Surface proton conductivity of dense nanocrystalline YSZ

Abstract

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 ∼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 °C in H₂O- and D₂O-wetted atmospheres (air, O₂ and 10% H₂ : 90% N₂). Transport in wet conditions below 50 °C is limited to the sample surface and occurs via proton hopping (Grötthus 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 °C in wet atmospheres comparable to that obtained at ∼300 °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.