Ab-initio design of microenvironments for eCO 2 R requires the full deployment of a multiscale approach, considering both the atomic-scale and transport effects of the electrolyte on the current densities. Therefore, we built an ab-initio multiscale model accounting for the cation role at all scales. Specifically,the cation transport from the OHP to the electrode surface was considered as a reaction step in the microkinetic model and the stabilization of the reaction intermediates in microenvironments composed of a cation and a surface site was evaluated. The model reproduces trends in experimental data for eCO 2 R on Ag in different alkali metal MHCO 3 electrolytes and quantifies the local operating conditions. We also considered an AEM-type solid electrolyte at different hydration levels. Using this methodology, we can design the catalyst-electrolyte microenvironments and predict the best electrolyte and operating conditions.