Correlation between Structural, Chemical, and Electrochemical Properties of La 0.6 Sr 0.4 CoO 3– d Nanopowders for Application in Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Lanthanum–strontium cobaltites are excellent mixed ionic and electronic conductors and exhibit good activity for the oxygen reduction reaction (ORR) at temperatures higher than 500 °C. This led to these materials being proposed as cathodes for solid-oxide fuel cell devices, either for the standard high temperature (SOFC, 900–1000 °C) or the intermediate temperature (IT-SOFC, 500–700 °C) range. Although it was determined that crystal structure and oxygen vacancies play a central role in these properties, little is known about cobalt speciation at SOFC or IT-SOFC working temperatures. In this study the cobalt speciation and local order of La0.6Sr0.4CoO3–d (LSC) is analyzed in detail for the first time in nano- and microstructured powders by means of in situ experiments using X-ray absorption spectroscopy (XAS) in the temperature range of 20–700 °C. The electrochemical performance of LSC-based cathodes in symmetrical cells under the same conditions is also studied using electrochemical impedance spectroscopy (EIS). Cobalt atoms can be found in any of the three possible oxidation states. Their relative amount depends on temperature and crystallite size. In crystallites of a few tens of nanometers, the ease with which Co atoms change their oxidation state is enhanced. This effect probably facilitates oxide transport in LSC-based cathodes.