Precatalyst Engineering Directs Reconstruction Into Coupled Defective Sites for Selective CO2‑to‑Formate Electroreduction.

Bismuth-based materials are promising for formate production from CO2 electroreduction, yet their rational design is hindered by an inability to control their dynamic reconstruction, which often leads to poorly defined active sites. Here, we propose a precatalyst engineering strategy wherein the crystallographic structure dictates the reconstruction pathway toward targeted active sites. Using bismuth oxyiodide as a model system, we demonstrate that layered BiOI transforms into metallic Bi favorable for the hydrogen evolution reaction. In contrast, the robust three-dimensional framework of non-stoichiometric Bi5O7I structurally preserves iodine, directing its reconstruction into a surface rich in coupled bismuth vacancies and iodine dopants. This uniquely defective configuration achieves a formate Faradaic efficiency of 96.8% at a high current density of 400 mA cm-2, while demonstrating stable operation for over 100 h with negligible activity loss at 200 mA cm-2. Combining in situ characterization with theoretical calculations, we elucidate the structure-dependent evolution mechanism. This work establishes a design principle for bismuth oxyhalide precatalysts to program reconstruction pathways for efficient CO2 electroreduction.