Cross-linked polybenzimidazoles containing hyperbranched cross-linkers and quaternary ammoniums as high-temperature proton exchange membranes: Enhanced stability and conductivity

High proton conductivity with sufficient stability for phosphoric acid (PA) doped polybenzimidazole membranes is critical for applications in fuel cells. Macromolecular cross-linkers with hyperbranched structures have large volumes and many functional groups, which can not only react with polymer backbones to form multiple cross-linked sites, but also provide favorable conditions for subsequent functional group modifications to regulate the interaction between polymers and PA; however, such cross-linkers are rarely exploited in fuel cells. Herein, a series of cross-linked polybenzimidazole membranes were successfully prepared based on a novel hyperbranched cross-linker and the incorporation of numerous quaternary ammonium groups. These cross-linked polybenzimidazole membranes containing a hyperbranched cross-linker and quaternary ammonium groups showed superior performance, in terms of mechanical properties, oxidative resistance and proton conductivity. The tensile strength of the PA doped cross-linked membranes was more than 20.0 MPa. These cross-linked membranes showed only a slight weight loss and no cracks after immersion in Fenton's reagent for 200 h, while the linear membrane was broken into pieces after immersion in Fenton's reagent for 100 h. With low acid loading, the membranes containing cross-linked polybenzimidazole with quaternary ammonium groups still exhibited good conductivity. As a result of the excellent comprehensive properties, the obtained fuel cell based on the membrane with 15% of the hyperbranched cross-linker showed a great power density of 260 mW cm−2, which was 36.8% higher than that of the fuel cell based on the corresponding linear membrane.