Electrocatalytic reduction of CO2 into valuable fuels and chemicals using renewable electricity offers a promising approach to sustainable production of fuels and chemicals. In 1985, Hori et al. discovered Cu's unique catalytic activity in electrochemically reducing CO2 to hydrocarbons. Later, in 2012, Kuhl et al. found that copper under CO2 reduction conditions could produce at least 16 different products including CO, C1, C2, and C3 products. However, the nature of the formation of this extensive range of products has been a subject of intense debate. Unraveling this mystery is critical to the designing of a reaction system with superior efficiency and selectivity towards desired products.