Mechanisms of Protonic Surface Transport in Porous Oxides: Example of YSZ

The electrical properties and the charge carrier mechanism of porous 8 mol % yttria-stabilized zirconia (8YSZ) ceramic samples have been investigated over wide ranges of relative humidity (RH) and temperature (25–400 °C). The presence of humidity introduces protonic surface conduction, and porous YSZ shows pure protonic conduction below ∼150 °C. H/D isotope effect studies combined with transport number measurements reveal a change in transport mechanism from structural diffusion (Grotthuss type) to vehicular transport when the relative humidity exceeds ∼60% RH, coinciding with a change in the adsorbed water layer from an “ice-like” to a “water-like” structure. Similarly, the activation energy for proton transport decreases from 0.43 to 0.28 eV when the relative humidity increases from 20% RH to 84% RH, reflecting changes in the enthalpies of formation and migration of charged species with increasing water layer thickness.