Activity Enhancement of Dense Strontium-Doped Lanthanum Manganite Thin Films under Cathodic Polarization: A Combined AES and XPS Study

La1-xSrxMnO3 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 La0.8Sr0.2MnO3 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 La0.8Sr0.2MnO3 thin films and microelectrodes were analyzed by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), respectively. Annealed, quenched La0.8Sr0.2MnO3 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 similar to 150 to similar 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.