Conventionally, Ni-based cermets are widely used for SOCs as porous fuel materials due to their excellent electrocatalytic activity, thermal and chemical stability, and cost-effectiveness. [13][14][15] However, redox cycles and carbon depositions on Ni will inevitably occur within the fuel electrode, deteriorating their long-term electrochemical performance and stability. [16] Alternatively, perovskite oxides (ABO 3 ) attract significant attention from the scientific community as new potential candidates, [3,[17][18][19][20] as they exhibit excellent ionic and electronic conductivity as well as redox stability. Among these, Sr 2 FeMoO 6-σ (SFM) double perovskites are especially promising, owing to their outstanding electronic conductivity and carbon deposition tolerance. Unfortunately, their comparatively poor catalytic activity would obstruct their practical applications since AO termination of perovskite oxides with low O-vacancy contents prevents full contact between analytes and the very active B-sites. [21] Therefore, new approaches to overcome such sluggish reaction kinetics are highly required for the efficient design of SOC-based catalysts.Infiltration [22][23][24] and in situ exsolution method [25][26][27][28][29] are both effective approaches to enhance the electrocatalytic activity of perovskite oxides. Although some promising results are obtained, the range of their performance enhancements is very limited because even though more active metal oxides interfaces are created, the sluggish reaction kinetics of the perovskite oxide matrix is still not intrinsically improved. Nevertheless, previous studies have showed that the enhanced reaction kinetics of the SFM matrix could be obviously achieved by element doping, such as Fe. [30][31][32] However, the related mechanism has been rarely investigated, and thus the origin behind the enhanced reaction kinetics is still not clear. Since the improved performance of SFM is closely related to its electronic and structural variations, [33,34] better unraveling of their relationships is urgently desirable, which is not only important for further performance improvement by structural optimizations, but also could provide a general guidance to design new electrocatalysts for high-performance solid oxide cells.Herein, the electronic structures of the SFM are investigated by partial replacement of Mo with Fe ions. The effect of this substitution on the crystal, physical, chemical, and The performance of Sr 2 FeMoO 6-σ double perovskites can be significantly enhanced by optimizing the ratio of Fe/Mo as a promising electrode material for solid oxide fuel/electrolysis cells. However, the intrinsic origin is still doubt for the improvement of Sr 2 FeMoO 6-σ sluggish electrocatalytic reaction kinetics. Herein, their electronic structures are investigated by partial replacement of Mo with Fe ions. As the Fe content in Sr 2 Fe 1+x Mo 1-x O 6-δ is increased, its oxidation state increases, which enhances the metal-oxygen hybridization and shifts its bulk O p band energy toward...
