Cross-linked sulfonated poly(ether ether ketone) electrolytes bearing pendent imidazole groups for high temperature proton exchange membrane fuel cells

An ionically cross-linked sulfonated poly(ether ether ketone) (SPEEK) bearing imidazole groups (SPEEK-Im) has been synthesized via the 1,1′-carbonyldiimidazole (CDI) activating reaction. By regulating the molar ratio of p-xylene dibromide to the imidazole groups of SPEEK-Im, a series of ionic-covalently cross-linked membranes (C-SPEEK-Im-x) can be achieved. Phosphoric acid (PA) doped membranes (PA-SPEEK-Im and PA-C-SPEEK-Im-x) were further prepared by a simple impregnation method for application in high temperature (>100 °C) fuel cells. The ionic interactions between the imidazole and sulfonic acid groups in SPEEK-Im improve the thermal and oxidative stability of the corresponding membrane while offering an elongation at break of 400%. After the ionic-covalent cross-linking process, the mechanical properties, and oxidative and dimensional stability of Im-C-SPEEK-x membranes have been greatly enhanced due to their more compact chemical structure. More importantly, the PA-Im-C-SPEEK-x membranes display high proton conductivity with a low PA doping level at high temperature. Notably, the proton conductivity of PA-Im-C-SPEEK-10 reaches 0.067 S cm−1 at 150 °C and 30% relative humidity. The proton exchange membrane fuel cell using PA-Im-C-SPEEK-20 exhibits a peak power density of 209 mW cm−2 at 130 °C in H2/O2 under anhydrous conditions. The results indicate that ionic-covalently cross-linked membranes with a relatively low PA doping level are suitable for use in high temperature fuel cells.