The oxygen reduction reaction (ORR) activity of a Cu-doped Ba_0.5Sr_0.5FeO_3−δ (Ba_0.5Sr_0.5Fe_1−xCu_xO_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 ($Pm\overline{3}m$). 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.

Conductive oxides are widely studied as cathode materials for electrochemical cells, such as solid oxide fuel cells (SOFCs), because of their chemical stability and high electrical conductivity at high temperatures (800–950 °C). The cathode is a key component of SOFCs, accounting for the greatest resistance loss among the SOFC components. It is important to precisely determine the conductivity of the cathode material, but it is difficult to achieve consistency among measurements because of errors caused by differences in the measurement methods and conditions employed by various research teams. In this study, the total electrical conductivity of an SOFC cathode material was measured by the DC 4-point method by investigating the geometrical parameters of the sample and the measurement terminal and the measurement device using La_0.8Sr_0.2MnO_3+d (LSM). The measurement variables included the spacing between the measurement terminals (1 and 2 cm), lead wire diameter (0.25 and 0.5 mm), specimen thickness (3, 4, and 5 mm), and the applied current (10, 50, and 100 mA). The larger the spacing between the measurement terminal and the thinner the specimen, the smaller the standard deviation.

The oxygen vacancy formation behavior and electrochemical and thermal properties of Ba_0.5Sr_0.5Fe_1−xMn_xO_3−δ (BSFMnx, x = 0–0.15) cathode materials were investigated. For thermogravimetric analysis, the weight decreased from 1.98% (x = 0) to 1.81% (x = 0.15) in the 400–950 °C range, which was due to oxygen loss from the lattice. The average oxidation state of the B-site increased, the O_ads/O_lat ratio decreased, and the binding energy of the O_lat peak increased with Mn doping. These results indicate that Mn doping increases the strength of the metal–oxygen bond and decreases the amount of oxygen vacancies in the lattice. The electrical conductivity of BSFMnx increased with the temperature due to the thermally activated small-polaron hopping mechanism showing a maximum value of 10.4 S cm⁻¹ (x = 0.15) at 450 °C. The area-specific resistance of BSFMn0.15 was 0.14 Ω cm² at 700 °C and the thermal expansion coefficient (TEC) gradually decreased to 12.7 × 10⁻⁶ K⁻¹, which is similar to that of Ce_0.8Sm_0.2O₂ (SDC) (12.2 × 10⁻⁶ K⁻¹). Mn doping increased the metal–oxygen bonding energy, which reduced the oxygen reduction reaction activity but improved the electrical conductivity and thermal stability with SDC.

The degradation behavior of yttria-stabilized zirconia by thermal aging was investigated in terms of phase transformation, local atomic structure, and electrical conductivity. The average grain size of 8YSZ was increased from 20.83 μm to 25.81 μm with increasing aging temperature. All 8YSZ samples degraded at different temperatures had a predominantly cubic structure. The (400) peak of 8YSZ deteriorated at 1300 and 1400 °C shifted to a high angle, and the peak of tetragonal was not indexed. For 8YSZ degraded at 1500 °C, the (400) peak shifted to a lower angle, and the peak of tetragonal was identified. Analysis of the local microstructure of aged 8YSZ using extended X-ray absorption fine structure showed that the intensity of the Zr-O peak gradually increased and that the intensity of the peak of cationic Zr decreased as the aging temperature increased. The changes in the peaks indicate that the oxygen vacancies were reduced and Y³⁺ ions escaped from the lattice, leading to the destabilization of 8YSZ. The activation energies of 8YSZ at 1300 °C and 1400 °C were derived to be 0.86 and 0.87 eV, respectively, and the activation energy of 8YSZ at 1500 °C increased significantly to 0.92 eV. With the thermal deterioration of 8YSZ, the cation (Y³⁺) escaped from the lattice and the number of oxygen vacancies decreased, resulting in the formation of a tetragonal structure and high activation energy at 1500 °C.

The phase formation and stabilization behaviors of calcia partially stabilized zirconia (Ca-PSZ) were investigated with regard to the CaO content and post-heat treatment. Sintered specimens were prepared by adding 2, 3, 4, and 5 mol% to CaO to ZrO₂, and post-heat treatment were conducted. In the X-ray diffraction pattern, the monoclinic peak decreased, the tetragonal peak increased upon CaO doping, and no CaZrO₃ peak was observed. Transmission electron microscopy images of the Ca-PSZ showed that the d-spacing of 4CSZ (200)_m extended from 0.260 nm to 0.266 nm subsequent to post-heat treatment. The coefficient of thermal expansion gradually increased in accordance with the dopant concentration, in addition, it increased even after the post-heat treatment. These results are related to the increase in tetragonal phase, which has a relatively higher coefficient of thermal expansion than that of the monoclinc phase. According to the Vickers hardness measurement, the hardness of all specimens increased gradually as the concentration of CaO increased, and the hardness of the 5CSZ was improved from 676 to 774 Hv by the post-heat treatment.