Here we demonstrate efficient CO2 electroreduction with Faraday efficiency as high as 90% in strongly acidic media (pH =1) by suppressing HER with alkali cations. This approach can be applied for three representative classes of catalysts, namely carbon supported SnO2, Au, and Cu nanoparticles (SnO2/C, Au/C and Cu/C), which give formic acid, CO, and hydrocarbons as main CO2 reduction products, respectively. Our simulation and analysis indicate that the alkali cations in the double layer of the cathode effectively shield the electric field in the diffusion layer and suppress the migration of hydronium ions towards the cathode, which lowers the concentration of hydronium ions in outer Helmholtz plane (OHP) and thus suppresses HER.