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Research Papers

Oxygen Partial Pressure Dependence of In Situ X-Ray Absorption Spectroscopy at Co and Fe K-Edge for (La0.6 Sr0.4 )(Co0.2 Fe0.8 )O3–δ

[+] Author and Article Information
Takanori Itoh1

 AGC Seimi Chemical Co., Ltd., 3-2-10 Chigasaki, Chigasaki City, Kanagawa 253-8585, Japan

Saori Shirasaki

 AGC Seimi Chemical Co., Ltd., 3-2-10 Chigasaki, Chigasaki City, Kanagawa 253-8585, Japan

Hironori Ofuchi, Sayaka Hirayama, Tetsuo Honma

 Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan

Masashi Mori

 Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka,Yokosuka, Kanagawa 240-0196, Japan

Masanobu Nakayama

 Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya City, Aichi 466-8555, Japan

1

Corresponding author.

J. Fuel Cell Sci. Technol 9(3), 031004 (Apr 20, 2012) (8 pages) doi:10.1115/1.4005608 History: Received August 31, 2011; Revised September 30, 2011; Published April 19, 2012; Online April 20, 2012

(La0.6 Sr0.4 )(Co0.2 Fe0.8 )O3–δ (LSCF) has been promised as a cathode material of solid oxide fuel cells at intermediate temperatures. Despite the many previous studies of LSCF that have been reported, the role of Co and Fe atoms in the oxygen ion conduction is still unclear. In this work, we aimed at presenting each valence, oxygen chemical diffusion coefficient (Dchem ) and activation energy (Ea ) related to Co and Fe in LSCF by in situ X-ray absorption spectroscopy (XAS) at high temperatures and during reduction. For quantitative analysis of X-ray absorption near edge structure (XANES) spectroscopy, these results indicated that the Co valence decreased more easily than the Fe valence. On the other hand, from relaxation plots of the Co and Fe valence during reduction, the values of Dchem and Ea related to Co and Fe were nearly equal. Considering equations showing the oxygen ion conductivity, these results would indicate that oxygen ion conductivity was contributed by Co with more oxygen vacancies rather than Fe. According to these results, a structural model with and without oxygen vacancies and the oxygen ion conduction mechanism of LSCF was speculated, that is, we found that oxygen ion conductivity was more closely related to Co than Fe in LSCF by direct observations of in situ XAS.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 4

Examples of EXAFS oscillations at the (a) Co and (b) Fe K-edge at 300 and 1000 K during reduction. Solid line: 1000 K during reduction, broken line: 300 K.

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Figure 7

Model of LSCF structure (a) without and (b) with the oxygen vacancy and the image of oxygen ion diffusion

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Figure 6

Arrhenius plots of Dchem related to Co and Fe valences during reduction with literature data (a) and (b) Katsuki , (La0.6 Sr0.4 )(Co0.8 Fe0.2 )O3−δ data by TG [12]; (c) Elshof , (La0.6 Sr0.4 )(Co0.6 Fe0.4 )O3−δ data by coulometric titration [15]; and (d) Elshof , (La0.6 Sr0.4 )(Co0.6 Fe0.4 )O3−δ data by conductivity relaxation [15]

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Figure 5

Plots of γ2 related to Co and Fe valences as a function of time during reduction at 900 and 1000 K

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Figure 3

Plots of (a) ΔE0 and (b) valence of Co and Fe as a function of time during reduction at 900 and 1000 K

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Figure 2

Examples of enlarged views around E0 of the (a) and (c) Co and (b) and (d) Fe K-edge during reduction with changing P(O2 ) from 1 to 10−4 atm at (a) and (b) 900 K and (c) and (d) 1000 K. The inset shows the XANES spectra at each edge.

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Figure 1

Rietveld refinement profile of the synchrotron X-ray diffraction data of (La0.6 Sr0.4 )(Co0.8 Fe0.2 )O3−δ corrected at 1000 K. The observed data, calculated pattern, and difference profile are shown as red plus symbols, the green line, and the continuous blue line below, respectively. The tick marks show the position of Bragg reflections predicted by the structural model. The wavelength of the incident synchrotron X-ray is 0.4989 Å.

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