An electrocatalytic carbon dioxide reduction reaction (CO 2 RR) is an appealing route to obtain the valueadded feedstocks and alleviate the energy crisis. However, how to achieve high-performance electrocatalysts for CO 2 reduction to formate is challenging owing to the poor intrinsic activity, insufficient conductivity, and low surface density of active sites. Herein, we fabricated an extremely active and selective hydrangealike superstructured micro/nanoreactor of ultrathin bismuth nanosheets through an in situ electrochemical topotactic transformation of hierarchical bismuth oxide formate (BiOCOOH). The resulted bismuth nanosheet superstructure is in the form of three-dimensional intercrossed networks of ultrathin nanosheets, forming an ordered open porous structure through self-assembly, which can be used as a micro/nanoreactor to enable a large electrochemically active surface area as well as high atomic utilization. Such a distinctive nanostructure endows the material with high electrocatalytic performances for CO 2 reduction to formate with near-unity Faradaic selectivity (>95%) in a wide potential window from −0.78 to −1.18 V. Furthermore, this micro/nanoreactor can give the high current densities over 300 mA cm −2 at low applied potentials without compromising selectivity in a flow cell reactor. Density functional theory (DFT) and in situ attenuated total reflection-infrared spectroscopy (in situ ATR-IR) were further conducted to interpret the CO 2 RR mechanisms.

The reduction of carbon dioxide (CO2) into value‐added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high‐performance electrocatalysts for CO2 reduction reaction (CO2RR) with high formate selectivity and good stability need to be improved. Earth‐abundant Bi has been demonstrated to be active for CO2RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi‐NSs) assembly via an in situ electrochemical transformation of (BiO)2CO3 nanostructures. The as‐prepared material exhibits high activity and selectivity for CO2RR to formate, with nearly 94% faradaic efficiency at −1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from −0.83 to −1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi‐NSs based CO2RR/methanol oxidation reaction (CO2RR/MOR) electrolytic system for overall CO2 splitting was constructed, evidencing the feasibility of its practical implementation.