Highly compatible acid–base blend membranes based on sulfonated poly(ether ether ketone) and poly(ether ether ketone-alt-benzimidazole) for fuel cells application

Miscible acid–base blend membranes were developed by incorporating poly(ether ether ketone-alt-benzimidazole) (PEEK-alt-BI) into highly sulfonated poly(ether ether ketone) (SPEEK). The blend membranes with PEEK-alt-BI content up to 20 wt% via optical observation were transparent, indicating an excellent miscibility between the two components. This was further verified by a well-defined morphology and nanoscale domain size of the membranes according to TEM and SEM images. The compatibility between SPEEK and PEEK-alt-BI could be ascribed to the acid–base interactions between the sulfonic acid groups in SPEEK and the benzimidazole groups in PEEK-alt-BI as well as the similarity in the molecular structures of SPEEK and PEEK-alt-BI. It was found that, the addition of PEEK-alt-BI dramatically improved thermal stability and oxidative stabilities, dimensional stability, and mechanical properties of SPEEK membrane. The proton conductivity of SPEEK/PEEK-alt-BI-3 (85/15) membrane is up to 0.087 S/cm at 80 °C, which is higher than that of Nafion117 measured in the same condition. Meanwhile, its methanol permeability is nearly seven times lower than that of Nafion117 at 25 °C.