Tuning the quaternization degree of N-phenyl-substituted polybenzimidazole membranes for enhanced HT-PEMFC performance

Development of polybenzimidazole (PBI)-based ion-pair membranes with enhanced conductivity and sufficient dimensional stability is crucial for the practical application of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). In this work, quaternized OPBI (QPBI) membranes with controlled N-phenyl substitution degrees of 50 %, 63 %, 73 %, and 93 % were synthesized by post-polymer modification. Increasing substitution degree enhanced ion-exchange capacity, phosphoric acid uptake (PAU), and proton conductivity, but also increased swelling and reduced dimensional stability. QPBI93 showed the highest conductivity of 84 m S cm −1 at 180 °C, whereas QPBI50 provided the best balance between conductivity and stability, with a conductivity of 33 m S cm −1 and a volume swelling of 80 %. Membrane-electrode assemblies incorporating the QPBI membranes and the PWN64 ionomer were evaluated against commercial Orion CMX membranes. QPBI50 achieved the highest maximum power density at 0.742 W cm −2 at 180 °C and H 2 /O 2 (1.5/2.5), while QPBI93 failed during operation due to severed swelling and poor dimensional stability. Electrochemical impedance spectroscopy further showed that the type of membrane affected not only the ohmic resistance, but also the charge-transfer resistance. These results demonstrate that moderate quaternization is an effective strategy for optimizing ion-pair membranes for HT-PEMFCs.