Macromolecular covalently cross-linked quaternary ammonium poly(ether ether ketone) with polybenzimidazole for anhydrous high temperature proton exchange membranes

Abstract

Poly(ether ether ketone) bearing benzyl bromide groups (Br–PEEK) was synthesized and a series of cross-linked membranes (Br–PEEK–x%PBI) based on Br–PEEK with polybenzimidazole (PBI) as a macromolecular cross-linker was prepared to improve the dimensional stability and tensile strength without reducing proton conductivity. X-ray photoelectron spectroscopy (XPS) confirmed the success of the cross-linking reaction. After being ammoniated, the quaternary ammonium PEEK membranes were immersed in phosphoric acid and anhydrous phosphoric acid doped membranes were obtained. The phosphoric acid doped membranes without PBI as the cross-linker had excess volume swelling and could not remain integrated. The other cross-linked membranes had good dimensional stabilities. Because PBI could absorb phosphoric acid, the proton conductivities of cross-linked membranes first increased and then decreased with the content of PBI increasing. The highest proton conductivity was 0.081 S cm⁻¹ at 200 °C for the PA–PEEK–20%PBI membrane. The dimensional stabilities, oxidative stabilities and tensile strength of PA–PEEK–x%PBI membranes improved. The PA–PEEK–30%PBI membrane could last for 7.5 h in 3 wt.% H₂O₂, 4 ppm Fe²⁺ Fenton solution at 80 °C before breaking into pieces. Energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) were used for detailed research.