AbstractBismuth-based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for converting CO₂ to formate. However, Bi^o-based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter stability issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electrocatalysts under operando CO₂RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl-containing subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi₂O₂(CO₃)_xCl_y via a cathodic potential-promoted anion-exchange process. The operando synthesized Bi₂O₂(CO₃)_xCl_y can tolerate − 1.0 V versus RHE, while for the wet-chemistry synthesized pure Bi₂O₂CO₃, the formation of metallic Bi^o occurs at − 0.6 V versus RHE. At − 0.8 V versus RHE, Bi₂O₂(CO₃)_xCl_y can readily attain a FE_HCOO- of 97.9%, much higher than that of the pure Bi₂O₂CO₃ (81.3%). DFT calculations indicate that differing from the pure Bi₂O₂CO₃-catalyzed CO₂RR, where formate is formed via a ^*OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOO⁻ over Bi₂O₂(CO₃)_xCl_y has proceeded via a ^*COOH intermediate step that only requires low energy input of 2.56 eV.