Quaternary Ammonium Polybenzimidazole/Polymeric Ionic Liquid Cross-Linked Membranes with High Proton Conductivity and Stability for HT-PEM Applications

Quaternary ammonium (QA) group is widely used in high-temperature proton exchange membranes (HT-PEMs) because of its own strong basicity and ability to form QA–diphosphate ion pairs with phosphoric acid (PA). In this work, compared with other introductions of QA groups in membranes, by adding QA sites to the polybenzimidazole (PBI) main chains, not only PA retention is improved but also the imidazole N–H sites are conserved, which allow them to participate in proton transfer and further improve proton conductivity. QMPBI was prepared by introducing QA groups into the main chains of methyl-containing polybenzimidazole (MPBI) through a series of modifications in order to improve PA uptake and retention. Then, QMPBI and presynthesized polymeric ionic liquid (PIL) containing two nitrogen-positive ion pairs undergo cross-linking to generate composite membranes with different quaternization ratios-QMPBI-X-PIL (where X = 30, 50, 70%). It is experimentally verified that the QMPBI-70%-PIL membrane at a quaternization ratio of 70% has the best proton conductivity and PA retention rate. The QMPBI-70%-PIL membrane had a proton conductivity of 126.22 mS cm^–1 at 180 °C and a maximum residual mass of 85.71% at 160 °C/0% RH after a 240 h PA retention test. At 160 °C without humidity, the peak power density of the QMPBI-70%-PIL membrane could reach 454.89 mW cm^–2. In conclusion, QMPBI-70%-PIL membranes are expected to be widely used in HT-PEMs.