinked membrane uc-BM 1 already failed within 100 h, while the cured c-BM 1 membrane was much more stable. A cured membrane with less PA and higher amount of the acid blend component (c-BM 3) gave a stable performance for over 1000 h, proving that thermally induced sulfone crosslinking strongly increases the stability.