Abstract: Bi-O species are considered as the active sites of Bi-based catalysts for the production of formic acid through electrochemical CO₂ reduction (ECO₂RR). However, huge challenges remain because of the chemically unstable Bi-O active sites under high reduction potentials. Herein, we report a Ce-doped metallic bismuth nanosheet (Ce-Bi NS) with enormous surface Bi-O species for ECO₂RR. Specifically, an in situ electrochemical reduction strategy was adopted to reduce the Ce-doped Bi₂O₃ precursor, preserving the Bi-O species on Ce-Bi NS. As such, the resultant Ce-Bi NS electrocatalysts exhibit excellent ECO₂RR performance with a Faradaic efficiency for formate of 95.8 % and partial formate current density of 1.08 A cm^-2, achieving a formate production rate of 20.1 mmol h^-1 cm^-2 in the flow cell. In situ Raman spectra, theoretical calculations, and electrochemical characterizations reveal that Ce doping induced the formation of abundant Bi-O species on the surface of Ce-Bi NS and can greatly promote CO₂ adsorption on the catalyst surface, facilitating the initial electron-transfer step of ECO₂RR for forming the key CO₂^- intermediates. Furthermore, a pure formic acid solution is produced using a membrane electrode assembly reactor containing a solid-state electrolyte and Ce-Bi NS as a catalyst, and the pure formic acid solution can be obtained continuously and stably for 200 h at 100 mA cm^-2. This work demonstrates the critical role of Bi-O species in ECO₂RR and offers a simple yet effective method for creating a stable Bi-O structure on ECO₂RR catalysts.

Key words: Electrocatalyst, CO₂ reduction, Cerium doping, Bismuth, Bi-O species