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

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.