Fabricating a MOF Material with Polybenzimidazole into an Efficient Proton Exchange Membrane

Metal organic frameworks (MOFs) have received considerable importance as proton conducting materials in recent times. However, most of the MOFs lack the ability to form film, which limits their application. In the present work, polybenzimidazole (PBI) composite membranes have been prepared by loading post synthetically modified (PSM) UiO-66-NH2 MOFs, denoted as PSM 1 and PSM 2 into an aryl ether-type polybenzimidazole (OPBI) polymer. The pristine OPBI, and MOF nanofiller loaded membranes were doped with phosphoric acid (PA) to prepare proton exchange membranes (PEMs). Use of thermally stable, hydrophilic MOFs resulted in enhanced proton conductivity, higher PA retention capacity, and increased stability against oxidative degradation for the composite membrane than the pristine OPBI polymer. The proton conductivities of the composite membranes (0.29 S cm-1 for PSM 1-10% and 0.308 S cm-1 for PSM 2-10% membranes at 160 °C, under anhydrous environment) were notably higher than the conductivities of the constituents and also higher than most of the MOF based polymer supported membranes. To the best of our knowledge, the PA doped PSM 2 loaded composite membrane shows the highest proton conductivity at 160 °C among all MOF based composite membranes. Extensive interfacial H-bonding plays the most crucial role behind the enhanced proton conductivities of the PA doped MOF containing polymer membranes reported here. This work clearly demonstrates the benefits of using rationally designed PSM 1 and PSM 2 MOFs as nanofiller to prepare OPBI supported membranes that can perform excellent proton conduction in a wide temperature range spanning up to 160 °C. This provides a generalized approach toward achieving an efficient proton conducting membrane for use in fuel cells.