Impedance spectroscopy study of gadolinia-doped ceria

The electrical properties of Gd0.1Ce0.9O1.95 were investigated in various gas atmospheres using a.c. impedance spectroscopy. In dry oxygen, impedance spectra consisted of two arcs at low temperatures corresponding to bulk and grain boundary behaviour, respectively, and only one arc at high temperatures (T>550 °C). The results showed a high oxygen ion conductivity in oxygen atmosphere more than 10−2 S/cm at 600 °C. In water atmospheres, the bulk arc in dry oxygen was reduced by 30 %, and an additional arc corresponding to the grain boundary effect appeared. It seems that the presence of water may decrease the bulk resistance, but causes an additional grain boundary resistance and thus increase in the total resistance. In hydrogen atmosphere, a conductivity enhancement up to one order of magnitude was observed. In hydrogen there is a reduction of Ce4+ to Ce3+ in the sample, which involves defect formation generating e.g. electrons, additional oxygen vacancies and at the same time protons in the lattice. These new charge carriers are responsible for a significant conductivity enhancement. Therefore, in a hydrogen atmosphere the material is a mixed (oxygen ion + proton + electron) conductor.