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Title: 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 (ABO 3 ) surfaces are detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H 2 O and CO 2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, Sr La ') by the positively charged oxygen vacancies (V $$••\atop{o}$$) enriched at the surface. Here we show that reducing the surface V $$••\atop{o}$$ concentration improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O 2 molecules. We take La 0.8 Sr 0.2 CoO 3 (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 V $$••\atop{o}$$ 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.

Authors:
 [1];  [2];  [1];  [3];  [4]
  1. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Lab. for Electrochemical Interfaces and Dept. of Nuclear Science and Engineering
  2. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Lab. for Electrochemical Interfaces and Dept. of Materials Science and Engineering
  3. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Advanced Light Source
  4. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Lab. for Electrochemical Interface, Dept. of Nuclear Science and Engineering, and Dept. of Materials Science and Engineering
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF)
OSTI Identifier:
1377444
Grant/Contract Number:  
AC02-05CH11231; DMR-1419807
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nature Materials
Additional Journal Information:
Journal Volume: 15; Journal Issue: 9; Journal ID: ISSN 1476-1122
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; fuel cells; structural properties

Citation Formats

Tsvetkov, Nikolai, Lu, Qiyang, Sun, Lixin, Crumlin, Ethan J., and Yildiz, Bilge. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. United States: N. p., 2016. Web. doi:10.1038/nmat4659.
Tsvetkov, Nikolai, Lu, Qiyang, Sun, Lixin, Crumlin, Ethan J., & Yildiz, Bilge. Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface. United States. doi:10.1038/nmat4659.
Tsvetkov, Nikolai, Lu, Qiyang, Sun, Lixin, Crumlin, Ethan J., and Yildiz, Bilge. Mon . "Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface". United States. doi:10.1038/nmat4659. https://www.osti.gov/servlets/purl/1377444.
@article{osti_1377444,
title = {Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface},
author = {Tsvetkov, Nikolai and Lu, Qiyang and Sun, Lixin and Crumlin, Ethan J. and Yildiz, Bilge},
abstractNote = {Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO 3 ) surfaces are detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H 2 O and CO 2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, Sr La ') by the positively charged oxygen vacancies (V $••\atop{o}$) enriched at the surface. Here we show that reducing the surface V $••\atop{o}$ concentration improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O 2 molecules. We take La 0.8 Sr 0.2 CoO 3 (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 V $••\atop{o}$ 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.},
doi = {10.1038/nmat4659},
journal = {Nature Materials},
number = 9,
volume = 15,
place = {United States},
year = {Mon Jun 13 00:00:00 EDT 2016},
month = {Mon Jun 13 00:00:00 EDT 2016}
}

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