The reaction of carbon dioxide with hydroxide to form carbonate in near-neutral or alkaline medium severely limits the energy and carbon efficiency of CO2 electroreduction. Here we show that by suppression of the otherwise predominant hydrogen evolution using alkali cations, efficient CO2 electroreduction can be conducted in acidic medium, overcoming the carbonate problem. The cation effects are general for three typical catalysts including carbon-supported tin oxide, gold and copper, leading to Faradaic efficiency as high as 90% for formic acid and CO formation. Our analysis suggests that hydrated alkali cations physisorbed on the cathode modify the distribution of electric field in the double layer, which impedes hydrogen evolution by suppression of migration of hydronium ions while at the same time promoting CO2 reduction by stabilization of key intermediates.