New Multiblock Copolymers Containing Quaternary Ammonium\nGroups with Ultramicroporous Structure for High-Temperature Proton\nExchange Membrane Fuel Cells

Polybenzimidazole (PBI) exhibits\nconsiderable advantages as a leading\nmembrane material for high-temperature proton exchange membrane fuel\ncells (HT-PEMFCs). However, their harsh synthesis conditions and high\nprocessing costs have greatly restricted the large-scale commercialization\nof HT-PEMFCs. Therefore, developing high-performance and durable membrane\nmaterials as alternatives to PBI has been recognized as the key technical\nchallenge for the advancement of HT-PEMFC technology. In this study,\na series of novel multiblock copolymers QPSBI-<i>b</i>-xTMA,\nconsisting of acidophobic pentafluorophenyl, acidophilic quaternary\nammonium groups, and high free-volume spirobisindane, were synthesized\nby a straightforward polymerization process involving two kinds of\nlow-molecular-weight oligomers with different structures. The resulting\nmultiblock membranes QPSBI-<i>b</i>-xTMA demonstrate well-defined\nmicroporous properties, and the PA-doped membranes exhibit a microphase\nseparation structure, which effectively facilitates proton conduction\n(75.45 mS cm<sup>–1</sup>@200 °C). The HT-PEMFCs based\non the QPSBI-<i>b</i>-xTMA/PA membrane can operate efficiently\nwithin the temperature range of 160–220 °C, achieving\na high peak power density of 0.84 W cm<sup>–2</sup> without\nexternal pressure and humidity. Notably, owing to the siphoning effect\nof the micropores and the strong quaternary ammonium-biphosphate ion\npairs, the fuel cell exhibits a stable performance at a high current\ndensity of 0.5 A cm<sup>–2</sup> at 160 °C, with a minimal\nvoltage degradation rate of merely 4.7 μV h<sup>–1</sup>. Thus, the newly developed QPSBI-<i>b</i>-<i>x</i>TMA/PA materials present a promising avenue for HT-PEMFC applications.