Grain boundaries in dense nanocrystalline ceria ceramics: exclusive pathways for proton conduction at room temperature

Low-temperature proton conduction in nanocrystalline ceramics of Gd-doped ceria with an average grain size of ca. 15 nm was investigated as a function of dopant concentration over the temperature range of 30 < theta 1 degrees C < 200. The proton conductivity, sigma(H+), was independent of the dopant concentration while the oxygen-ion conductivity, sigma(O2-), of these solid electroytes (SEs) displayed the expected dependence. Evidently, the defect chemistry of the bulk phase plays no significant role in determining sigma(H+). These results are thus consistent with our earlier conclusion that the proton conduction in the nanocrystalline SEs takes place exclusively along the grain boundaries. Furthermore, these results suggest the possibility that materials with sufficiently large interface/surface areas may conduct protons under wet atmosphere at such low temperatures regardless of their bulk electrical characteristics.