The second important consequence of the formation of the EDL is the drop in pH in the double layer region due to both, a higher concentration of positively charged H + , and a depletion of negatively charged OH -from the region measuring a few nm from the electrode surface (Figures and). The pH at the OHP drops to a threshold value and the profile extends into the double layer region at higher voltages analogous to what is observed for the accumulation of the K + ions in Figure . Assuming point species and not considering volume exclusion within the PNP model, leads to an unphysically low pH value at the OHP (Figure ). Figures show the pH and OH -concentration profiles for varying values of total current densities at a fixed potential and Faradaic efficiency distribution computed using the GMPNP model. The results obtained using the reaction-diffusion (RD) model are shown in Figures and for comparison. As expected, the pH at the OHP as well as at its peak value are found to be proportional to the total current density and the pH attains a maximum value beyond the EDL at a distance of ∼5 nm from the OHP (Figures and). The RD model predicts a considerably higher pH at x=0 and also, as expected, does not capture the drop in the pH due to migration effects very close to the electrode surface as predicted by the GMPNP model (also see Figure ). Similarly, the OH -concentration calculated using GMPNP is completely depleted at the OHP at high voltages (Figures and) whereas the peak value beyond the EDL is proportional to the total current density. The RD model however, predicts a ∼4 times higher rise in OH -concentration at the electrode relative to the peak value predicted by GMPNP and does not account for its depletion due to electrostatic repulsion in the EDL (also see Figure ). We have also considered a limiting proton current case where no more than 10% of the bulk concentration of H + is allowed to accumulate at the OHP and the protons get consumed via the catalytic reactions ( ) and ( ) in the SI. Adding the flux of protons at the catalyst surface leads to the pH profiles for all current densities to overlap and a pH value of 8 is obtained at the OHP with a maximum value of ∼9.7 beyond the EDL (Figure S5 in SI).The current attributed to the H + flux for H 2 and CO production is <0.5% of the total current density for all cases considered. Numerous studies of CO 2 ER systems have shown a strong correlation between pH of the electrolyte in the vicinity of the catalyst (or the so-called local pH) and the overall performance, especially the catalyst selectivity . Our results demonstrate that the EDL leads to pH and OH -concentration profiles which do not monotonically increase towards the catalyst surface but in fact, become considerably diminished in the immediate vicinity of the OHP up to a distance of ∼5 nm. Since the OH -concentra-tion is depleted from the EDL region, the pH at the OHP at high applied potentials will have a stronger dependence on the rate of H + consumption in catalytic processes than the total current density. Therefore, we believe that there is a strong need to explicitly define what is meant by the term 'local' in the context of pH and concentrations of solution species when discussing mechanistic insights and relative performance of CO 2 ER systems. In addition, Figure shows the extent of the deviation of homogeneous reactions ( ), ( ) and ( ) from equilibrium at steadystate as defined by the equations ( ), ( ) and ( ) in the SI. The buffer reaction of CO 2 to HCO - 3 (reaction ( )) is too slow to reach equilibrium and is deficient of HCO - 3 in the entire Nernst domain. Therefore even though the concentration of OH -rises due to CO 2 ER, the competing rates of diffusion and reaction kinetics imply that CO 2 is still present in excess in reaction (8). This is in line to what has been shown previously using spectroscopic measurements . The HCO - 3 to CO 2 - 3 buffer reaction (reaction (7)) remains in equilibrium in the entire domain outside the EDL which is expected considering that k a1 is 6 order of magnitudes bigger than k b1 . The water-dissociation reaction (reaction ( )) also deviates from its equilibrium due to competing reaction and mass transport rates. It should be noted that the results shown in Figure remain qualitatively similar even if a reaction-diffusion model is considered. The dramatic variation of the pH profile as a function of distance from the OHP (Figure ) as well as the deviation of the buffer reactions from their equilibrium are important factors that should be considered in experimental studies aiming to measure pH in CO 2 ER systems using for e.g. spectroscopic techniques.