The effects of cation type and concentration have been explored for various electrocatalytic reactions. Similar surface charge effects can also explain cation effects in the HER, ORR 58 and OER. Huang et al. rationalized cationdependent kinetics of the HER/HOR by considering the influence of cations on the interfacial water structure and H-bonding network. Furthermore, Qin et al. proposed that the CO2RR proceeds through an inner-sphere electron transfer pathway in presence of alkali cations and, in contrast, through an outer-sphere electron transfer pathway in systems without cations. Most of the above mechanisms assume that the binding energies of adsorbed intermediates are affected by the electric field, which is then modulated by the cations. Following this line of thermodynamic binding-energy approach, we would expect opposite sequences of cation size effects for metals on the left and right legs of the volcano plot. Specifically, for metals on the right leg of the volcano plot, the activity follows the sequence of Li + < Na + < K + < Cs + . Conversely, for metals on the left leg of the volcano plot, the activity should follow the sequence of Li + > Na + > K + > Cs + . Xue et al. observed opposite trends of cation size effects on the HER at Pt or Au, which adsorb hydrogen too strongly or too weakly, respectively. However, opposite trends are absent for the CO2RR. In particular, for CO2RR to HCOOH, Sn locates at the peak of the volcano plot. Therefore, enhancing the adsorption of the key intermediate, *OCHO, would be expected to decrease the activity. In contrast, since Ag lies at the right leg of the volcano plot, enhancing the adsorption of *OCHO would increase its activity. However, experiments have shown that the CO2RR to HCOOH follows the sequence of Li + < Na + < K + < Cs + at both Sn and Ag. This discrepancy has motivated us to look beyond the binding-energy approach and introduce an electrostatic factor, i.e., the cation overcrowding effect, into consideration. This mechanism was previously acknowledged by Frumkin et al. in the study of peroxydisulfate anion reduction when the surface charge is very negative. We demonstrated that the cation overcrowding effect offers an alternative or at least complementary explanation to previously observed cation effects.