Dynamic In Situ Surface Reoxidation Dictates Bi-O Structures in Reconstructed Bismuth Nanosheets for Selective CO₂ Electroreduction to Formate.

The Bi-O structure in bismuth (Bi)-based catalysts is crucial for activity and selectivity in the CO₂ electroreduction (CO₂RR) 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 Bi₂SeO₅ 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-Ol) or defect oxygen (Bi-Ov), respectively. The Bi-Ol catalyst achieves a Faradaic efficiency of 91.2% for formate, representing a 3.9-fold enhancement in formate selectivity compared with the Bi-Ov 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 CO₂RR electrocatalysts.