Oxygen Vacancy and Valence Band Structure of Ba(0.5)Sr(0.5)Fe(1−x)Cu(x)O(3−δ) (x = 0–0.15) with Enhanced ORR Activity for IT-SOFCs

The oxygen reduction reaction (ORR) activity of a Cu-doped Ba(0.5)Sr(0.5)FeO(3−δ) (Ba(0.5)Sr(0.5)Fe(1−x)Cu(x)O(3−δ), BSFCux, x = 0, 0.05, 0.10, 0.15) perovskite cathode was investigated in terms of oxygen vacancy formation and valence band structure. The BSFCux (x = 0, 0.05, 0.10, 0.15) crystallized in a cubic perovskite structure ([Formula: see text]). By thermogravimetric analysis and surface chemical analysis, it was confirmed that the concentration of oxygen vacancies in the lattice increased with Cu doping. The average oxidation state of B-site ions decreased from 3.583 (x = 0) to 3.210 (x = 0.15), and the valence band maximum shifted from −0.133 eV (x = 0) to −0.222 eV (x = 0.15). The electrical conductivity of BSFCux increased with temperature because of the thermally activated small polaron hopping mechanism showing a maximum value of 64.12 S cm(−1) (x = 0.15) at 500 °C. The ASR value as an indicator of ORR activity decreased by 72.6% from 0.135 Ω cm(2) (x = 0) to 0.037 Ω cm(2) (x = 0.15) at 700 °C. The Cu doping increased oxygen vacancy concentration and electron concentration in the valence band to promote electron exchange with adsorbed oxygen, thereby improving ORR activity.