SPEEK/CMABPBI Ionic and Self-Covalent Cross-Linked Composite Membrane: A Method to Comprehensively Enhance the Properties of High-Temperature Proton Exchange Membranes

To use a minimized cross-linking agent to best improve the performance of a membrane, chloromethylated poly(2,5-benzimidazole) (CMABPBI) is designed to achieve self-covalent cross-linking. Poly(2,5-benzimidazole) (ABPBI)-based high-temperature proton exchange membranes are prepared by blending CMABPBI with sulfonated poly(ether ether ketone) (SPEEK). CMABPBI self-covalent cross-linking formed basic sites benefiting ionic and hydrogen bonds. The combination of covalent cross-linking and ionic and hydrogen bonds enhances the comprehensive properties of the composite membranes, including temperature resistance, durability, mechanical properties, oxidation resistance, methanol resistance, and dimensional stability. Above all, the proton conductivity of the membrane, especially at high temperature and low humidity, is high when those properties are good. The proton conductivity of SPEEK/CMABPBI (20%) at 170 °C and 100% relative humidity (RH) is 0.193 S cm–1. At 180 °C, the proton conductivity is 0.08 S cm–1 at 50% RH and 0.0082 S cm–1 at 0% RH. The results indicate that the SPEEK/CMABPBI ionic and self-covalent cross-linked composite membrane is a potential candidate for direct methanol fuel cells.