Proton conducting porous membranes based on poly(benzimidazole) and poly(acrylic acid) blends for high temperature proton exchange membranes

Abstract Poly(benzimidazole) (PBI): poly(acrylic acid) (PAA) blend membranes were prepared with different number-average molecular weights ( M ¯ n ) and molar ratios of PAA as novel proton exchange membranes. PAA titration behavior showed that by increasing the M ¯ n , apparent acidity of PAA decreases. The interaction between PBI and PAA was confirmed by FTIR and DSC. The cross-section SEM photographs showed that pore structure can be formed in the membrane by increasing M ¯ n and molar ratio of PAA. The membranes showed lower methanol permeability (1.06–1.91×10 −9  cm 2 .s −1 ) in comparison to the Nafion and mechanical properties were comparable with phosphoric acid (PA)-doped PBI membranes. An increasing trend in proton conductivity of the membranes with M ¯ n and molar ratio of PAA was observed that is attributed to the increasing the water uptake, amount of carboxylic acid groups of PAA and pores in structure where more hydrogen bonding network between PBI and PAA can be formed. Also, by increasing the M ¯ n , acidity of PAA was decreased, resulting in more proton transport by hopping mechanism. PBI:PAA100KD(1:4) (membrane with PAA of 100,000 g.mol −1 and PBI:PAA molar ratio of 1:4) blend membrane showed the highest proton conductivities of 5.0 mS.cm −1 at 150 °C in the anhydrous state; 7.5 mS.cm −1 at 80 °C in the humidified state and 500.0 mS.cm −1 at 150 °C in the H 3 PO 4 -doped form.