Doped lanthanum cobaltite and its derivatives are used as cathode materials in solid oxide fuel cell (SOFC). For a high catalytic activity, the improvement of properties governing the cathode performance is important. It is generally regarded that one of the important factors is the electronic structure of cathode material. Recently, the influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was reported [1]. In addition, the heterostructured oxide interface ((La,Sr)₂CoO_4±δ/La_1-xSr_xCoO_3-δ (LSC)) and thin-film heterostructure interface (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ/gadolinium-doped CeO₂ (GDC)) enhanced the surface oxygen exchange kinetics and ORR activity [2,3]. These results indicate the possibility that the structural change near interface changes the electronic structure of cathode materials. Detailed understandings of electronic structure of doped lanthanum cobaltite near the interface is important to design the high catalytic cathode materials. In this study, we analyzed the stability of oxygen vacancy in LSC of LSC/GDC interface by using the density functional theory (DFT) calculation.

Computational Details

We used LaO-terminated (001) surface of cubic LaCoO₃ crystal structure. Half of La atoms were substituted by Sr atoms in LaO layers in LSC slab model (La₆Sr₆Co₈O₂₈). As a surface of CeO₂, we used (111) surface model (Ce₁₄O₂₈) from the surface stability. GDC surface model was prepared based on the previous DFT calculation [4]. The interface of LSC and GDC was modeled from CoO₂-terminated LSC(001) and GDC(110) surfaces. The initial spin configuration of Co³⁺ ion was set as intermediate spin state. GGA-PAW potentials were applied with cutoff energy of 600 eV and 1×1×1 k-points. We employed GGA+U to improve the description of Co 3d and Ce 4f electrons. All DFT calculations were performed by using VASP.

Results and Discussion

We first analyzed the charge distribution near LSC/GDC interface. In case of LSC/CeO₂ interface, the electron transferred from CeO₂ to LSC. We clearly observed the larger electron transfer from GDC to LSC. These results indicate that the LSC region near CeO₂ or GDC interface shows the negatively charged structure. We analyzed the oxygen vacancy formation energy in LSC. The oxygen vacancy formation energy from surface at LSC/GDC interface was more stable than that of LSC model. Although the oxygen vacancy formation energy from 2^ndlayer in LSC model was stable, unstable oxygen vacancy formation energy was obtained in the LSC/GDC interface model. One of the reasons of oxygen vacancy formation energy difference between LSC and LSC/GDC interface is an effect of electron transfer from GDC to LSC. The atomic position change at interface is also influenced the oxygen vacancy formation energy. To see the relation between oxygen vacancy formation energy and distance from interface, we prepared larger LSC/GDC interface models. Details are discussed in the presentation.

Acknowledgement

This work was supported by CREST, Japan Science and Technology Agency. All calculations were performed on the HA8000 computer systems in Research Institute for Information Technology, Kyushu University. The activities of Advanced Automotive Research Collaborative Laboratory in Hiroshima University are supported by Mazda Corporation. The activities of INAMORI Frontier Research Center in Kyushu University are supported by Kyocera Corporation.

References

[1] N. Tsvetkov, Q. Li, and B. Yildiz, ACS Nano, 9, 1613 (2015).

[2] D. Lee, Y. Lee, W. T. Hong, M. D. Biegalski, D. Morgan, and Y. Shao-Horn, J. Mater. Chem. A, 27, 7910 (2015).

[3] E. M. Hopper, E. Perret, B. J. Ingram, H. You, K. Chang, P. M. Baldo, P. H. Fuoss, and J. A. Eastman, J. Phys. Chem. C, 119, 19915 (2015).

[4] X. Aparicio-Anglès, A. Roldan, and N. H. de Leeuw, Chem. Mater., 27, 7910 (2015).