Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface
Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface
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DOI：
10.1038/nmat4659
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发表时间：
2016-09-01
期刊：
影响因子：
41.2
通讯作者：
Yildiz, Bilge
中科院分区：
文献类型：
--
作者：
Tsvetkov, Nikolai;Lu, Qiyang;Yildiz, Bilge
钙钛矿氧化物（ABO（3））表面上的异价掺杂剂的偏析和相分离对能量转换系统（例如固体氧化物燃料/电解电池和用于热化学H2O和CO2裂解的催化剂）的性能是有害的。钙钛矿氧化物表面的不稳定性背后的一个关键原因是在表面处富集的带正电的氧空位（V-o（中心点））对带负电的A位掺杂剂（例如Sr '（La））的静电吸引。在这里，我们表明，减少表面的V-O（中心点中心点）的浓度，提高了氧表面交换动力学和稳定性显着，虽然相反的公认的理解，表面氧空位促进与O-2分子的反应。我们以La0.8Sr0.2CoO3（LSC）为模型钙钛矿氧化物，在LSC的B位上添加比Co还原性更强和更弱的阳离子对其表面进行改性。利用常压X射线吸收谱和光电子能谱，我们证明了难还原阳离子的主要作用是抑制Sr的富集和相分离，同时降低V-O（中心点）的浓度，使LSC在其表面更易氧化。因此，我们发现，这些还原性较低的阳离子显著提高了稳定性，通过将Hf添加到LSC上，在530 ℃下在空气中54小时后的氧交换动力学快达30倍。最后，结果揭示了添加阳离子的二元氧化物的氧交换动力学和氧空位形成焓之间的“火山”关系。这种火山关系突出了最佳表面氧空位浓度的存在，该浓度平衡氧交换动力学的增益和化学稳定性损失。
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.