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,Sr' _La) by the positively charged oxygen vacancies (Vo) enriched at the surface. Here we show that reducing the surface Vo 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 La_0.8Sr_0.2CoO₃ (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 chemical stability loss.

Nature Materials (2016) doi:10.1038/nmat4659
Received 03 November 2015 Accepted 03 May 2016 Published online 13 June 2016

http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4659.html

钙钛矿型氧化物（ABO₃）表面分离和相分离对于能量转化系统（如：固态氧化物燃料电池/电解池和热化学H2O和CO2的分解催化）的性能具有不利影响。钙钛矿型氧化物不稳定的一个关键因素是带有负电A位点掺杂物被带有正电的氧空位吸引，从而引起表面的分离。尽管与传统上理解的表面氧空位有利于反应进行相悖，美国Yildiz课题组通过减少表面的氧空位浓度提高了氧的表面交换和稳定性。“火山”关系图表明优化的表面氧空位浓度协调了氧交换运动学和化学稳定性的损失。（Nature Materials DOI: 10.1038/NMAT4659）(新材料在线）