In situ Synchrotron X-ray Studies of Dense Thin-Film Strontium-Doped Lanthanum Manganite Solid Oxide Fuel Cell Cathodes
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In situ Synchrotron X-ray Studies of Dense Thin-Film Strontium-Doped Lanthanum Manganite Solid Oxide Fuel Cell Cathodes Kee-Chul Chang1, Brian Ingram2, Balasubramaniam Kavaipatti3, Bilge Yildiz4, Daniel Hennessy1, Paul Salvador3, Nadia Leyarovska5 and Hoydoo You1 1 Materials Science Division, Argonne National Laboratory, 9700 S Cass Ave., Argonne, IL 60439, U.S.A 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S Cass Ave., Argonne, IL 60439, U.S.A 3 Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, U.S.A 4 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, U.S.A 5 X-ray Science Divison, Advanced Photon Source, Argonne National Laboratory, 9700 S Cass Ave., Argonne, IL 60439, U.S.A ABSTRACT Using a model cathode-electrolyte system composed of epitaxial thin-films of La1(LSM) on single crystal yttria-stabilized zirconia (YSZ), we investigated changes of the cation concentration profile in the LSM during heating and under applied potential using grazing incidence x-rays. Pulsed laser deposition (PLD) was used to grow epitaxial LSM(011) on YSZ(111). At room temperature, we find that Sr segregates to form Sr enriched nanoparticles. When the sample heated to 700°C, Sr is slowly reincorporated into the film. We also find different amounts of Sr segregation as the X-ray beam is moved across the sample. The variation in the amount of Sr segregation is greater on the sample that has been subject to 72 hours of applied potential, suggesting that the electrochemistry plays a role in the Sr segregation. xSrxMnO3-
INTRODUCTION The solid oxide fuel cell (SOFC) has advantages of high efficiency and fuel-flexibility but is not yet economically competitive enough to gain widespread acceptance. Between the cathode, anode and electrolyte of SOFC, it is well known that the cathode resistance dominates the overall cell resistance and hence has the most room for improvement. Therefore, efforts to improve SOFC performance have been focused on the cathode but a more fundamental understanding of the mechanism and kinetics of the oxygen reduction reaction at the cathode is still lacking. We investigate a model system of pulsed laser deposited LSM on a single crystal YSZ electrolyte. Although LSM is widely used as cathodes for industrial SOFC in a porous form, such electrodes has complex electrochemical reaction pathways with no easy means for site specific X-ray characterization. Our model system was chosen to simplify reaction pathways as well as making the electrode amendable to grazing incidence X-ray characterization techniques. This paper focuses on the question of cation motion inside the cathode under long term applied potential. For perovskite LSM, defect thermodynamic models [1,2], based on precision balance weight measurements on a powder sample subject to certain temperature and oxygen
partial pressure [3,4], predict that the oxygen is supe
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