Converting CO2 emissions, powered by renewable electricity, to produce fuels and chemicals provides an elegant route towards a carbon-neutral energy cycle. Progress in the understanding and synthesis of Cu catalysts has spurred the explosive development of electrochemical CO2 reduction (CO2RR) technology to produce hydrocarbons and oxygenates; however, Cu, as the predominant catalyst, often exhibits limited selectivity and activity towards a specific product, leading to low productivity and substantial post-reaction purification. Here, we present a single-atom Pb-alloyed Cu catalyst (Pb1Cu) that can exclusively (~96% Faradaic efficiency) convert CO2 into formate with high activity in excess of 1 A cm–2. The Pb1Cu electrocatalyst converts CO2 into formate on the modulated Cu sites rather than on the isolated Pb. In situ spectroscopic evidence and theoretical calculations revealed that the activated Cu sites of the Pb1Cu catalyst regulate the first protonation step of the CO2RR and divert the CO2RR towards a HCOO* path rather than a COOH* path, thus thwarting the possibility of other products. We further showcase the continuous production of a pure formic acid solution at 100 mA cm–2 over 180 h using a solid electrolyte reactor and Pb1Cu.