The overcrowding effect describes how cations accumulating exceedingly near the negatively charged surface diminish the space for reactants and influence the local electrostatic potential and electric field. Specifically, the free space for species other than cations is (1 -𝑁 𝐴 𝑎 𝑐 3 𝑐 c ), with 𝑎 𝑐 being the effective diameter and 𝑐 c the concentration of cations. Theory and simulations accounting for the cation size have shown that this effect is more pronounced for cations with a larger hydrated size, 2 as depicted in Figure . Consequently, the concentration of reactant, e.g., CO2 for CO2RR and OH -for OER, at the surface follows the order Li + < Na + < K + (Figure ), which results in the same order of activity. Despite its simplicity, this rationale was shown to be relevant in explaining the cation effects in the CO2RR at Ag (Figure ), and the OER at Ni-based catalysts. Various experimental techniques have been employed to detect the local reactant concentration or the local pH, such as surface enhanced infrared absorption spectroscopy, Raman spectroscopy, scanning electrochemical microscopy. However, it is essential to acknowledge that in these experiments, the term "surface" typically refers to somewhere within the diffusion layer. The distance of the probe position to the catalyst surface varies from hundreds of nanometers to hundreds of micrometers, which is still far out from the reaction zone within the EDL. This discrepancy in the designation of the "surface" concentration, as depicted in Figure , may lead to confusion. For instance, Malkani et al. observed that the "surface" CO2 concentration follows the sequence of Li + > Na + > K + . At first glance, this shows the opposite trend to our simulations in Figure . This superficial inconsistency can be resolved by distinguishing the "surface"