Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

• Published in: Journal of the Electrochemical Society Vol. 158; no. 2; p. B99
• Main Authors: Piper, L. F. J., Preston, A. R. H., Cho, S.-W., DeMasi, A., Chen, B., Laverock, J., Smith, K. E., Miara, L. J., Davis, J. N., Basu, S. N., Pal, U., Gopalan, S., Saraf, Laxmikant, Kaspar, Tiffany, Matsuura, A. Y., Glans, P.-A., Guo, J.-H.
• Format: Journal Article
• Language: English
• Published: United States 01.01.2011
• Subjects: 30 DIRECT ENERGY CONVERSION; AIR; CATHODES; CHEMICAL COMPOSITION; DIFFUSION; ELECTRONIC STRUCTURE; Environmental Molecular Sciences Laboratory; FABRICATION; LANTHANUM; MODIFICATIONS; OXYGEN; PHOTOEMISSION; QUENCHING; REACTION KINETICS; SCATTERING; SOLID OXIDE FUEL CELLS; SPECTROSCOPY; X-RAY PHOTOELECTRON SPECTROSCOPY
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.3519075

The modification of the Mn charge-state, chemical composition and electronic structure of La0.8Sr0.2MnO3 (LSMO) cathodes for solid oxide fuel cell (SOFC) applications remains an area of interest, due to the poorly understood enhanced catalytic activity (often referred to as the "burn-in" phenomenon) observed after many hours of operation. Using a combination of core-level X-ray photoemission spectroscopy (XPS), X-ray emission/absorption spectroscopy (XES/XAS), resonant inelastic X-ray scattering (RIXS) and resonant photoemission spectroscopy (RPES), we have monitored the evolution of these properties in LSMO at various stages of fabrication and operation. By rapidly quenching and sealing in vacuum, we were able to directly compare the pristine (as-fabricated) LSMO with both "heat-treated" (800°C in air, and no bias) and "burnt-in" (800°C in air, -1 V bias) LSMO cathodes i.e. before and after the activation observed in our electrochemical impendence spectroscopy measurements. Comparison between the O K-edge XAS/XES and Mn L3,2-edge XAS of pristine and “burnt-in” LSMO cathodes revealed a severe change in the oxygen environment along with a reduced Mn2+ presence near the surface following activation. The change in the oxygen environment is attributed to SrxMnyOz formation, along with possible passive SrO and Mn3O4 species. We present evidence from our “heat-treated” samples that SrxMnyOz regions form at elevated temperatures in air before the application of a cathodic bias. Our core-level XPS, Mn L3,2-edge RIXS and Mn L3 RPES studies of “heat-treated” and pristine LSMO determined that SOFC environments result in La-deficiency (severest near the surface) and stronger Mn4+ contribution, leading to the increased insulating character of the cathode prior to activation. The passive Mn2+ species near the surface and increased hole-doping (>0.6) of the LSMO upon exposure to the operating environment are considered responsible for the initially poor performance of the SOFC. Meanwhile, the improved oxygen reduction following the application of a cathodic bias is considered to be due to enhanced bulk oxygen-ion diffusion resulting from the migration of Mn2+ ions towards the LSMO/electrolyte interface and the SrxMnyOz regions facilitating enhanced bulk oxygen reduction reaction kinetics.