Our study provides a comprehensive understanding of the mechanism underlying eCO 2 R on Ag surfaces in a wide range of electrolyte environments. We elucidated key trends in eCO 2 R by explicitly considering the role of cations across different length scales and identifying the catalytic active site as a microenvironment composed of the Ag surface sites and partially dehydrated cations. The differences observed in the performances of various electrolytes depend not only on the energetics of the system but also on transport properties. Specifically, the cation accumulation at OHP promotes the formation of microenvironments but induces a CO 2 transport limitation due to size exclusion effects. The operating conditions for eCO 2 R need to be optimized accounting for both these aspects. a descriptive parameter of the number of microenvironments at OHP. This plot loosely resembles a volcano, but includes both transport and kinetic effects. At low buffer concentrations, cations with smaller hydrated diameters (Cs + , K + ) reach higher current densities with an increase in the number of microenvironments if j CO is in the kinetic regime. At higher buffer concentrations, when the CO 2 transport is unfavorable, the system switches to the transport limited side. Cations with larger hydrated diameters such as Li + quickly reach CO 2 transport limitations at typical operating potentials, making them unsuitable for reaching higher current densities.