A nanosheet CH3COO(BiO) topotactically converted into nanocomposite of bismuth clusters and Bi2O2CO3 for highly efficient electrocatalytic reduction of CO2 to formate

•The CH3COO(BiO) exhibits high ECO2RR activity and stability.•The CH3COO(BiO) transforms to Bi clusters-Bi2O2CO3 during electrolysis.•The Bi clusters-Bi2O2CO3 has unique charge redistribution.•The Bi3+ in Bi2O2CO3 facilate the CO2 activation to *CO2.•The Bi in Bi cluster accelerates the proton-coupled electron transfer. Hydrogen is a promising clean energy source, and formic acid serves as a significant hydrogen storage material. The electrocatalytic reduction of carbon dioxide (ECO2RR) is a promising approach to produce fromate. However, the challenge lies in developing high-performance electrocatalysts for the conversion of CO2 to formate, due to the limited intrinsic activity, conductivity, and the low density of active sites. In this work, we have fabricated highly active and selective CH3COO(BiO) (BiOAc) nanosheets. These nanosheets undergo a transformation into Bi clusters-BiO2CO3 during electrolysis, endowing the material with superior electrocatalytic capabilities for CO2 reduction to formate, with a Faradaic selectivity of over 90 % across a wide potential window from −0.8 to −1.3 V. Additionally, this catalyst can sustain high current densities above 80 mA cm−2 at low applied potentials without compromising selectivity in a H type electron reactor. Both density functional theory (DFT) and in situ attenuated total reflection-infrared spectroscopy (in situ ATR-IR) findings indicate that the Bi3+ with reduced density facilitate the activation of CO2 to *CO2, while the charge density-enriched Bi0 atoms in Bi cluster promote the capture of HCO3– and enhance subsequent proton-coupled electron transfer reactions. Collectively, these factors significantly reduce the energy barrier for *OCHO formation on BiOAc, thereby enhancing its catalytic activity and Faradaic efficiency for the electroreduction of CO2.