Effects of Yttrium Doping on Oxygen Conductivity in Ba(Fe, Co, Zr, Y)O_{3-δ} Cathode Materials for Proton Ceramic Fuel Cells

Proton ceramic fuel cells (PCFCs) achieve high efficiency at reduced operating temperatures, but their performance is often limited by slow oxygen reduction reaction (ORR) kinetics at the cathode. The BaCoFeZrY (BCFZY) perovskite family is a promising triple-conducting air-electrode material, yet the role of Y dopants in governing oxygen transport remains unclear. In this study, we examine the effect of Y content on oxygen conductivity in three compositions: BCFZ, BCFZY0.1, and BCFY. Oxygen conductivity was evaluated from the product of oxygen tracer diffusivity and oxygen defect concentration. Ab initio molecular dynamics simulations were used to determine tracer diffusivity and migration energies, while defect concentrations were estimated from reference data. Y doping slightly decreases oxygen conductivity from BCFZ to BCFZY0.1, from 337 to 203 mS/cm at 500 C, with activation energies of 0.155 and 0.172 eV. BCFY shows much lower conductivity (99 mS/cm) and a higher activation energy of 0.261 eV. Computed conductivities are higher and more Arrhenius-like than experimental values, suggesting that microstructural features such as grain boundaries strongly limit oxygen transport in real materials. A series-circuit model combining bulk conductivity and fitted grain-boundary parameters provides semi-quantitative agreement with experiment. These results clarify the role of Y doping in oxygen transport and provide insight for optimizing cathode performance in PCFCs.