The effect of different alkali cations can also be explained by the model in fig. . The size of hydrated alkali cations decreased from Li to Cs. As the size of hydrated cations decreases, more alkali cations can accumulate in the OHP, leading to a stronger electric field in Stern layer and a weaker electric field in diffuse layer. This explains how the size of hydrated alkali cations affects the competition between HER and CO2 reduction in strongly acidic media. It is noteworthy that the partial current density of CO2 reduction could significantly exceed the diffusion limitation of hydronium ions in the presence of K + (fig. ), implying that water molecules are the proton source for CO2 reduction. Thus, CO2 reduction leads to the formation of OH -ions, which further react with hydronium ions near the electrode and suppress HER. Due to this effect, Faradaic efficiency of CO2 reduction reached 90% for SnO2/C and Au/C catalysts (fig. ).