In addition to hydrodynamics, the presence of bulk reactions occurring in the thin boundary layer can impact the apparent Tafel slopes. Figure and S8 illustrate the effect of the HER current density on apparent CO kinetics, demonstrating that at high current densities for HER, the apparent CO kinetic parameters can be quite inaccurate. However, this competition for reactant CO2 is indirect and is a result of consumption of HER-generated OH -anions to form HCO3 -and CO3 2-anions in the boundary layer, as opposed to direct electrochemical consumption.