Identifying the Origin of the Limiting Process in a Double Perovskite PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ Thin-Film Electrode for Solid Oxide Fuel Cells

Oxygen reduction reaction in a double perovskite material, PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically well-defined dense thin-film (0.8–2 μm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (E_OV) energies. For example, E_OV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (E_OV = 297 kJ/mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (E_OV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10^–12 cm² s^–1), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10^–10 cm² s^–1) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba²⁺ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.