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

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 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, Sr'(La)) by the positively charged oxygen vacancies (V-o(center dot center dot)) enriched at the surface. Here we show that reducing the surface V-o(center dot center dot) 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 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 V-o(center dot center dot) 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 degrees 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.