Interplay between Composition, Structure, and Properties of New H<sub>3</sub>PO<sub>4</sub>‑Doped PBI<sub>4</sub>N–HfO<sub>2</sub> Nanocomposite Membranes for High-Temperature Proton Exchange Membrane Fuel Cells

Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly­[2,2′-(<i>m</i>-phenylene)-5,5′-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0–18 wt %) have been prepared. The structure–property relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the acid-doped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 °C and a nanofiller level <i>x</i> ≥ 0.04, of 9.0 × 10<sup>–2</sup> S cm<sup>–1</sup> for [PBI4N­(HfO<sub>2</sub>)<sub><i>x</i></sub>]­(H<sub>3</sub>PO<sub>4</sub>)<sub><i>y</i></sub>. This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs).