Electrochemical CO2 reduction will be a key player in net-zero technologies, yet its industrial implementation is limited. Improvements by fine-tuning the microenvironments, electrolyte environments around the catalytic sites, have been scarce due to the interplay between electrode kinetics and transport. Here, we couple atomistic insights with continuum transport via ab-initio multiscale modeling, explicitly including electrolyte effects at all scales. The model was validated on Ag planar electrodes in several liquid electrolytes and the current dependence with voltage aligns with experimental observations. We show that a balance between CO2 diffusion and cation accumulation needs be achieved to obtain optimal rates. In ionomers, this limitation can be overcome since organic cations-based microenvironments are present at a fixed concentration, but water management becomes critical. Our approach paves the way towards rational microenvironment design in electrochemical CO2 conversion.