An Active and Stable High‐Entropy Ruddlesden‐Popper Type La_1.4Sr_0.6Co_0.2Fe_0.2Ni_0.2Mn_0.2Cu_0.2O_4±δ Oxygen Electrode for Reversible Solid Oxide Cells

Abstract Insufficient catalytic activity and stability of oxygen electrodes are major challenges for the widespread use of reversible solid oxide cells (Re‐SOCs). Here, a Ruddlesden‐Popper‐structured high‐entropy La_1.4Sr_0.6Co_0.2Fe_0.2Ni_0.2Mn_0.2Cu_0.2O_4±δ (RP‐LSCFNMC) oxygen electrode with fast oxygen reduction and emission reaction kinetics, inhibited Sr segregation and favorable thermal expansion efficient is reported. A Re‐SOC with the RP‐LSCFNMC oxygen electrode achieves an encouraging peak power density of 1.74 W cm⁻² in the fuel cell mode and a remarkable current density of 2.10 A cm⁻² at 1.3 V in the water electrolysis mode at 800 °C. The Re‐SOC also shows excellent stability, with no Sr segregation observed after 120 h of testing in both the fuel cell and electrolysis modes at 750 °C. Furthermore, the improved activity and stability of the RP‐LSCFNMC oxygen electrode are confirmed through a combination of experiments and density functional theory‐based calculations. These findings make the high‐entropy RP‐LSCFNMC oxide a promising oxygen electrode candidate for advanced Re‐SOCs.