Benzylbiphenyl-regulated poly(aromatic ethylimidazole)s toward high-performance high temperature proton exchange membranes in fuel cells.

Abstract

The development of high temperature proton exchange membranes (HT-PEMs) with simultaneously enhanced proton transport and mechanical robustness remains a major challenge for HT-PEM fuel cells. Herein, we report a molecular backbone engineering strategy based on the incorporation of 4-benzylbiphenyl (BBP) units into poly(p-terphenyl ethylimidazole) frameworks to regulate chain conformation and membrane properties. A series of BBP-containing P(TPx%-BBPy%-EIm) copolymers with systematically varied compositions are synthesized via a superacid-catalyzed Friedel-Crafts polymerization, enabling an in-depth investigation of the relationship between backbone architecture and membrane performance. Experimental characterization and theoretical calculations reveal that the sp3-hybridized methylene bridge in BBP disrupts continuous π-conjugation and induces a twisted, flexible polymer configuration, which enhances phosphoric acid (PA) affinity, promotes microphase separation, and facilitates proton conduction. Among the resulting membranes, P(TP70%-BBP30%-EIm)/214%PA achieves an optimized balance between conductivity and mechanical integrity, exhibiting a tensile strength of 15.7 MPa and a proton conductivity of 104 mS cm-1 at 180 °C under anhydrous conditions. When integrated into an H2/O2 membrane electrode assembly, this membrane delivers a peak power density of 783 mW cm-2 at 180 °C under non-humidified and zero-backpressure conditions, which also display a 144-h durability under 200 mA cm-2 at 160 °C. This work establishes flexible benzylbiphenyl-containing poly(aromatic ethylimidazole) polymers as a promising platform for HT-PEMs in fuel cells and also provides a molecular design strategy for developing advanced membrane materials for other new energy devices.