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Polybenzimidazole (PBI) has becomea popular polymer of choicefor the preparation of membranes for potential use in high-temperatureproton exchange membrane polymer fuel cells. Phosphoric acid-dopedcomposite membranes of poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole](PBI4N) impregnated with hafnium oxide nanofiller with varying contentlevels (0–18 wt %) have been prepared. The structure–propertyrelationships of both the undoped and acid-doped composite membranesare studied using thermogravimetric analysis, modulated differentialscanning calorimetry, dynamic mechanical analysis, wide-angle X-rayscattering, infrared spectroscopy, and broadband electrical spectroscopy.Results indicate that the presence of nanofiller improves the thermaland mechanical properties of the undoped membranes and facilitatesa greater level of acid uptake. The degree of acid dissociation withinthe acid-doped membranes is found to increase with increasing nanofillercontent. This results in a conductivity, at 215 °C and a nanofillerlevel x≥ 0.04, of 9.0 × 10–2S cm–1for [PBI4N(HfO2)x](H3PO4)y. This renders nanocomposite membranes of this type as goodcandidates for use in high temperature proton exchange membrane fuelcells (HT-PEMFCs). [ABSTRACT FROM AUTHOR]