Understanding the active species on the catalyst surface under realistic reaction conditions is crucial for rationally designing efficient catalysts [1].Electrochemical CO2 reduction reaction (eCO2-RR) is considered as a promising way to convert greenhouse gas CO2 to useful fuel and chemicals [2], At present, many efforts have been devoted into hunting for high-performance eCO2RR catalysts,while the true nature of the active species on the surface of the electrocatalyst is often unclear [3,4].Taking metal oxides as exampies, many studies show that the metallic surface is the ultimate form under reaction conditions, while there are some studies arguing that the presence of oxygen species on the catalyst surface/subsurface could affect the electrocatalyst properties [5,6].