(Invited) Molecular Understanding of Oxygen Exchange in Solid Oxide Fuel Cell Cathodes

Materials with the ability to rapidly exchange oxygen with the environment play a critical role in many existing and developing technologies, e.g., solid oxide fuel cells (SOFCs), gas separation membranes, oxygen sensors, chemical looping devices, and memristors.  In particular, reducing the working temperature of SOFCs, which is critical to their increased commercialization, is significantly inhibited by the slow oxygen exchange kinetics at the cathode.  The oxygen exchange is an essential step in the overall oxygen reduction reaction (ORR), where O₂ (gas) combines with 4e- to incorporate as 2O^2- into the electrolyte, facilitated by the cathode.  Oxygen exchange rates are typically quantified by the oxygen exchange coefficient, K*, which represents the response of the rate of oxygen exchange to a driving force. 

In our work we are developing techniques to predict K* and provide a molecular scale understanding of processes involved in oxygen surface exchange, with the long-term goal of designing more active oxygen exchange materials.  Our work has focused on perovskite materials.  A few years ago we identified the distance of the bulk oxygen 2p-band center to the Fermi level as an easily calculated descriptor for oxygen exchange in electronic conducing perovskites.[1]  These results suggested that overall oxyphilicity plays a critical role in determining K* and provides a tool for high-throughput ab initio screening of promising materials.  We will report on recent predictions of active oxygen exchange systems using these screening approaches. 

In more recent work we have initiated an effort to model oxygen exchange at the molecular level for the (La,Sr)CoO_3-d (LSC) system.  Using ab inito energetics and the elementary kinetics formalism developed by Adler, et al.[2]  We have modeled the possible reaction steps on the LSC (001) surface.  This modeling has yielded valuable insights in the rate limiting steps of oxygen exchange through a perovskite surface, the role of surface termination and Sr segregation on K*,[3, 4] and the absolute limits of surface exchange rates.

Acknowledgement

We gratefully acknowledge financial support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0001284, and computing support from NSF National Center for Supercomputing Applications (NCSA - DMR060007).

References

[1] Y. L. Lee, D. Morgan, J. Kleis, and J. Rossmeisl, Ab initio based modeling of perovskites for solid oxide fuel cell cathodes, Abstracts of Papers of the American Chemical Society 241 (2011).

[2] S. B. Adler, X. Y. Chen, and J. R. Wilson, Mechanisms and rate laws for oxygen exchange on mixed-conducting oxide surfaces, Journal of Catalysis 245, p. 91-109 (2007).

[3] Z. Feng, Y. Yacob, M. J. Gadre, Y.-L. Lee, W. T. Hong, H. Zhou, M. D. Biegalski, H. M. Christen, S. B. Adler, D. Morgan, and Y. Shao-Horn, Anomalous Interface and Surface Strontium Segregation in (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ Heterostructured Thin Films, Journal of Physical Chemistry Letters 5, p. 1027−1034 (2014).

[4] E. Crumlin, E. Mutoro, Z. Liu, M. E. Grass, M. Biegalski, Y.-L. Lee, D. Morgan, H. Christen, H. Bluhm, and Y. Shao-Horn, Surface Strontium Enrichment on Highly Active Perovskites for Oxygen Electrocatalysis in Solid Oxide Fuel Cells, Energy & Environmental Science 5, p. 6081-6088 (2012).