Quaternized polybenzimidazole ion-pair blend membranes for high performance and stability in high-temperature proton exchange membrane fuel cells

Abstract

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) require effective and selective proton-transporting membranes, which should additionally be affordably accessible. Quaternized polybenzimidazole may enable such proton transport if it can be functionally introduced into an adequate membrane conformation. Here, we post-modified polybenzimidazole (PBI) via N-alkylation/arylation, creating a positively charged polymer backbone that improves solubility and processability in organic solvents. The thermal stability of the N-phenyl-substituted OPBI (Ph-OPBI) (400 °C) was remarkably higher than that of the N-methyl-substituted PBI (Me-OPBI) (200 °C). Both quarternized OPBIs formed stable and processable membranes after blending with 20 wt% phosphonated poly(pentafluorostyrene) (PWN). Ex-situ impedance tests revealed high proton conductivity, namely, 96 mS cm⁻¹ for Ph-OPBI-PWN (167 % doping) and 142 mS cm⁻¹ for Me-OPBI-PWN (313 % doping) at 180 °C. The blended membranes based on Ph-OPBI exhibited a superior performance compared to Me-OPBI and unmodified OPBI with a maximum power density of ∼0.55 W cm⁻² at 200 °C (H₂/air, without humidification). Accelerated stress tests (AST) of the blended membranes based on the Ph-OPBI membrane also revealed remarkable stability, exhibiting non-detectable performance degradation over 100 cycles (∼288 h). Hence, the N-phenyl substituted OPBI opens a promising way to engineer membranes with improved performance and accessible for long-term operation in HT-PEMFC applications.