La 1 − x Sr x Mn O 3 cathodes used in solid oxide fuel cells have exhibited enhanced catalytic activity after polarization; however, the origin of this enhancement is not understood. In this study, dense La 0.8 Sr 0.2 Mn O 3 thin-film microelectrodes fabricated by sputtering were cathodically or anodically polarized, and electrochemical impedance spectroscopy was used to examine the changes in impedance for oxygen reduction reaction (ORR) before and after polarization. Scanning electron and atomic force microscopy revealed surface microstructural changes after polarization. Surface compositions of La 0.8 Sr 0.2 Mn O 3 thin films and microelectrodes were analyzed by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), respectively. Annealed, quenched La 0.8 Sr 0.2 Mn O 3 were found to have surfaces significantly enriched in La. Cathodic polarization considerably decreased electrode impedance, which was associated with reduced surface La and increased surface Sr and Mn fractions on the cathode adjacent to the three-phase boundaries. Surface particles of ∼ 150 to ∼ 240 nm appeared on the surface regions, where changes in the surface chemical composition of microelectrodes were detected. Acid-etching similarly lowered surface La contents. However, only cracks were found and no change in the surface composition was detected after anodic polarization. The mechanism of ORR activity enhancement induced by cathodic and anodic polarization was discussed.
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O’ et al. (2009) studied this question.
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