Production of formate or CO from electrochemical CO2 reduction reactions (eCO2RRs) represents a promising way to utilize CO2 with future low-carbon electricity to produce value-added chemicals and fuels. Herein, a series of Cu–Sn composite catalysts were designed for eCO2RRs. The reduction products could be tuned selectively from formate to CO by varying the Cu/Sn composition. The catalyst Cu1Sn1 with a CuSn alloy core and a SnO shell structure doped with a small amount of Cu gives a maximum faradic efficiency (FE) of 95.4 for formate at −1.2 V. In contrast, the single-Sn-atom-doped Cu of Cu20Sn1 is selective to CO with a maximum FE of 95.3% at −1.0 V. The DFT results reveal that the existence of small amounts of Cu or Sn single atoms in these two catalysts is critical to reducing the reaction free energies of CO2 reduction, resulting in the selective formation of formate and CO, respectively.
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