Effects of yttrium doping on oxygen conductivity in Ba(Fe, Co, Zr, Y)O_3-δ cathode materials for proton ceramic fuel...

Proton ceramic fuel cells (PCFCs) operate efficiently at lower operating temperatures than solid oxide fuel cells, but their performance is often limited by cathodic oxygen reduction reaction (ORR) kinetics. BaCo_xFe_yZr_zY_1-x-y-zO_3-δ (BCFZY) perovskites are promising triple-conducting cathodes for PCFCs, although the role of acceptor dopants such as Y on oxygen conductivity remains unclear. In this study, we investigate the effects of Y doping on oxygen conductivity in BaFe_0.4Co_0.4Zr_0.2O_3-δ (BCFZ), BaFe_0.4Co_0.4Zr_0.1Y_0.1O_3-δ (BCFZY0.1) and BaFe_0.4Co_0.4Y_0.2O_3-δ (BCFY). The oxygen conductivity was analysed from oxygen tracer diffusivity and oxygen concentration. Ab initio molecular dynamics (AIMD) simulations were used to calculate oxygen tracer diffusivity and migration energies, while oxygen defect concentrations were estimated from literature data. Our results indicate that Y doping slightly decreases the oxygen conductivity from BCFZ to BCFZY0.1 with values of 337 ± 105 mS cm^-1 and 203 ± 81 mS cm^-1 at 500 °C, respectively, with corresponding activation energies of 0.155 ± 0.06 eV and 0.172 ± 0.006 eV. In contrast, BCFY exhibits a significantly lower conductivity (99 ± 40 mS cm^-1) and a higher activation energy of 0.261 ± 0.006 eV. A comparison with experimental data shows that the computed conductivities are substantially higher and exhibit more Arrhenius-like behavior in their temperature dependence than available experimental results, suggesting that microstructural effects such as grain boundaries strongly influence oxygen transport in experiments. Consistent with this hypothesis, a series circuit model of our calculated bulk conductivity and fitted grain boundary transport parameters resulted in semi-quantitative agreement with experimental conductivities and grain boundary behaviour consistent with that typically observed in oxides. These findings provide insights into the defect chemistry and ORR kinetics of BCFZY materials for PCFC cathode design.