Electrochemical CO2 reduction is expected to become a key player in net-zero technologies, yet its industrial implementation is currently limited. Improvements based on fine-tuning microenvironments (that is, electrolyte environments around 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 modelling, explicitly including electrolyte effects at all scales. The resulting model is validated on silver 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 to be achieved to obtain optimal rates. In ionomers, this limitation can be overcome since organic cation-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.