Active Reaction Sites for Oxygen Reduction in La0.9Sr0.1MnO3 /  YSZ  Electrodes

Active reaction sites for 02 reduction in La0. ~Sr01MnO3 electrode have been characterized by addressing the origin of cathodic polarization effects on this electrode material. Cathodic polarization (up to - 1. 2 V vs. Pt reference electrode} several effects on O2 reduction kinetics. First, the O2 reduction rate was favorably increased when the perovskite was cathodically polarized. Second, in situ x-ray photoelectron spectroscopy results indicated that the Mn ions electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent higher O~ pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies in the O2 reduction reaction. However, it has been explained from the Po2-dependent hysteresis phenomena that, though those surface sites are active in the O2 reduction~ their activity is less than that of the three-phase boundary since additional diffusional processes are required for the former sites. Consequently, the three-phase boundary sites the major reaction sites at lower O2 pressure, which leads to a small hysteresis. However, at higher 02 pressure, the sites also participate in the reaction, resulting in a larger hysteresis.