Local Structure and Dynamics of Imidazole Molecules in Proton-Conducting Poly(vinylphosphonic acid)–Imidazole Composite Material

Many proton-conducting polymers, including imidazole and its derivatives, have been developed for their applications in solid electrolyte fuel cells. It is thought that proton conduction is caused by the Grotthuss mechanism associated with the reorientational motion of imidazole molecules in these materials. However, there are still very few analyses of the relation between proton conduction and the reorientational motion of imidazole molecules following the detailed investigation of the molecular motion of imidazole. In the present work, molecular motions and their influence on proton conductivity were investigated for poly(vinylphosphonic acid) (PVPA)–imidazole (Im) composite material (PVPA/xIm, where x is the molar ratio of Im to polymer repeat unit) by solid-state NMR. Between 20 and 50 °C, the Im molecule undergoing pseudoisotropic rotation with small anisotropy accounted for more than 90% and that undergoing rotational vibration for less than 10% for PVPA/2Im. The tight segment, where Im molecules undergo rotational vibration, prevents long-range proton conduction. Above 60 °C, efficient proton conduction accompanied by pseudoisotropic rotation of the Im molecules was confirmed in PVPA/2Im.