High-Performance Ether-Free Poly(terphenyl-<i>co</i>-9,9-dimethylfluorene-ethylimidazole) Membranes for High Temperature Proton Exchange Membrane Fuel Cells

High-temperature proton exchange membranes (HT-PEMs) are essential components in HT-PEM fuel cells. Although phosphoric acid (PA)-doped polybenzimidazole (PBI) membranes are widely used, the development of HT-PEMs that are more cost-effective, easier to synthesize, and offer enhanced physicochemical performance remains a critical research focus. In this study, we report a new class of ether-free poly(terphenyl-co-9,9′-dimethylfluorene-ethylimidazole) copolymers synthesized via a superacid-catalyzed Friedel–Crafts hydroxyalkylation reaction. Rigid p-terphenyl (TP) and sterically twisted 9,9′-dimethylfluorene (DMF) segments were systematically copolymerized with 1-ethyl-2-imidazolecarbaldehyde (EtIm) to modulate the polymer backbone architecture and membrane performance. The incorporation of DMF units significantly enhances acid doping capacity and proton conductivity while maintaining good mechanical integrity. Among the copolymers, the optimized P(75%TP-25%DMF-EtIm) membrane achieves an acid doping content of 201%, a high proton conductivity of 124 mS cm–1 at 180 °C, and a tensile strength of 6.02 MPa. When applied in an H2–O2 fuel cell under anhydrous conditions and without backpressure, this membrane delivers an impressive peak power density of 1060 mW cm–2 at 200 °C. These results demonstrate the great promise of ether-free P(x%TP-y%DMF-EtIm) based copolymers as next-generation HT-PEMs, offering a compelling alternative to conventional PBI membranes for high-performance fuel cell applications.