Dynamic In Situ Surface Reoxidation Dictates Bi–O Structures in Reconstructed Bismuth Nanosheets for Selective CO 2 Electroreduction to Formate

The Bi–O structure in bismuth (Bi)‐based catalysts is crucial for activity and selectivity in the CO 2 electroreduction (CO 2 RR) to formate. However, the stability of the Bi–O structure and the true active species remain subjects of significant debate, since Bi‐based catalysts undergo structural reconstruction to metallic Bi. This discrepancy may stem from the extreme ease of Bi oxidation, highlighting the importance of understanding the evolution of the Bi–O structure under in situ conditions. To address this, we engineered Bi catalysts via electrochemical reconstruction of Bi 2 SeO 5 nanosheet precursors and utilized in situ spectroscopies to unequivocally demonstrate that an active oxidized layer forms dynamically during reconstruction. The precursor determines the resulting Bi–O structure abundant in lattice oxygen (Bi–O l ) or defect oxygen (Bi–O v ), respectively. The Bi–O l catalyst achieves a Faradaic efficiency of 91.2% for formate, representing a 3.9‐fold enhancement in formate selectivity compared with the Bi–O v catalyst. Mechanistic studies reveal that the lattice oxygen governs catalytic kinetics by optimizing *OCHO adsorption, while defect oxygen preferentially promotes the competing hydrogen evolution by enhancing water adsorption. This study provides critical mechanistic insights for the rational design of next‐generation CO 2 RR electrocatalysts.