Optimizing Reversible Exsolution and Phase Transformation in Double Perovskite Sr2Fe1.5‐xCoxMo0.5O6‐δ Electrodes for High‐Performance Symmetric Solid Oxide Cells

Abstract 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 Sr 2 Fe 1.5‐x Co x Mo 0.5 O 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. Sr 2 Fe 1.2 Co 0.3 Mo 0.5 O 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 H 2 fuel cell mode and a current density of 2.33 A cm −2 at 1.6 V and 800 °C in H 2 O 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.