The proton reduction reaction, responsible for decreasing the CO FE at less cathodic potentials , is a strong function of pH. As shown in Figure (c), the pH OHP decreases initially with potential due to the double layer charging, and later increases due to the strong OH -flux from the electrochemical reactions and the size exclusion effect of cations crowding the OHP. The larger ∆ϕ OHP at higher buffer concentrations (due to high [K + ] OHP ) leads to a less negative local potential in the diffusion layer, hindering H + migration through a cation-shielding effect (Figure ) . ), leading to a drop in FEs. Our model effectively predicts and explain the well-known bell curve shape of the Faradaic efficiency, in line with experimental observations (Figure ).