High-performance poly(aromatic pyridine) copolymers with crown ether moieties for high temperature polymer electrolyte membrane fuel cells

Abstract

This study concentrates on the development of high temperature polymer electrolyte membranes (HT-PEMs), which are essential components for HT-PEM fuel cells (HT-PEMFCs). Although the phosphoric acid (PA)-doped polybenzimidazole (PBI) has been regarded as the successful HT-PEM, this system still suffers from several challenges, including the use of carcinogenic monomers, complex synthesis procedures, and poor solubility in organic solvents. To develop more cost-effective, readily synthesized and high-performance alternatives, this study employs a simply superacid-catalyzed Friedel-Crafts reaction to synthesize a series of poly(triphenyl-co-dibenzo-18-crown-6 pyridine) copolymers, denoted as P(TP_x%-co-CE_y%), using p-triphenyl, dibenzo-18-crown-6 and 4-acetylpyridine as monomers. The copolymerized hydrophilic and bulky crown ether unites introduce large free volumes and multiple interaction sites with PA molecules, as elucidated by theoretical calculations. Meanwhile microphase separation structures are formed as confirmed by atomic force microscope (AFM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Thus P(TP_x%-co-CE_y%) membranes exhibit excellent PA absorption and proton conduction abilities. For example, after immersing in 85 wt% PA at 30 °C, the P(TP_91%-co-CE_9%) membrane achieves a PA doping content of 205% and a high conductivity of 0.138 S cm⁻¹ at 180 °C, while maintaining a tensile strength of 7.5 MPa at room temperature. Without humidification and backpressure, the peak power density of an H₂-O₂ cell equipped with P(TP_91%-co-CE_9%)/205%PA reaches nearly 1200 mW cm⁻², representing one of the highest performances reported for PA-doped HT-PEMs to date. This work demonstrates the enormous potential of poly(triphenyl-co-crown ether pyridine) membranes in the HT-PEMFC applications.