Precatalyst Engineering Directs Reconstruction Into Coupled Defective Sites for Selective CO 2 ‑to‑Formate Electroreduction

ABSTRACT Bismuth‐based materials are promising for formate production from CO 2 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 Bi 5 O 7 I 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 CO 2 electroreduction.