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

Abstract

The surface oxygen exchange kinetics occurring on dense La0.8Sr0.2MnO3 (65nm thick) and La0.8Sr0.2FeO3 (110nm 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, kq and kchem , in the range of temperatures from 790to660°C and oxygen partial pressures from 10−5to1atm . The magnitudes of kq and kchem were found comparable for both La0.8Sr0.2MnO3 and La0.8Sr0.2FeO3 , and a PO2 dependence m fell in the range from ∼0.2 to ∼0.3 . The thermodynamic enhancement factor γ 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.