Complex Impedance and Solid-State NMR Studies on a Novel High Proton Conductor Na-RUB-18

Measurements of complex impedance and 2H NMR spectra were performed on a layer silicate compound, Na-RUB-18 (sodium octosilicate, NaSi4O8(OH)·nH2O), to investigate hydrogen dynamics in crystals. A high proton conductivity amounting to ca. 10−1 S m−1 was observed at room temperature under 100% relative humidity. Activation energies of the proton conduction were estimated to be 125 and 18 kJ mol−1 below and above 290 K, respectively. 2H NMR spectra observed on a deuterated analog revealed 1-D proton conduction through hydrogen-bond networks formed among silanol groups on the layer surface and water molecules in the interlayer space. This material was discovered to be a new type of solid proton-conductor contributed by very weak acids in contrast to the strong-acid type ones so far reported.