Synergistic Behavior of Phosphonated and Sulfonated Groups on Proton Conductivity and Their Performance for High-Temperature Proton Exchange Membrane Fuel Cells (PEMFCs)

Some new polybenzimidazoles (PBIs) containing different acidic groups (−SO₃H and −PO₃H₂) as proton donors and amphoteric groups (imidazole and −PO₃H₂) as proton transmitters were synthesized. The proton conductivity was 0.015 S·cm^–1 for PBI1 and 0.075 S·cm^–1 for PBI2 at ambient temperatures in water. At 140 °C and RH = 30%, the proton conductivity equaled 0.126 S·cm^–1 for PBI2 and 0.003 S·cm^–1 for Nafion. Moreover, PBI2 showed high proton conductivity under anhydrous conditions at 140 °C. It exhibited a porous morphology due to its sulfonated hydrophilic side chains. Theoretical studies showed that hydrogen bonding (HB) interactions of side chains and main chain groups leads to the formation of pores in the polymer structure. The presence of pores and hydrophilic groups in polymer structure increases water uptake and the durability of water molecules at high temperatures. The formation of a great HB network between acidic and amphoteric groups helps proton transport under anhydrous conditions