We next probed whether other alkali cations such as Li + , Na + and Cs + have the similar effect on the competition between HER and CO2 reduction as K + . Indeed, in 0.1 M HOTf + 0.4 M MOTf (M = Li, Na, Cs), plateaus of current densities corresponding to the diffusion-limited reduction of hydronium ions were observed (fig. ), indicating that these alkali cations also suppressed the migration of hydronium ions. We then measured CO2 electroreduction on SnO2/C and Cu/C in acidic solutions containing these alkali cations (fig. ). All alkali cations promoted CO2 reduction by inhibiting HER, but the effects are variable. On SnO2/C the Faradaic efficiency of CO2 reduction increased in the order Li < Na < K < Cs for both catalysts (fig. ), and the partial current densities of formic acid and CO increased in the same order (fig. ). On Cu/C the partial current density of ethylene increased in a similar order of Li < Na < K ≈ Cs (fig. ). In the study of Huang et al on CO2 reduction in strongly acidic medium with Cu catalysts, 15 they proposed that a high cathodic current density would lead to hydronium depletion and pH increase near the cathode. Water instead of hydronium reduction then contributes to the majority of HER currents. Alkali cations were then proposed to favor CO2 reduction over water reduction, although the details of this promotion were not studied. This mechanistic hypothesis does not take into account the migration of hydronium ions under an electric field during HER. It cannot explain why in alkali cation-free solutions the HER current density can exceed the diffusion limit of hydronium ions and why the addition of alkali cations can suppress the HER current density to a limiting value, as observed in our study.