Active oxygen materials that can rapidly transport and 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 reduction reaction at the cathode. The steps in the ORR and related oxygen transport are complex enough that it still remains an open challenge how to design improved cathode catalytic activities using either intuition or computational materials design tools.

In this presentation we summarize our recent results on using the Oxygen 2p-band (or just “p-band”) as a descriptor for many important properties related to SOFC cathode performance for both perovskite and 214 Ruddlesden-Popper oxides. We demonstrate that the p-band, which is an easy to calculate bulk electronic structure descriptor, correlates with properties that include oxygen vacancy and interstitial defect energies [1, 2], oxygen diffusion coefficients [3, 4], surface exchange coefficients [1, 3], and cathode area specific resistance [1] (e.g., see Figure 1). This descriptor is therefore a powerful tool for active oxide materials design, and we describe its use to search for promising new SOFC cathode materials.

The p-band descriptor highlights some of the intriguing differences in trends of defect chemistry and surface exchange behavior between the perovskite and 214 Ruddlesden-Popper phases. Specifically, the dominance of vacancy and interstitial defects, respectfully, in the oxygen exchange and transport in these two classes of materials leads to opposite trends in their behavior with p-band and generally in the behavior with oxygen bond strength and state of oxidation. We show the results of detailed calculations of the defect behavior in 214 Ruddlesden-Popper La_2-xSr_xMO_4+d (M = Co, Ni, Cu) compounds and demonstrate how the crossover in dominant defects with doping can be understood in terms of simple trends in oxidation chemistry. We further demonstrate that the previously identified but somewhat elusive peroxide defects are stable in Ruddlesden-Popper phases over some doping domains and can play a role in the oxygen transport in these materials.

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, Department of Energy (DOE), National Energy Technology Laboratory (NETL), Solid State Energy Conversion Alliance (SECA) Core Technology Program (Funding Opportunity Number DEFE0009435), and the National Science Foundation Software Infrastructure for Sustained Innovation (SI²) (award no. 1148011). Computing in this work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by U.S. National Science Foundation with grant number ACI-1053575, and the National Energy Research Scientific Computing Center (NERSC) allocation of the CNMS at ORNL (CNMS2013-292).

References

[1] Y. L. Lee, J. Kleis, J. Rossmeisl, Y. Shao-Horn, and D. Morgan, Prediction of solid oxide fuel cell cathode activity with first-principles descriptors, Energy & Environmental Science 4, p. 3966-3970 (2011).

[2] W. Xie, Y.-L. Lee, Y. Shao-Horn, and D. Morgan, Oxygen Point Defect Chemistry in Ruddlesden-Popper Oxides (La1-xSrx)(2)MO4 +/-delta (M = Co, Ni, Cu), Journal of Physical Chemistry Letters 7, p. 1939-1944 (2016).

[3] Y. L. Lee, D. Lee, X. R. Wang, H. N. Lee, D. Morgan, and Y. Shao-Horn, Kinetics of Oxygen Surface Exchange on Epitaxial Ruddlesden-Popper Phases and Correlations to First-Principles Descriptors, Journal of Physical Chemistry Letters 7, p. 244-249 (2016).

[4] T. T. Mayeshiba and D. D. Morgan, Factors controlling oxygen migration barriers in perovskites, Solid State Ionics 296, p. 71-77 (2016).