Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface - pp1010 - 1016
Nikolai Tsvetkov, Qiyang Lu, Lixin Sun, Ethan J. Crumlin & Bilge Yildiz
doi:10.1038/nmat4659
The chemical instability of perovskite oxide surfaces limits their energy conversion performance. Significantly enhanced electrochemical stability in a model perovskite electrocatalyst has now been achieved using less reducible cations.
Nikolai Tsvetkov1,2†, Qiyang Lu1,3†, Lixin Sun1,2, Ethan J. Crumlin4 and Bilge Yildiz1,2,3*
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 H2Oand
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, Sr0La) by the positively charged oxygen vacancies (V
O ) enriched at the surface. Here we
show that reducing the surface V 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 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 VO 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
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