Rational design of microporous polybenzimidazole framework for efficient proton exchange membrane fuel cells

The preparation of polymeric anhydrous proton conducting membranes is critical for the development of high-temperature proton-exchange membranes (HT-PEMs) for use in fuel cells but remains a significant scientific challenge to date. Polybenzimidazole (PBI) is a highly stable engineered plastic with excellent thermochemical stability, which demonstrates its suitability as an HT-PEM. However, the application of this material is limited due to its leaching of phosphoric acid (PA) and slow and low proton conduction. Herein, we demonstrate a feasible strategy to address these key issues by synthesising a new class of three-dimensional (3D) iptycene-based ladder-like porous pyridine-bridged oxypolybenzimidazoles (IPyPBIs) as self-standing, highly flexible HT-PEMs via the facile polycondensation between newly designed Y- and H-shaped scaffolds of iptycene-containing aryl ether diacids and pyridine-functionalized tetraamine building blocks. The as-synthesized polymers possessed high molecular weights, excellent thermal-chemical stability, hierarchical intrinsic porosity ( ca. 12.1 Å) and excellent solubility in various solvents, and thus excellent processability for the facile fabrication of PEMs. Furthermore, the IPyPBI walls were found to trigger multiple hydrogen-bonding interactions with PA molecules to lock and stabilize the PA network inside the pores, thereby favouring superior PA-holding capability (as high as 32 mol of PA/repeat units) in the resulting membranes. Consequently, these PA-loaded IPyPBI HT-PEMs exhibited stable ultrahigh proton conductivity (up to 0.24 S cm −1 at 180 °C and 0% humidity) and crossed the upper proton conductivity (0.1 S cm −1 ) limit of traditional PA-loaded PBI. The single cell made from these PEMs displayed a good peak power density of 0.28 W cm −2 at 160 °C in H 2 /O 2 . Overall, this work paves the way to achieve the targeted properties of PBIs through the predesign and functionalization of their porous surface and highlights the immense potential of microchannel-forming PBIs as a rigid platform for fast proton transportation. Ion-conducting membranes from three-dimensional iptycene-based polybenzimidazoles developed for the first time to fabricate efficient proton exchange membranes with ordered nanochannels, enabling ultrafast and stable anhydrous proton conduction.