Copolymers of Pyridine-bridged polybenzimidazole for the use in high temperature PEM fuel cell

Abstract

The development of phosphoric acid (PA)-loaded polybenzimidazole (PBI) proton exchange membranes (PEMs) possessing high proton-conductivity and stability (both chemically and dimensionally) is highly desirable but remains a significant challenge. A series of random copolymers of pyridine-functionalized bulky fluorophenylene-containing flexible arylether polybenzimidazoles were designed and synthesized in good yields with different molecular weights and fabricated as membranes for the use in high-temperature PEM fuel cell (HTPEMFC). These membranes showed hydrophilic-hydrophobic microphase-separated structures, confirmed by transmission electron microscopy (TEM), owing to the combination of rigid and numerous heteroatoms including N, O and F which were integrated into polymer skeletons. The copolymer backbone with a microphase-separated path endowed the membranes with high PA-loading capacity (up to 25.4 mol of PA/repeat unit) and high-efficiency proton conduction. Noticeably, the constructed co-network membranes exhibited a high proton conductivity (up to 0.17 S cm−1 at 180 °C) under anhydrous conditions. The H2/O2 single-cell membrane electrode assembly (MEA) based on copolymer (Co-P3) PEM exhibited a maximum peak power density (PPD) of 332 mW cm−2 at a current density of 722 mA cm−2 at 160 °C. This performance is much better than that of 60 wt% PA-loaded PyOPBI membrane (108 mW cm−2 with a current density of 209 mA cm−2) and 85 wt% PA-loaded Ph(CF3)-PyOPBI membrane (239 mW cm−2 with current density of 629 mA cm−2) under the identical fuel cell performance measurement conditions. In addition, these membranes showed remarkable performance in oxidative and thermal–mechanical stability. Overall, the current results demonstrate that the combination of pendant rigid and flexible random co-structures are promising membrane materials for use in HT-PEMFC.