Interfacially Tuned <scp>PEEK</scp> ‐Based Composite Bipolar Membranes for High‐Temperature Fuel Cells

ABSTRACT Hydrocarbon‐based bipolar membranes (BPMs) have gained considerable attention for high‐temperature electrochemical systems because of their tunable physicochemical properties and lower cost than conventional perfluorinated membranes. However, their performance is often limited by poor interfacial morphology and high junction resistance. This work develops interfacially engineered PEEK‐based composite BPMs using a structure–process–performance approach. Sulfonated poly(ether ether ketone) (sPEEK) proton exchange membranes (PEM) modified with TiO 2 nanoparticles or poly(ether sulfone) (PES) are integrated with crosslinked quaternary ammonium PEEK (cQAPEEK) anion exchange membranes (AEM). Thermo‐mechanical hot‐press lamination parameters are systematically optimized using response surface methodology (RSM) to improve interfacial compactness, polymer chain interpenetration, and ionic transport pathways. The optimized sPEEK/TiO 2 0.5–cQAPEEK72h BPM exhibits an ionic conductivity of 8.39 mS cm −1 and achieves the highest maximum power density of 53.12 mW cm −2 at 80°C and 100% relative humidity. SEM analysis shows a compact and well‐bonded PEM/AEM interface. Together with the electrochemical results, these findings suggest that improved interfacial morphology and adequate ionic conductivity enhance hydration management and fuel cell performance. This study establishes a clear relationship between membrane composition, hot‐press processing, interfacial morphology, and electrochemical performance, providing a practical strategy for developing durable, fluorine‐free BPMs for high‐temperature fuel cell applications.