The electrocatalytic reduction of CO₂ (CO₂R) presents a sustainable pathway for producing fuels and chemicals. Cations have a strong influence on the reaction activity and selectivity, and there are currently three theories regarding their main role: (1) to modify the local electric field, (2) to buffer the interfacial pH and (3) to stabilize reaction intermediates. Here, we tested these theories, and were able to define the main mechanism that explains how cations interact in the CO₂R reaction. Cyclic voltammetry and Scanning Electrochemical Microscopy (SECM) measurements were performed without metal cations and in the presence of Cs⁺. On gold, CO₂R does not take place at all in pure 1 mM H₂SO₄, unless Cs⁺ ions are added to the electrolyte (Fig. a). We used SECM to locally probe if CO is produced on other common CO₂R catalysts, namely copper and silver. Without a cation in solution, CO₂R does not take place at all, regardless the metal surface. These remarkable observations allow us to define the main role of cations in the CO₂R reaction, and to propose a new reaction mechanism in which the cation is actually what enables the reaction to happen, by forming a complex with CO₂ and allowing the first reaction intermediate to be formed (Fig b). We supported this model with Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, which confirmed that CO₂R happens only if the CO₂^– intermediate is stabilized via a cation-oxygen chemical bond, which occurs depending on alkali cation solvation shell and, thus, on radius. From the system design point-of-view, it is desired to find charged species that have an even larger stabilizing effect on CO₂^–_(ads) than Cs⁺. Based on that, we have also performed CO₂R in acidic media in electrolytes containing: Li⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Nd³⁺ and Ce³⁺. We find via experiments and DFT/AIMD that acidic cations with a moderate hydration radius, promote CO₂R in acidic media/low overpotentials, while the non-acidic weakly hydrated Cs⁺ is the cation that leads to more CO in alkaline media/high overpotentials. These differences come from the promotional effects acidic cations have on water reduction, only allowing these species to favor the selectivity towards CO₂R before the onset of this reaction. In this talk, we elucidate through experiments and simulations the main role of cations on CO₂R, and also which cation properties are important when designing an optimal electrolyte for this system.