Oxygen reduction reaction over (Ba,Sr)6RE2Co4O15–Ba(Ce,Pr,Y)O3 composite cathodes for proton-conducting ceramic fuel cells

Abstract

In this study, the effect of elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)₆RE₂Co₄O₁₅ (RE = La, Pr, Nd, Sm, Gd) and the composite effect with BaCe_0.5Pr_0.3Y_0.2O_3−δ on the activity for the oxygen reduction reaction were studied to develop high-performance cathodes for proton-conducting ceramic fuel cells. The polarization resistance of (Ba_6−xSr_x)RE₂Co₄O₁₅ electrodes decreased with an increase in the Ba/Sr ratio, while the activity did not change systematically along the periodic table when the rare-earth element was substituted. Although the polarization resistance of Ba₅SrGd₂Co₄O₁₅ was about one order of magnitude lower than that of Ba₄Sr₂Sm₂Co₄O₁₅ at 500 °C, the composite of Ba₅SrGd₂Co₄O₁₅–BaCe_0.5Pr_0.3Y_0.2O_3−δ (30 : 70 wt%) exhibited comparable performance and activation energy to Ba₄Sr₂Sm₂Co₄O₁₅–BaCe_0.5Pr_0.3Y_0.2O_3−δ (30 : 70 wt%): polarization resistance – 0.20 Ω cm² and 0.52 Ω cm² at 600 °C and 500 °C, respectively, and activation energy – 61.8 kJ mol⁻¹. Then, the reason for the high performance of the Ba₅SrGd₂Co₄O₁₅–BaCe_0.5Pr_0.3Y_0.2O_3−δ (30 : 70 wt%) composite was studied in detail, especially from the viewpoint of elemental interdiffusion. Finally, the plausible oxygen reduction reaction mechanism on this composite was proposed.