In view of the formation of CO, we suggest it is formed by a radical elimination reaction. In our scheme, two radicals (O=C • -O-Cu) must be adjacent for dimerization. However, the majority of radicals (O=C • -O-Cu) are isolated because the electrochemical CO2 reduction requires a media to dissolve gaseous CO2 and those media could reduce the statistical probability of two O=C • -O-Cu radical forming on neighboring surface Cu atoms. This probably is the main reason that C2 products usually have lower yields than CO and C1 products.