The multiscale approach employed in this study, including the role of cations in CO 2 reduction, is described in Figure and experiments reveal very little effect on acidic HER currents across monovalent cations , therefore the effect of cation on H + reduction is not considered. In this work, mechanisms 1 and 2 were always considered, while mechanisms 3 and 4 were only included in the less negative potentials range. We also consider the HCOOH formation for K + and Li + buffers on Ag(111). As the discussions on the role of carbonic acid and bicarbonate ions as proton donors to produce H 2 and their contribution to CO 2 R is not conclusive, we have not considered these effects.