Stabilizing Perovskite Phase via A‐Site Charge Density Modulation During Metal In Situ Exsolution for Robust CO ₂ Electrolysis

Perovskite in situ exsolution is an effective approach for fabricating robust heterostructures with superior catalytic properties, but the concomitant phase transformation occurring in parent perovskite matrix often cause compromised structural integrity and diminished catalytic activity. Here a series of high‐charge density Ca‐doped Sr 2− x Ca x Fe 1.3 Ni 0.2 Mo 0.5 O 6−δ (Ca x SFNM, x ≤ 0.5) is synthesized, and treated them in reducing atmospheres to in situ exsolve FeNi 3 nanoalloys (FeNi 3 @Ca x SFNM). The phase structure progressively evolves during exsolution as Ca content decreases, among which FeNi 3 @Ca 0.5 SFNM preserves its double perovskite structure with maximal oxygen vacancy concentration, whereas other counterparts exhibit stepwise structural reconstruction. Moreover, increased oxygen vacancies strengthen their surface interactions with CO 2 , conferring FeNi 3 @Ca 0.5 SFNM with exceptional CO 2 electrolysis performance, where a current density of 1.05 A cm −2 and a CO Faraday efficiency of 95.38%, coupled with a minimal decay rate of only 0.8 mA cm −2 h −1 during 200 h of test, are obtained at 850 °C and 1.5 V, surpassing others with varying phase transitions. Theoretical calculations reveal that relative to Sr 2+ , Ca 2+ enhances electronic coupling of A‐O‐B sites and B 3d‐O 2p orbital hybridization, ultimately reinforcing B─O bond covalency to suppress phase transition and oxygen vacancy loss upon exsolution.