Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells

Silane-cross-linked polybenzimidazole (PBI) membranes with high proton conductivity and excellent mechanical properties were successfully prepared by using a silane monomer, γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560), as a cross-linker. Fourier transform infrared spectroscopy and solubility tests were used to characterize and confirm the cross-linked structure in the membranes. The silane-cross-linked membranes displayed excellent chemical stability and improved mechanical strength. Especially at high temperature (130 °C), where the tensile strength value was in the range of 68.6 to 99.3 MPa, while that of the pristine PBI was 61.7 MPa. Moreover, the proton conductivity was significantly enhanced because the silane-cross-linked structure in the membranes could absorb more phosphoric acid. Considering the tradeoff of mechanical properties and proton conductivity, 3% KH560 in weight was demonstrated to be the optimum content in the membranes, for instance, the SCPBI-3/7.95 PA (the cross-linker content was 3 wt% and the PA doping level was 7.95) had a proton conductivity of 0.081 S cm^-1 and that of the SCPBI-3/9.07 PA was 0.114 S cm^-1 at 200 °C, while that of pristine PBI was 0.015 S cm^-1 at 200 °C.