Electrochemically converting CO2 into valuable chemicals and fuels in acidic media is argued as a promising energy- and carbon-efficient route. Although several key roles of alkali cations have been unveiled, the alkali cation trends for CO2 reduction remain largely elusive. With decreasing cation size from Cs+ to Li+, here we show that the apparent proton diffusion coefficient in 3.0 M Li+ is tens-fold lower than in 3.0 M K+ and 3.0 M Cs+ acidic electrolytes. Although Li+ has the strongest inhibition ability for proton transport, it acts the worst for both the CO2-to-CO conversion and partial current density on Au catalysts. Unexpectedly, K+ with a higher proton transport performs the best for CO2-to-CO conversion. We thus revisit the roles of alkali cations and find that hydrated K+ can stabilize hydrogen radicals benefiting CO2 conversion at the electrode interface while for Li+ this is not the case. This study proposes that cation-stabilized atomic hydrogen assists in activating CO2 via a reverse water-gas shift route under electrochemical conditions.