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

Abstract

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 H₂/O₂. 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.