Highly active doped ternary oxides, including perovskites, are common functional materials in energy conversion, catalysis, and information processing applications . At elevated temperatures related to synthesis or operation, however, surface segregation of dopant cations is a common problem of state-of-the-art materials, such as the Sr-doped La-based transition metal perovskites (La,Sr)CoO3, (La,Sr)FeO3, and (La,Sr)MnO3, used for example as solid oxide fuel cell cathodes. In these oxides, Sr segregation under operating conditions leads to degradation via the formation of an inactive surface oxide [2,3], blocking the oxygen reduction reaction . This dopant segregation occurs due to two driving forces: strain and electrostatics . While the size mismatch between Sr and La cations causes strain that can be released via segregation, the electrostatic driving force originates in the attraction of negatively charged dopants to the surface layer enriched in positively charged oxygen vacancies.
Here we relate the segregation in perovskite oxide cathodes to their surface properties, comparing the effects on surface structure and composition of as-grown thin films and metal-doped surfaces. We address the electrostatic driving force of segregation by depositing metals of different reducibility using electron-beam evaporation, thus modifying the local bond strength of oxygen in the surface region. Similar doping has proven successful in a previous study on solution-based metal deposition on (La,Sr)CoO3 . Here we demonstrate that this effect is not limited to one material and deposition method, but can be generalized to (La,Sr)MnO3, an intrinsically different type of cathode material, modified using metal evaporation. Using in-situ ambient pressure photoelectron spectroscopy and X-ray absorption spectroscopy, we observe an improved stability under the influence of metals such as Hf and Ti. In combination with scanning probe microscopy and electrochemical performance measurements, we analyze the details of surface doping on structure and stability and explore its limitations. Identifying the nature of the surface phase and the changes due to metal deposition will provide a foundation for targeted approaches to inhibit segregation, facilitating the pathway towards higher long-term stability of doped perovskite oxide surfaces.
Support by the Fonds zur Förderung der Wissenschaftlichen Forschung (FWF, project J4099) and the Air Force Office of Scientific Research (AFOSR) is gratefully acknowledged.
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