ACTIVE REACTION SITES FOR OXYGEN REDUCTION IN LA0.9SR0.1MNO3 YSZ ELECTRODES/

Active reaction sites for O-2 reduction in La0.9Sr0.1MnO3 electrode ha ve been characterized by addressing the origin of the cathodic polariz ation effects on this electrode material. Cathodic polarization (up to -1.2 V vs. Pt reference electrode) had several effects on O-2 reducti on kinetics. First, the O-2 reduction rate was favorably increased whe n the perovskite electrode was cathodically polarized. Second, in situ x-ray photoelectron spectroscopy results indicated that the Mn ions a re electrochemically reduced and concomitantly the oxygen stoichiometr y decreases. Reduction of Mn ions was further demonstrated in the cycl ic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the per ovskite/YSZ/Pt system, and the hysteresis phenomena were more prominen t at higher O-2 pressure. We interpreted these findings to mean that t he internal and/or external surface oxide vacancies participate in the O-2 reduction reaction. However, it has been explained from the P-O2- dependent hysteresis phenomena that, even though those surface sites a re active in the O-2 reduction, their activity is less than that of th e three-phase boundary sites since additional diffusional processes ar e required for the former sites. Consequently, the three-phase boundar y sites are the major reaction sites at lower O-2 pressure, which lead s to a small hysteresis. However, at higher O-2 pressure, the surface sites also participate in the reaction, resulting in a larger hysteres is.