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Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface

Abstract

Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO3) surfaces are detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H2O and CO2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, ) by the positively charged oxygen vacancies () enriched at the surface. Here we show that reducing the surface concentration improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O2 molecules. We take La0.8Sr0.2CoO3 (LSC) as a model perovskite oxide, and modify its surface with additive cations that are more and less reducible than Co on the B-site of LSC. By using ambient-pressure X-ray absorption and photoelectron spectroscopy, we proved that the dominant role of the less reducible cations is to suppress the enrichment and phase separation of Sr while reducing the concentration of and making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30 times faster oxygen exchange kinetics after 54 h in air at 530 °C achieved by Hf addition onto LSC. Finally, the results revealed a ‘volcano’ relation between the oxygen exchange kinetics and the oxygen vacancy formation enthalpy of the binary oxides of the additive cations. This volcano relation highlights the existence of an optimum surface oxygen vacancy concentration that balances the gain in oxygen exchange kinetics and the chemical stability loss.

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Figure 1: Surface oxygen exchange kinetics and stability on LSC dense thin film cathodes.
Figure 2: Surface chemical stability on LSC dense thin films.
Figure 3: Oxidation state of Co based on Co L2,3-edge XAS on LSC dense thin films.
Figure 4: Oxidation state on LSC based on the valence band and O K-edge.
Figure 5: Coordination environment of Ti on LSC-Ti15.
Figure 6: Dependence of oxygen surface exchange kinetics on the reducibility of the LSC surface.

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Acknowledgements

The authors are grateful for funding support from the NSF CAREER Award of the National Science Foundation, Division of Materials Research, Ceramics Program, Grant No. 1055583, and from the National Aeronautics and Space Administration (NASA) in support of the Mars Oxygen ISRU Experiment (MOXIE), an instrument on the Mars 2020 rover mission. We thank M. Youssef for useful discussions on the defects in LSC and Q. Liu for experiment assistance at Advanced Light Source Beamline 9.3.2. The authors also acknowledge the use of the Center for Materials Science and Engineering, an MRSEC Shared Experimental Facility of the NSF at MIT, supported by the NSF under award number DMR-1419807. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231.

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N.T. and Q.L. prepared the samples. N.T. performed electrochemical measurements. Q.L., N.T., B.Y. and E.J.C. performed XPS and XAS measurements. All authors analysed and discussed the results and wrote the paper. B.Y. designed and supervised the research.

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Correspondence to Nikolai Tsvetkov or Bilge Yildiz.

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Tsvetkov, N., Lu, Q., Sun, L. et al. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. Nature Mater 15, 1010–1016 (2016). https://doi.org/10.1038/nmat4659

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