Abstract
Active reaction sites for 
reduction in 
electrode have been characterized by addressing the origin of the cathodic polarization effects on this electrode material. Cathodic polarization (up to −1.2 V vs. Pt reference electrode) had several effects on 
reduction kinetics. First, the 
reduction rate was favorably increased when the perovskite electrode was cathodically polarized. Second, in situ x‐ray photoelectron spectroscopy results indicated that the Mn ions are electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further demonstrated in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent at higher 
pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the 
reduction reaction. However, it has been explained from the 
hysteresis phenomena that, even though those surface sites are active in the 
reduction, their activity is less than that of the three‐phase boundary sites since additional diffusional processes are required for the former sites. Consequently, the three‐phase boundary sites are the major reaction sites at lower 
pressure, which leads to a small hysteresis. However, at higher 
pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.