Mechanism of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 cathode degradation
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Eric D. Wachsmana) University of Maryland Energy Research Center, University of Maryland, College Park, Maryland 20742 (Received 13 January 2012; accepted 13 June 2012)
Elemental enrichment behavior on the surface of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was investigated in order to understand potential degradation mechanism of solid oxide fuel cell cathodes. Surface morphological changes were examined using scanning electron microscopy after heat treatment in the temperature range of 600–900 °C. Submicron-sized precipitates were formed on grain surfaces after heat treatment. Their shapes appeared to be aligned along the surface orientations of the underlying grains. Auger electron spectroscopy and transmission electron microscopy characterization revealed that the precipitate was strontium (Sr)-oxygen (O) based. The formation of Sr–O precipitates was found to increase with increasing temperature and oxygen partial pressure. A defect chemistry model is presented based on the observed phenomena. I. INTRODUCTION
Lanthanum strontium cobalt ferrite, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), has been widely used as a solid oxide fuel cell (SOFC) cathode. Due to partial substitution of strontium (Sr) for lanthanum (La) and reduction of transition metals, it has a substantial oxygen vacancy concentration at high temperatures, and therefore exhibits high ionic and electronic conductivity resulting in low SOFC cathode polarization. Long-term stability is also an important requirement for the commercialization of SOFCs. However, Simner et al.1 measured power density of anode-supported cells utilizing LSCF cathodes and observed decreased cell performance with time at 750 °C. Similarly, Tietz et al.2 reported degradation of LSCF cathode-based cells during long-term testing. Several studies report that Sr in LSCF tends to segregate to electrode surfaces or interfaces.1,3–5 As a result, the transition metal concentration was found to substantially decrease in the outermost surface layers. It is generally believed that B-site transition metals play a critical role for catalytic activity of ABO3 perovskite materials,6–9 and heterogeneous interaction is primarily determined by surface composition and structure.10 Consequently, the formation of Sr-enriched layers on LSCF surfaces is a potential degradation mechanism for LSCF cathode-based cells. Thus, the development of LSCF SOFC cathodes requires an understanding of Sr surface enrichment behavior. In the work presented herein, surface chemical compositional changes of LSCF at high temperatures were investigated Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.222
A. Sample preparation
A dense LSCF sample was prepared for analysis by uniaxially pressing La0.6Sr0.4Co0.2Fe0.8O3-d powder (Praxair Specialty Ceramics, Woodinville, WA) and sintering the pellet at 1400 °C for 4 h in air. The sintered body was cut and then polished with a series of SiC abrasive papers and diamond abrasives (Leco, St. Joseph, MI) down to 1 lm. All samples were cleaned with deionized (D
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