High surface proton conduction of a-axis oriented CeO_2−δ thin film on (200) YSZ substrate

We have prepared a-axis oriented CeO_2−δ thin films prepared on yttrium-stabilized zirconium (200) single crystal substrates by RF magnetron sputtering using a ceramic target. The amount of oxygen vacancies in the CeO_2−δ thin films was quantitatively evaluated by X-ray absorption spectroscopy and defect chemical analysis. In terms of its electronic structure, the O 1s photoemission spectrum of the wet-annealed film shows an O-H bond peak, which suggests proton conduction. The a-axis oriented CeO_2−δ thin film annealed at 600 °C in a wet atmosphere exhibit high proton conductivity of more than 10¹ Scm⁻¹K at medium temperature range from 300 °C to 550 °C by thermal activation process and surface proton conductivity of σT = ∼7.0 × 10⁻¹ Scm⁻¹K by Grotthuss mechanism below 50 °C. The enhancements of proton conductivities are considered to be due to large amount of oxygen vacancies of ∼3.3% at the surface of wet-annealed CeO_2−δ thin film. These results indicate that the wet-annealed a-axis oriented CeO_2−δ thin films can be applied as an electrolyte for solid oxide fuel cell operating at medium temperature range or electric double layer transistor operating below 50 °C.