Sulfonated Polyphosphazene-Blended Self-Cross-Linked Polybenzimidazole-Based High-Temperature Proton Exchange Membranes: High Efficiency in Proton Transport at Low Humidity

Polybenzimidazole (PBI) and polyphosphazene derivatives show favorable thermal stability in proton exchange membrane applications. Self-covalent cross-linked chloromethylated polybenzimidazole (cCM-mPBI) is blended with sulfonated polynaphthoxyphosphazene (SPNPP) at varying weight ratios for preparation of high-temperature proton exchange membranes. The structure and properties are studied using infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy. Results indicate that the self-covalent cross-linking structure improves the mechanical properties of the composite membranes and facilitates formation of a greater level of ionic and hydrogen bonds. The high content of sulfonic acid groups of SPNPP, good compatibility with CM-mPBI, more alkaline sites exposed by self-cross-linking, the exemption of nonconductive cross-linking agents, and a more compact hydrogen bond network have a synergistic effect on making the composite membrane exhibit good proton conductivity. The proton conductivity at an SPNPP doping level of 40 wt % is 0.152 S cm–1 at 170 °C and 100% relative humidity (RH). More importantly, the cCM-mPBI/SPNPP (40%) membrane also shows high proton transport efficiency at lower RH values of 50 and 0% with proton conductivities of 0.075 and 0.042 S cm–1, respectively. The results show that cCM-mPBI/SPNPP is a promising material to be applied as a high-temperature proton exchange membrane.