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

The surface oxygen exchange kinetics occurring on dense La0.8Sr0.2MnO3 (65 nm thick) and La0.8Sr0.2FeO3 (110 nm thick) thin films were investigated by electrochemical impedance spectroscopy (EIS). Rutherford backscattering spectroscopy revealed that the bulk film compositions were consistent with the nominal stoichiometry, with La0.8Sr0.2FeO3 having a slight Fe deficiency. Surface compositions of La0.8Sr0.2MnO3 and La0.8Sr0.2FeO3 were enriched in La using X-ray photoelectron spectroscopy and Auger electron spectroscopy. EIS data were utilized to determine the surface oxygen exchange coefficients, k(q) and k(chem), in the range of temperatures from 790 to 660 degrees C and oxygen partial pressures from 10(-5) to 1 atm. The magnitudes of k(q) and k(chem) were found comparable for both La0.8Sr0.2MnO3 and La0.8Sr0.2FeO3, and a Po-2 dependence m fell in the range from similar to 0.2 to similar to 0.3. The thermodynamic enhancement factor gamma was consistently higher for La0.8Sr0.2MnO3 than La0.8Sr0.2FeO3, which was in reasonable agreement with estimates based on thermogravimetric data of powder materials reported previously. The chemical capacitance for La0.8Sr0.2FeO3 was approximately 1 order of magnitude larger than La0.8Sr0.2MnO3, which indicates a larger oxygen vacancy content. The rate-limiting steps of surface oxygen exchange on La0.8Sr0.2MnO3 and La0.8Sr0.2FeO3 were discussed with regard to previously proposed models. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3123214] All rights reserved.