(Invited) Theoretical Approach for Understanding Oxygen Reduction at the Cathode Surface of Solid Oxide Fuel Cell

Doped lanthanum cobaltite and its derivatives are used as cathode materials in solid oxide fuel cell (SOFC), thus the cathodic reaction mechanism has been extensively investigated. As results, it is generally accepted that the dissociative adsorption of oxygen at the cathode surface is one of rate-determining steps. Therefore, it is important to understand factors governing the surface kinetics. Preceding experimental and theoretical studies have pointed out the correlation between electronic structure of doped lanthanum cobaltites and oxygen dissociation properties on surface, formation energy of oxygen vacancy, and oxygen diffusivity [1-6]. In our preceding study [7], we focused on the change of spin states in LaCoO₃ to have a basis for analyzing catalytic properties of the material at the operation temperature. Further analyses in collaboration with experimental observation have clarified the electronic properties of LaCoO₃ at the operation temperature on the basis of density functional theory calculations taking into account the statistical thermodynamics [8]. 

 As an extension of our preceding studies, we will discuss how the electronic structure affects the surface kinetics in this study. We adopted the La_0.5Sr_0.5O_3-δ as cathode material and prepared the (001) surface model with the LaO termination. The model consists of five layers with the total 48 atoms. For the electronic structure calculations, we used Vienna Ab initio Simulation Package (VASP) based on the density functional theory. U parameter is adopted as correction for electronic correlation calculation. We investigated the oxygen adsorption energy, vacancy formation energy and oxygen dissociation energy by changing the spin states of Co in La_0.5Sr_0.5O_3-δ. We observed dependency of the calculated energies and the spin states. Based on the calculated properties, we will discuss the kinetic process of oxygen reduction at La_0.5Sr_0.5O_3-δ surface together with the perspectives of future analysis. 

References

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[3] W. T. Hong, M. Gadre, Y. Lee, M. D. Biegalski, H. M. Christen, D. Morgan, and Y. Shao-Horn, J. Phys. Chem. Lett. 4, 2493 (2013).

[4] S. Mukhopadhyay, M. W. Finnis, and N. M. Harrison, Phys. Rev. B 87, 125132 (2013).

[5] A. Kushima, S. Yip, and B. Yildiz, Phys. Rev. B 82, 115435 (2010).

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[7] T. Ishimoto, Y. Ito, H. Kohno, M. Koyama, ECS Trans., 57(1) (2013) 2655-2660.

[8] T. Ishimoto, Y. Ito, T. Tada, R. Oike, T. Nakamura, K. Amezawa, M. Koyama, submitted

Acknowledgement

The activities of INAMORI Frontier Research Center is supported by KYOCERA Corporation.