Ether-Free Poly(9,9-dimethylfluorene- co -ethylimidazole- co -oxindole) Copolymers for High-Temperature Polymer Electrolyte Membrane Fuel Cell Applications

Abstract High-temperature proton exchange membranes (HT-PEMs) are key components for high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes have been normally regarded as the benchmark materials, but their practical application is still limited by several drawbacks. Developing low-cost, easily processable, and high-performance alternative HT-PEMs therefore remains highly desirable. Herein, a range of fully aromatic and ether-free poly(9,9-dimethylfluorene-co-ethylimidazole-co-oxindole) copolymers are successfully synthesized via superacid-catalyzed Friedel–Crafts hydroxyalkylation using low-cost and highly reactive 1-ethyl-1H-imidazole-2-formaldehyde (EtIm) as the functional monomer. Twisted and bulky 9,9-dimethylfluorene (DMF) units are introduced to increase backbone free volume and promote PA uptake, while rigid oxindole segments were incorporated to improve mechanical strength, dimensional stability, and oxidative durability. By systematically tuning the EtIm/isatin molar ratio, the correlation between polymer backbone architecture and properties of membranes was comprehensively examined. The optimized P(DMF-80%EtIm-20%OxI) exhibited the superior physicochemical properties, possessing a proton conductivity of 119.3 mS cm–1 at 180 °C and a tensile strength of 4.6 MPa after PA doping, together with excellent thermal and chemical stability. More importantly, the corresponding H2–O2 single cell delivered a maximum power density of 846 mW cm–2 at 180 °C under nonhumidified and ambient-pressure conditions. These results demonstrate that the regulation of backbone rigidity, free volume, and basic functional groups is an effective strategy for designing advanced HT-PEMs for fuel cell applications.