Decoupling the Roles of B‐Site Cation Defects and Oxygen Vacancies in Exsolved Perovskites for Robust CO 2 Electrolysis in Solid‐State Cells

Construction of a strong metal‐oxide heterointerface via perovskite in situ exsolution synchronously generates oxygen vacancies (Vö) and B‐site cation defects. However, their individual contributions are often conflated and overshadowed by the overall high performance toward CO 2 electrolysis in solid‐state electrolyzers, obscuring their specific roles in modulating perovskite properties. Here we designed a series of Sr 2 Fe 1.35 Mo 0.45 Ni 0.2 O 6−δ (SFMN) with different levels of B‐site Fe/Ni defects and Vö. Cross‐scale experimental and theoretical results revealed that increased B‐site Fe/Ni defects and Vö strengthened Fe 3d−O 2p orbital hybridization, thereby enhancing electrical conductivity, but this trend reversed beyond a certain threshold. Then, in situ topotactic exsolution was performed for SFMN to refill B‐site cation defects while maintaining Vö concentration intact. The resulting sample without B‐site cation defects exhibited a faster oxygen‐ion transport capability than conventionally exsolved SFMN with rich B‐site defects, achieving a high current density of 2.40 A cm −2 at 1.5 V and 850°C, while avoiding SrCO 3 formation. Theoretical calculations demonstrated that refilling B‐site cation defects in exsolved SFMN thermodynamically accelerated CO 2 adsorption and activation, as well as suppressed Sr segregation. This study unravels the crucial impact of B‐site defects on perovskite properties and provides insights for the future development of robust perovskite electrocatalysts.