Benzimidazole grafted polybenzimidazole cross-linked membranes with excellent PA stability for high-temperature proton exchange membrane applications

Benzimidazole grafted polybenzimidazole cross-linked membranes (CPBIm-X) with outstanding phosphoric acid (PA) stability are successfully prepared. 2-chloromethyl benzimidazole (CMBelm) is grafted onto PBI mainchains and using 3-glycidoxypropyltrimethoxysilane (KH560) as a cross-linker. The benzimidazole sidechains can not only increase the basic sites but also allow the membrane to achieve higher phosphoric acid uptakes without sacrificing mechanical strength. Moreover, compared with the imidazole rings in the PBI backbone, the side chain with imidazole rings are flexible, which benefit the proton transportation and lower the activation energy. In addition, the mechanical strength of cross-linked membranes is excellent. For example, the tensile strength value of CPBIm-5 is 101.4 MPa, while that of the PBIm is 79.0 MPa. The proton conductivity is enhanced because the hydrolysis of KH560 resulting SiOSi networks structure, which can absorb more phosphoric acid. The SiOSi networks in the matrix can efficiently improve the stability of phosphoric acid (PA), the remaining weight of CPBIm-5 is 57% PA. Taking into account performance comprehensively, the weight of 5% KH560 is the optimum content. For example, the proton conductivity of CPBIm-5 is 0.092 S cm−1 at 180 °C. Compared to the pristine PBI, it is almost three-fold in proton conductivity.