In view of the problems of easy oxidation and carbon deposition of conventional electrode materials in the actual working environment of SOEC, as well as the lack of understanding of the structure-activity relationship between oxygen vacancies and electrode performance of the existing materials, a dual regulation strategy of simultaneous doping of fluoride ion (F^–) and introducing A-site defects in Sr₂FeCo_0.7Mo_0.3O_6-δ oxide was investigated. The crystal structure, electronic structure, chemical stability and other aspects of electrode materials were optimized to overcome the limitations of traditional materials and develop high-performance SOEC cathode materials. The influence mechanism of the dual regulation strategy on the key factors (i.e., oxygen vacancy concentration, ion transport capacity and surface chemical properties) of the material was explored. It was expected that the adsorption and activation ability of the material to CO₂ could be enhanced, the electrode overpotential could be reduced, and the electrode reaction rate could be improved via increasing the oxygen vacancy concentration, enhancing the oxygen ion transport capacity and optimizing the surface chemical state, thereby significantly improving the catalytic activity and electrolysis efficiency of CO₂RR, realizing the efficient conversion and utilization of carbon dioxide, and providing technical support for alleviating greenhouse gas emissions and energy crisis.

A series of Sr₂FeCo_0.7Mo_0.3O_6-δ based electrode materials and related powders were prepared by an improved EDTA-citric acid composite sol-gel method. In the preparation process, the raw material ratio and reaction conditions were accurately controlled. For the preparation of A-site defect samples by adjusting the stoichiometric ratio of strontium source, and for the preparation of fluorine-containing samples by replacing part of Sr (NO₃)₂ with SrF₂ as fluorine source and strontium source, all the samples were calcined under the same conditions to ensure the comparability of experimental results. Also, the LSGM electrolyte sheet was prepared by a tape casting method, and the LDC buffer layer was prepared by a sol-gel method. The symmetrical battery was assembled by a screen printing process. The printing thickness and sintering temperature of each functional layer were strictly controlled to ensure the structural integrity and interface bonding quality of the battery.

The prepared materials are analyzed by advanced characterization techniques and experimental methods. The analysis of X-ray diffraction patterns clearly shows the crystal structure change of the material. Fluorine doping causes a lattice expansion, while the introduction of A-site defects causes a lattice contraction, and effectively inhibits the precipitation of SrF₂ secondary phase. The analysis of X-ray photoelectron spectroscopy further reveals a significant change in the chemical state of the material surface. The fluorine element is effectively incorporated into the lattice, and the lattice Sr ratio is greatly increased, which effectively reduces the surface segregation of Sr and enhanced the stability of the material. Also, the decrease of the valence state of the B-site transition metal increases the number of d-orbital electrons, thus improving the electrocatalytic activity of the material. The increase of the relative content of adsorbed oxygen also indicates that the concentration of oxygen vacancies increases, which creates favorable conditions for ion transport and CO₂ adsorption activation. The analysis of CO₂ adsorption and thermal stability, temperature-programmed desorption and thermogravimetric experiments shows that the synergistic effect of fluorine doping and A-site defects greatly improves the desorption temperature, desorption amount and anti-carbonation ability of the material for CO₂. The desorption amount of SFCMAF sample is 2.25 times greater than that of SFCM sample. The electron paramagnetic resonance characterization also confirms that SFCMAF has the maximum oxygen vacancy concentration, providing a great support for efficient CO₂ electrolysis reaction.

The results of electrochemical performance test indicate that the current density of SFCMAF material is 66.7% higher than that of the original material, and the performance of each sample improves and the amplitude is similar for CO₂/H₂O co-electrolysis under the working conditions of 850 ℃ and 2 V. The analysis of electrochemical impedance spectroscopy and distribution relaxation time shows that the polarization resistance of SFCMAF reduces and the surface catalytic activity enhances. The SFCMAF electrode has a superior stability in both short-term and long-term stability tests, with only 7% decay in 120 h long-term test. The calculations based on the density functional theory indicate the essential reasons for the performance improvement in micron scale. SFCMAF has a stronger ability to adsorb and activate CO₂. Fluorine doping and A-site defects synergistically optimize the electronic structure, reduce the oxygen vacancy formation energy, and change the electronic structure of the active site, which is highly consistent with the experimental results, and provides a solid theoretical guidance for further optimizing the material properties.

This study developed a high-performance SOEC cathode material via introducing fluoride ion doping and A-site defects in Sr₂FeCo_0.7Mo_0.3O_6-δ matrix. The modified material maintained a basic double perovskite structure with significant changes in lattice parameters. The dual-regulation strategy improved the surface chemical properties, CO₂ adsorption, and electrochemical performance. The oxygen vacancy concentration increased, and the polarization resistance decreased. The theoretical calculations revealed the mechanism of performance improvement. This strategy could provide some ideas for designing efficient SOEC cathode materials and validate the effectiveness of anion doping combined with cation defect regulation.