Isao Kagomiya

Mixed ionic–electronic conducting (MIEC) perovskite oxides are good choices for air electrodes of solid oxide fuel cells (SOFCs), proton-conducting ceramic fuel cells (PCFCs), and/or oxygen permeation membranes. The composition of BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZrY) and its modified variants reveal encouraging results for air electrodes of both SOFCs and PCFCs. To further tailor the MIEC properties of the BCFY series, this study focuses on BaCo0.4Fe0.4Ni0.1Y0.1O3-δ (BCFNiY) and BaCo0.4Fe0.4Mo0.1Y0.1O3-δ (BCFMoY). These compositions were prepared through the citrate-based liquid mixing method. X-ray powder diffraction and corresponding Rietveld refinement studies confirm that the main phase is a cubic perovskite structure, although a secondary phase of <10 wt % appears in both compositions. At 800 °C, the total conductivities of BCFNiY and BCFMoY are 5.90 and 4.88 S cm–1, respectively. The impressive total conductivity of BCFNiY is higher than that of the promising cathode BCFZrY. At the same temperature, the oxygen permeabilities of BCFNiY and BCFMoY are 0.44 and 0.29 μmol cm–2 s–1, respectively. The significantly higher oxygen permeability of BCFNiY compared to that of BCFMoY can be attributed to its lower absolute value of average bond energy (ABE), larger critical radius (rc), and increased lattice-free volume (LFV). These factors collectively facilitate oxide-ion conduction and contribute to the enhanced oxygen permeation properties of the Ni-substituted composition. These results highlight BCFNiY as a promising MIEC perovskite for oxygen permeation membranes and air electrodes in next-generation SOFCs and PCFCs.

Lead-free dielectric ceramics are environmentally friendly candidates for energy-storage capacitors, while the recoverable energy density of ferroelectric perovskites is known to be enhanced in the vicinity of the Curie point. As such, (1–x)Ba _0.85 Ca _0.15 Zr _0.1 Ti _0.9 O ₃ – x SrTiO ₃ ceramics with x = 0–0.5 were prepared by a conventional solid-state reaction, and the effect of the ST solid-solution ratio on the energy-storage behavior was examined at a fixed field of 30 kV cm ⁻¹ . At room temperature, the recoverable energy density W rec reached approximately 133 mJ cm ⁻³ at x = 0.3 and the efficiency η increased steadily from about 25% to 90%. The temperature-dependent measurements revealed that W rec peaked near the respective Curie point of each composition, with a comparable peak value of approximately 140 mJ cm ⁻³ for all compositions. These results indicate that Sr does not intrinsically improve the achievable energy density, but instead shifts the temperature window over which the optimal performance is obtained.

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High oxygen permeability of mixed conductive La0.65Ca0.35FeO3-delta (LCF) is applicable to pure oxygen gas generators and cathodes for solid oxide fuel cells, etc.; however, lower surface exchange reactions at temperatures below 800 degrees C reduce permeability. To understand the microscopic surface reaction mechanism, operando soft X-ray photoelectron spectroscopy of an LCF film surface was conducted during the evolution and incorporation of oxygen. LCF film was prepared on yttria-stabilized zirconia and a current was applied throughout the film at similar to 600 degrees C. From operando X-ray photoelectron spectra, surface oxide species involved in the surface exchange reaction obviously appeared on the film during the evolution of oxygen from the surface. The number of surface oxide species abruptly decreased during incorporation of oxygen. By applying the current from a negative to positive value, the numbers of surface oxide species and ligand holes near Fe3+ ions on the surface both significantly increased. The results infer that ligand holes in the Fe 3d-O 2p hybrid orbitals correspond to active reaction sites at which surface oxide species change to oxygen molecules. Increasing the number of active reaction sites is key to improving oxygen evolution of mixed conductive oxides.

The layered perovskite LaSr ₃ Fe ₃ O ₁₀ (LSF) easily hydrates under ambient air. The hydrated LSF transports hydroxides in the middle‐temperature range of 100 °C–250 °C. This study used X‐ray absorption fine structure analysis on normal and hydrated LSF from room temperature to 350 °C to evaluate the relationship between local structures and ion transport characteristics. The H ₂ O molecules are mainly intercalated into rock‐salt interlayers, which are close to the La1 and O3 sites. The generated OH ⁻ ions are found at O3 sites, which are closer to Sr ions than La ions. The intercalated H ₂ O molecules and formed OH ⁻ ions expand the spaces of the rock‐salt interlayers. The temperature dependence of the local crystal structure was also studied in this study. According to the findings, the presence of OH ⁻ ions on the O3 sites, which are closer to Sr ions, plays a significant role in OH ⁻ transport in the medium temperature range.