We considered the case of eCO 2 R on Ag(111) in a CO 2 -saturated KHCO 3 buffer for validation. Different starting concentrations of KHCO 3 were examined to identify the best operating conditions in a H-cell type reactor. Current densities for CO 2 R via H 2 O (mechanism 1) are given in Figure (a) (see figure for the current densities obtained from all the mechanisms). From the degree of rate control analysis in Figure we determine two areas. At less negative potentials, the CO 2 to COOH step is rate limiting (kinetic-limited), while at more negative potentials, the adsorption of CO 2 from the double layer is limiting (transport-limited). This shift in the rate-determining step corresponds to the peak in current densities, which divides the potential in two regions. At lower magnitude potentials, in the kinetically limited region (more than about -1.4 V RHE ), higher cation concentration favors CO 2 RR, while at higher magnitude potentials, in the transport limited region (below about -1.4 V RHE ) the trend is inverted, with lower buffer concentrations leading to higher current densities. For the 0.1 M KHCO 3 system, current densities are about two orders of magnitude lower than in the experimental data .