Toward a New Type of Anhydrous Organic Proton Conductor Based on Immobilized Imidazole

Extensive studies on a model system (imidazoleterminated ethylene oxide oligomers doped with small amounts of strong acids) for a proton-conducting polymer functioning without a liquid phase, but instead using imidazole tethered to the backbone via flexible spacers as a proton solvent, are presented and the parameters governing conductivity and its mechanism are discussed. Temperature-dependent conductivities are well described by free-volume considerations (VTF-behavior). Thus, besides a high density of imidazole moieties, a low Tg is in favor of high proton conductivity, which experimentally is shown to be predominantly due to structure diffusion. The available free volume is suggested to correlate with the rate of hydrogen bond breaking and forming processes within the dynamical hydrogen bond networks, which generally limit the rate of long-range diffusion of protonic defects via structure diffusion. The equilibrium constants for the protonation of imidazole by two different dopants in an oligo(ethylene oxide) environment are determined by NMR, indicating complete dissociation of strong acids such as trifluoroacetic acid. The crystal structure obtained from one of the model compounds is dominated by hydrogen bond interactions interconnecting the imidazole units and suggesting an easy proton migration within the imidazole-rich domains.