Electrocatalysis, whose reaction venue locates at the catalyst-electrolyte interface, is controlled by the electron transfer across the electric double layer (EDL), envisaging a mechanistic link between the electron transfer rate and the EDL structure. To precisely control EDL structure, there are two points for engineering: electrolyte engineering and electrode engineering. A fine example of electrolyte engineering is in the CO₂ reduction reaction, of which rate shows a strong dependence on the alkali metal cation (M+) identity, but there is yet to be a unified molecular picture for that. Using quantum-mechanics-based atom-scale simulation, we herein scrutinize the M+- coupling capability to possible intermediates, and establish H+- and M+-associated ET mechanisms for CH₄ and CO/C₂H₄ formations, respectively. These theoretical scenarios are successfully underpinned by Nernstian shifts of polarization curves with the H+ or M+ concentrations and the first-order kinetics of CO/C₂H₄ formation on the electrode surface charge density. Another strategy, electrode engineering, is a change in the potential of zero charge (EPZC), an intrinsic property of electrode material and able to touch EDL structure. Endowing oxygen functional groups on transition metal single-atom catalysts, i.e., CoNC, we change the EPZC of CoNC, consequently modifying surface charge density which modulates oxygen reduction reaction activity, and selectivity towards H₂O₂.