Blend Membranes Consisting of Sulfonated Poly(ether ether ketone) and Polysulfone Bearing 4-Nitrobenzimidazole for Direct Methanol Fuel Cells

Low-cost blend membranes consisting of sulfonated poly(ether ether ketone) (SPEEK) (an acidic polymer) and polysulfone-4-nitrobenzimidazole (PSf-NBIm) (a basic polymer) have been prepared with various PSf-NBIm contents (0-5.0 wt %) and characterized by ion-exchange capacity, proton conductivity, and water uptake measurements. The blend membranes with PSf-NBIm contents of 2.5 and 5.0 wt % show higher proton conductivity and lower water uptake in water and methanol/water solutions at a given temperature than plain SPEEK. The blend membranes also exhibit better electrochemical performance and lower methanol crossover in a direct methanol fuel cell than plain SPEEK and Nafion 115 membranes due to an enhancement in proton conductivity through acid-base interactions and an insertion of the benzimidazole side groups into the ionic clusters of SPEEK, as indicated by small-angle X-ray scattering. The maximum power density of the blend membrane with a PSf-NBIm content of 2.5 wt % is 1.5 times higher than that of the Nafion 115 in 1 M methanol solution at 80 degrees C.