Polybenzimidazole composite with acidic surfactant like molecules: A unique approach to develop PEM for fuel cell

Abstract

In the present work, proton exchange membrane (PEM) based on series of polybenzimidazole (PBI) composites are prepared with acidic surfactant like molecules (ASMs) with an objective to improve properties of PEM especially proton conductivity. Composites are obtained by homogenizing poly (4, 4′-diphenylether-5, 5′-bibenzimidazole) (OPBI) in three different ASMs namely camphorsulfonic acid (CSA), p-toluenesulfonic acid (PTSA) and mono-n-dodecyl phosphate (MDP). FT-IR and solid-state NMR studies indicate the presence of interactions between OPBI and ASMs which are necessary for obtaining homogeneous composite membranes. Mechanical reinforcement is observed in case of composite membranes and storage modulus increases with increasing ASM loading in the composite. The detailed thermal analysis shows that phosphoric acid loaded (PA) composite membranes have higher thermal stability than the PA loaded pristine OPBI, it increases with increasing loading of ASM and it largely depends upon the type of ASM in the composite. Though OPBI is an amorphous polymer but ASMs self-organizes themselves in the polymer matrix owing to the strong interaction between OPBI and ASMs. As a result composite membranes display the crystalline character which in turn significantly influences the morphological features of the composites. Fibrillar to porous morphology are observed in composites depending on the type and loading of ASM. This morphological features and the crystalline nature of the composites are found to be responsible for mechanical reinforcement and significant increase in PA loading. The PA doped pristine OPBI, OPBI/CSA-20%, OPBI/PTSA-20% and OPBI/MDP-20% composite membranes proton conductivities are 8.6 × 10−2 S/cm, 2.82 × 10−1 S/cm, 1.71 × 10−1 S/cm and 1.43 × 10−1 S/cm, respectively at 180 °C. OPBI/ASM composite membranes also display very low acid leaching in comparison to pristine OPBI owing to the formation of strong interaction between PA and polymer chains through ASMs.