Fluorinated Poly(imide- <i>co</i> -benzimidazole) Copolymer as a High-Temperature Proton Exchange Membrane for Fuel Cells

High-temperature proton exchange membranes (HT-PEMs) play a significant role in high-temperature proton exchange membrane fuel cell (HT-PEMFC) applications. In this work, a series of fluorinated poly(imide-co-benzimidazole) (PFIBIx) copolymers were synthesized as potential HT-PEM materials. The introduction of benzimidazole groups into the backbone of the polymers significantly enhanced PA uptake and retention of the membranes via acid–base interactions, endowing the PFIBIx membranes with a proton conductivity of 60–82 mS cm–1 at 160 °C. Meanwhile, the introduction of fluorinated groups effectively suppressed excessive membrane swelling due to their acid-phobic nature. The PFIBIx membranes also exhibited excellent mechanical properties and oxidation stability. Notably, the single fuel cell with PFIBI40, despite having a relatively lower conductivity of 67 mS cm–1 at 160 °C, delivered a maximum power density of 210 mW cm–2. This enhanced performance is attributed to its superior stability and overall balanced properties, demonstrating that PFIBI40 is a highly promising candidate for HT-PEM applications.