Oxygen Surface Exchange Kinetics on Sr-Substituted Lanthanum Manganite and Ferrite Thin-Film Microelectrodes

Abstract

The surface oxygen exchange kinetics occurring on dense La0.8 Sr0.2Mn O3 (65 nm thick) and La0.8 Sr0.2 Fe O3 (110 nm thick) thin filmswere investigated by electrochemical impedance spectroscopy (EIS).Rutherford backscattering spectroscopy revealed that the bulk filmcompositions were consistent with the nominal stoichiometry, withLa0.8 Sr0.2 Fe O3 having a slight Fe deficiency. Surface compositionsof La0.8 Sr0.2 Mn O3 and La0.8 Sr0.2 Fe O3 were enriched in La usingX-ray photoelectron spectroscopy and Auger electron spectroscopy.EIS data were utilized to determine the surface oxygen exchange coefficients,kq and kchem, in the range of temperatures from 790 to 660°C andoxygen partial pressures from 10-5 to 1 atm. The magnitudes of kqand kchem were found comparable for both La0.8 Sr0.2 Mn O3 and La0.8Sr0.2 Fe O3, and a P O2 dependence m fell in the range from ∼0.2to ∼0.3. The thermodynamic enhancement factor γ was consistentlyhigher for La0.8 Sr0.2 Mn O3 than La0.8 Sr0.2 Fe O3, which was inreasonable agreement with estimates based on thermogravimetric dataof powder materials reported previously. The chemical capacitancefor La0.8 Sr0.2 Fe O3 was approximately 1 order of magnitude largerthan La0.8 Sr0.2 Mn O3, which indicates a larger oxygen vacancy content.The rate-limiting steps of surface oxygen exchange on La0.8 Sr0.2Mn O3 and La0.8 Sr0.2 Fe O3 were discussed with regard to previouslyproposed models. © 2009 The Electrochemical Society.