Overcoming tradeoff between catalytic activity and stability of perovskite oxides
Perovskite oxides (ABO3) have become a bustling area of research in recent years, with potential applications in areas ranging from fuel cell electrodes, to nonvolatile memory chips for computers, to solar thermochemical fuel production through the splitting of water and carbon dioxide. But the relative instability of the material’s surface over time has been one of the major limitations to use of highly reactive perovskite oxides. In this talk Dr. Tsvetkov will present his recent investigations, conducted in MIT, on developing of a practical and physically-based way of treating the surface of perovskite oxides, to make them more durable and improve their performance.
One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, ) by the positively charged oxygen vacancies () enriched at the surface causing the surface reactions to slow down by more than an order of magnitude. In our work we show that reducing the surface 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 took 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 and making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30x acceleration of the oxygen exchange kinetics, after 54 hours in air at 550 oC 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.
Thus, surface treatment could solve one of the major challenges that has hindered widespread deployment of fuel cell technology that, when operated reversibly, can present a promising alternative to batteries for renewable-energy storage. These new findings are being reported in June 2016 in the journal Nature Materials.