Reversible symmetrical solid oxide cells (RS-SOCs) exhibit significant potential in the fields of high-efficiency energy conversion and storage, due to their high energy conversion efficiency and simplified manufacturing processes. The key to their performance lies in the development of robust electrode materials that can function effectively as both the fuel electrode under reducing atmospheres and the air electrode under oxidizing atmospheres, during reversible operation between solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes. Sr₂Fe_1.5Mo_0.5O_6-δ (SFM)-based materials have attracted considerable attention in this field, owing to their excellent mixed ionic-electronic conductivity, high structural stability and remarkable tolerance to sulfur poisoning and carbon deposition. This study was to focus on Sr_1.85Fe_1.3Co_0.2Mo_0.5O_6-δ (SFCM), a modified composition, in which partial substitution with Co to further enhance electrochemical activity, while maintaining sufficiently high stability. The primary objectives are to synthesize high-purity SFCM powder via an optimized method, comprehensively characterize its phase evolution and structural stability under both oxidizing and reducing conditions relevant to SOC operation, and ultimately evaluate its practical electrochemical performance as a symmetrical electrode in a single cell.

The material was prepared using an EDTA-citrate dual-complexing sol-gel method. Crystal structure and phase evolution of the SFCM material were studied by using X-ray diffraction (XRD). XRD analysis was also performed on samples thermally reduced at 900 ℃ under a reducing atmosphere to simulate the fuel-electrode environment. Microstructure and electrode/electrolyte interface of the cells were examined by using scanning electron microscopy (SEM). The SFCM electrode was composited with gadolinium-doped ceria (GDC) to form an SFCM-GDC composite electrode, which increases the triple-phase boundary length and mitigates potential chemical reactions with the electrolyte. Electrochemical impedance spectroscopy (EIS) was employed to analyze polarization resistance of the electrodes. Finally, performance of the single cell was tested in SOFC mode using 3 vol.% H₂O-97 vol.% H₂ as fuel and air as oxidant and in SOEC mode under an applied voltage for electrolysis of a 50 vol.% H₂-50 vol.% CO₂ gas mixture.

XRD results confirmed that the powder calcined in air at 1200 ℃ is of a pure double perovskite structure, as the phase of Sr₂Fe_1.5Mo_0.5O₆, indicating success in synthesis of the target materials. After reduction at 900 ℃, significant phase evolution was revealed. While the primary double perovskite structure was largely retained, new phases emerged, including a distinct double perovskite phase identified as Sr₃FeMoO₇ and a metallic cubic phase corresponding to a CoFe alloy. This in-situ exsolution of alloy nanoparticles under reducing conditions is highly beneficial, as it can significantly enhance the electrocatalytic activity for fuel oxidation and reduction reactions. SEM results showed that the SFCM-GDC composite electrode adhered well to the SSZ electrolyte after sintering, forming a porous electrode layer with strong connectivity, which is crucial for gas diffusion and charge-transfer processes. Electrochemical tests demonstrated promising performance. In SOFC mode at 800 ℃, the symmetrical single cell with SFCM-GDC electrodes achieved a maximum power density of 0.52 W·cm^-2, when fueled with humidified hydrogen, indicating promising anodic and cathodic activity. More notably, in SOEC mode at 800 ℃ under an electrolysis atmosphere (50 vol.% H₂-50 vol.% CO₂), the single cell reached a current density of 1.106 A·cm^-2 at an applied voltage of 1.5 V.

SFCM material synthesized using the EDTA-citrate sol-gel method demonstrates excellent potential as a symmetrical electrode for RS-SOCs. The material exhibits remarkable structural adaptability, maintaining a stable double perovskite framework in air and undergoing favorable in-situ phase evolution under reducing conditions to form catalytically active CoFe alloy nanoparticles along with a stable Sr₃FeMoO₇ phase. This bifunctional character underpins its robust electrochemical performance. The fabricated symmetrical cell with an SFCM-GDC composite electrode on an SSZ electrolyte delivered competitive power output in fuel cell mode. More importantly, high current density was realized in electrolysis mode at 800 ℃. These results validate SFCM as a highly active and stable electrode material suitable for both power generation and fuel synthesis in reversible solid oxide cells. It is confirmed that strategic cationic substitution (Co for Fe) in the Sr-Fe-Mo-O system, combined with composite electrode engineering, is an effective approach for developing high-performance symmetrical electrodes.