Polybenzimidazole and butylsulfonate grafted polybenzimidazole blends for proton exchange membrane fuel cells

In this work, we synthesize two poly(benzimidazole) (PBIs), i.e., medium molecular weight PBIm and high molecular weight PBIh with 〈Mw〉 = 9.16 × 104 and 1.60 × 105 g/mol, respectively, and two butylsulfonate grafted PBIs (PBI-BSs), i.e., PBIm-BS and PBIh-BS. In the PBIs, 22 mole% of imidazole groups of PBIm and 17 mole% of imidazole groups of PBIh are grafted with butyl sulfonate groups. The PBIh/PBIh-BS and PBIm/PBIm-BS blend membranes are prepared with various weight (wt) ratios of PBI/PBI-BS. We demonstrate that PBIh/PBIh-BS blend membranes have better mechanical properties, phosphoric acid doping level, proton conductivity, and fuel cell performance than neat PBI and PBIm/PBIm-BS blend membranes. The influences of PBI molecular weight and the Lewis acid–base interaction of PBI-BS butyl sulfonic acids with imidazole groups on the membrane properties and fuel cell performances of PBI/PBI-BS blends are discussed.

► Two PBIs, with 〈Mw〉 = 9.16 × 104 (PBIh) and 1.60 × 105 g/mol (PBIm), were synthesized.
► Two butylsulfonate grafted PBIs, PBIm-BS (with 22 mole%) and PBIh-BS (with 17 mole%), were synthesized.
► The PBIh/PBIh-BS membranes have better fuel cell performance than PBI and PBIm/PBIm-BS membranes.
► Three factors control the membrane properties, namely: hydrogen bonds, acid–base interactions, and excess free volume.