Sulfonated Polyphosphazene-Blended Self-Cross-Linked Polybenzimidazole-Based High-Temperature Proton Exchange Membranes: High Efficiency in Proton Transport at Low Humidity
Weihui Cui, Peng Sun, Ping Li, Lei Zhang, Xulei Zhi, Qiang Liu, Zhongfang Li
Abstract
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.