Optimizing Reversible Exsolution and Phase Transformation in Double Perovskite Sr₂Fe_1.5‐xCo_xMo_0.5O_6‐δ Electrodes for High‐Performance Symmetric Solid Oxide Cells

Double perovskite (DP) oxides are promising electrode materials for symmetric solid oxide cells (SSOCs) due to their excellent electrochemical activity and stability. B-site cation doping in DP oxides affects the reversibility of phase transformation and exsolution, which plays a crucial role in the catalyst recovery. Yet, few studies have been conducted on this topic. In this study, the Sr₂Fe_1.5-xCo_xMo_0.5O_6-δ (CSFM, x = 0, 0.1, 0.3, 0.5) DP system demonstrates modulated exsolution and phase transformation reversibility by manipulating the oxygen vacancy concentration. The correlation between Co-doping level and oxygen vacancy concentration is investigated to optimize the exsolution and phase transformation properties. Sr₂Fe_1.2Co_0.3Mo_0.5O_6-δ (3CSFM) exhibits reversible transformation between DP and Ruddlesden-Popper phases with a high density of exsolved CoFe nanoparticles under redox atmospheres. The quasi-symmetric cell with 3CSFM shows a peak power density of 1.27 W cm^-2 at 850 °C in H₂ fuel cell mode and a current density of 2.33 A cm^-2 at 1.6 V and 800 °C in H₂O electrolysis mode. The 3CSFM electrode exhibits robust stability during continuous operation for ≈700 h. These results demonstrate the significant role of B-site doping in designing DP materials capable of dynamic phase transformation in diverse environments.