Quaternary Ammonium\nPolybenzimidazole/Polymeric Ionic\nLiquid Cross-Linked Membranes with High Proton Conductivity and Stability\nfor HT-PEM Applications

Quaternary ammonium (QA) group is widely used in high-temperature\nproton exchange membranes (HT-PEMs) because of its own strong basicity\nand ability to form QA–diphosphate ion pairs with phosphoric\nacid (PA). In this work, compared with other introductions of QA groups\nin membranes, by adding QA sites to the polybenzimidazole (PBI) main\nchains, not only PA retention is improved but also the imidazole N–H\nsites are conserved, which allow them to participate in proton transfer\nand further improve proton conductivity. QMPBI was prepared by introducing\nQA groups into the main chains of methyl-containing polybenzimidazole\n(MPBI) through a series of modifications in order to improve PA uptake\nand retention. Then, QMPBI and presynthesized polymeric ionic liquid\n(PIL) containing two nitrogen-positive ion pairs undergo cross-linking\nto generate composite membranes with different quaternization ratios-QMPBI-<i>X</i>-PIL (where <i>X</i> = 30, 50, 70%). It is experimentally\nverified that the QMPBI-70%-PIL membrane at a quaternization ratio\nof 70% has the best proton conductivity and PA retention rate. The\nQMPBI-70%-PIL membrane had a proton conductivity of 126.22 mS cm<sup>–1</sup> at 180 °C and a maximum residual mass of 85.71%\nat 160 °C/0% RH after a 240 h PA retention test. At 160 °C\nwithout humidity, the peak power density of the QMPBI-70%-PIL membrane\ncould reach 454.89 mW cm<sup>–2</sup>. In conclusion, QMPBI-70%-PIL\nmembranes are expected to be widely used in HT-PEMs.