Pyridine-Bridged Polybenzimidazole for Use in High-Temperature PEM Fuel Cells

Abstract

Although pyridine bridged oxypolybenzimidazole (PyOPBI) membranes are considered to be promising high-temperature proton exchange membrane (HT-PEM) materials that have the potential to overcome many obstacles such as solubility, membrane processability, cost, etc., of the mainstream conventional polybenzimidazole (PBI)-based HT-PEM, the weak structural stability of PyOPBI in concentrated phosphoric acid (PA) and poor dimensional and mechanical stability have been the crucial issues restraining the performance. To mitigate these bottlenecks, in this work, we successfully synthesized three types of PyOPBIs with flexible aryl ether backbones and bulky substituents by polycondensation reaction of various aryl diacids and pyridine-bridged tetraamine 2,6-bis(3′,4′-diaminophenyl)-4-phenylpyridine (PyTAB) in Eaton's reagent followed by casting as HT-PEMs. Three designed bulky substitute containing PyOPBI membranes showed considerably high PA loading capacity (16-22 mol of PA/repeat unit) and proton conductivity (0.04-0.078 S/cm) at 180 °C as compared to earlier reported unsubstituted PyOPBI membranes (14 mol of PA/repeat unit and 0.007 S/cm at 180 °C). In addition, the obtained membranes showcased good chemical, mechanical, thermal, and long-term conductivity stabilities and outstanding stability in concentrated PA. The pendent groups and the bulkiness of the backbone are believed to be the cause behind better stability and facilitating proton transport that results in higher proton conductivity. The single cell made from the membrane electrode assembly of these bulky substituted PyOPBI membranes displayed a peak power density in the range of 144-240 mW cm-2 under H2/O2 at 160 °C, which is considerably higher than that for unsubstituted PyOPBI membrane (90.4 mW cm-2). Overall, the current results provide an effective strategy to explore the benefits of structural modulation of PyOPBI using various structurally divergent diacids to enhance HT-PEM properties and suggest a scalable route for the advancement of PBI-based HT-PEM fuel cells.