Formation and migration of oxygen vacancies in (La,Ba,Sr)(Co,Fe)O3-δ perovskites

E. Kotomin, Yu. A. Mastrikov*, M. M. Kuklja**, R. Merkle

* Institute for Solid State Physics, University of Latvia, Riga ** Materials Science and Engineering, University of Maryland, USA

Mixed conducting (La,Ba,Sr)(Co,Fe)O3-δ perovskites are used as cathode materials in solid oxide fuel cells. Ionic conductivity is not only important for the transport of O through the electrode material, but also decisive for fast kinetics of the oxygen incorporation surface reaction. Thus the reasons for the significantly lower oxygen migration barriers in (Ba,Sr)(Co,Fe)O3-δ compared to (La,Sr)(Co,Fe)O3-δ were investigated by DFT calculations. For (Ba,Sr)(Co,Fe)O3-δ, not only geometrical constraints affect the barrier, but also the O vacancy formation energy (Fig. 1(a)) - i.e. the energy of a redox reaction. This can be understood when the transition state is analyzed in detail: at the saddle point the migrating oxygen O* transfers electron density to the neighboring transition metal cation (decreasing the size of O* and thus the migration barrier), and this electron transfer is easier for perovskites in which also the vacancy formation energy is lower (Fig. 1b).

Fig, 1(a) Oxygen migration barriers for (Ba,Sr)(Co,Fe)O2.875 with different overall cation compositions and local cation configurations; (b) migration barrier and electron transfer as function of O vacancy formation energy.  From Kuklja et al (2013) with permission

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E. A. Kotomin, R. Merkle, Y. A. Mastrikov, M. M. Kuklja, and J. Maier: Energy Conversion—Solid Oxide Fuel Cells: First-Principles Modeling of Elementary Processes In Computational Approaches to Energy Materials, A. Walsh, A. A. Sokol, and C. R. A. Catlow, eds., John Wiley & Sons, May 2013.

M. M. Kuklja, E. A. Kotomin, R. Merkle, Yu. A. Mastrikov, and J. Maier: Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells Physical Chemistry Chemical Physics 15(15), 5443–5471 (2013). DOI: 10.1039/C3CP44363A

Y. A. Mastrikov, R. Merkle, E. A. Kotomin, M. M. Kuklja, and J. Maier: Formation and migration of oxygen vacancies in La1-xSrxCo1-yFeyO3-δ perovskites: insight from ab initio calculations and comparison with Ba1-xSrxCo1-yFeyO3-δ Physical Chemistry Chemical Physics 15(3), 911–918 (2013). DOI: 10.1039/C2CP43557H

R. Merkle, Yu. A. Mastrikov, E. A. Kotomin, M. M. Kuklja, and J. Maier: First Principles Calculations of Oxygen Vacancy Formation and Migration in Ba1-xSrxCo1-yFeyO3-δ Perovskites Journal of The Electrochemical Society 159(2), B219–B226 (2012). DOI: 10.1149/2.077202jes

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