Surface proton conductivity of dense nanocrystalline YSZ

Ionic transport in nanocrystalline solid oxides is of considerable current interest. Several studies have reported room-temperature proton conductivity in nanoscaled 8 mol% Y2O3-doped zirconia (YSZ), although the location of the transport species is not clear. In this study, nanocrystalline YSZ with a grain size of similar to 50 nm was prepared by spark-plasma sintering of nanoscaled commercial powder to a density of >97% of the theoretical value. Impedance spectroscopy was employed to analyze the electrical behavior in the temperature range 25-600 degrees C in H2O- and D2O-wetted atmospheres (air, O-2 and 10% H-2 : 90% N-2). Transport in wet conditions below 50 degrees C is limited to the sample surface and occurs via proton hopping (Grotthus mechanism), as demonstrated by a conductive H+/D+ isotope effect. The impedance in these conditions is dominated by a single arc which can be modelled with parallel paths for proton transport on the lateral sample surface and a capacitance with values of the order of those of the grain interior. Surface proton transport is considerable, exhibiting a resistance at 26 degrees C in wet atmospheres comparable to that obtained at similar to 300 degrees C. In contrast, transport by volumetric processes (grain, grain boundary or nanopores) was demonstrated to be insignificant by experiments involving conductivity measurements with a coated lateral surface, with electrode configurations of different areas, and emf measurements in a water-vapour concentration cell.