Cerium Reduction at the Interface between Ceria and Yttria-stabilised Zirconia and Implications for Interfacial Oxygen Non-stoichiometry
CeO2 and Y2O3-stabilized zirconia (YSZ) are two candidates for electrolyte materials in solid oxide fuel cells because of their high ion conductivities. Recent experiments have shown that hetero-structures consisting of alternating layers of Gd-doped CeO2 and YSZ exhibit even higher ionic conductivity than either of the bulk materials . However, the mechanism for the high ionic conductivity of these hetero-structures is still unclear. Theoretical calculations indicate that the oxygen vacancy formation energy is considerably reduced at the interfaces and oxygen vacancies, expected to segregate to the interfaces, might provide highways for rapid ion conduction . In this project, we investigate the structure and chemistry of interfaces between CeO2 and YSZ using scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS).
We observe that the CeO2 film and the YSZ substrate have a cubic on cubic orientation relationship. No reaction layers or other phases are seen at the interface. Periodical misfit dislocations are observed at the interface, with extra atomic planes appearing in YSZ (see figure). Since the presence of Ce3+ is seen as evidence of oxygen vacancy formation, oxygen states of cerium ions near the interface were investigated by EELS. This investigation revealed that most of the Ce ions are reduced from Ce4+ to Ce3+ in the interface region, with a gradient over several nanometers. Information on such local non-stoichiometries is crucial not only for understanding charge transport in these hetero-structures but also for understanding ceria catalytic properties .
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 M. Fronzi, S. Cereda, Y. Tateyama, A. De Vita, E. Traversa, Physical Review B 86 (2012) 085407
 K. Song, H. Schmid, V. Srot, E. Gilardi, G. Gregori, K. Du, J. Maier, P.A. van Aken, submitted to APL Materials (2014)
This research has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant no. 312483 (ESTEEM2).