Regulating oxygen-reduction kinetics in solid oxide fuel cells via bilayer double-perovskite composite cathodes

Solid oxide fuel cells (SOFCs) are promising energy-conversion devices owing to their high efficiency and fuel flexibility, but their practical performance is still constrained by sluggish cathodic oxygen reduction reaction (ORR) kinetics and stability degradation. To address this issue, a bilayer composite cathode was designed to decouple and synergistically optimize surface catalytic activity and charge-transport capability. In this architecture, Sr₂Ru_0.5M_1.5O_6-δ (M = Co, Fe) oxides were used as surface functional layers to promote oxygen-related surface reactions, while a composite layer consisting of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Ce0_.9Gd_0.1O_2-δ (GDC) served as the current-collecting and charge-transporting backbone. Among the investigated cathodes, the Sr₂Ru_0.5M_1.5O_6-δ/LSCF–GDC bilayer cathode delivered a 27.8% higher peak power density than the single-layer LSCF–GDC cathode. Impedance and distribution of relaxation times (DRT) analyses reveal that the bilayer configuration effectively reduces cathodic polarization and facilitates low-frequency interfacial reaction processes. Combined experimental and density functional theory (DFT) analyses further clarify the atomic-scale origin of the enhanced activity by correlating oxygen vacancy formation, oxygen adsorption, and charge-transfer behavior. Under CO₂-containing conditions, the bilayer cathode suppresses impedance growth and mitigates polarization resistance increase. Long-term tests further confirm its improved electrochemical stability, demonstrating the effectiveness of bilayer cathode engineering for regulating ORR kinetics, polarization behavior, and CO₂ tolerance in SOFCs.