Oxygen Vacancy and Valence Band Structure of Ba0.5Sr0.5Fe1−xCuxO3−δ (x = 0–0.15) with Enhanced ORR Activity for IT-SOFCs

Abstract

The oxygen reduction reaction (ORR) activity of a Cu-doped Ba0.5Sr0.5FeO3−δ (Ba0.5Sr0.5Fe1−xCuxO3−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 presented.) ). 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 cm2 (x = 0) to 0.037 Ω cm2 (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.