To assess the impact of mass transport and bulk reactions on observed partial current densities, a sensitivity analysis was carried out (Section S5). By this means, the sensitivity of the product partial current densities to changes in the kinetic parameters (𝛼 5,$ and 𝑖 5," ) is determined (Figure ). This procedure enables an assessment of the degree of sensitivity between the outputs and input parameters in the model (i.e., how much an output parameter changes when an input parameter is changed). When an output is positively sensitive to an input, an increase in the input results in an increase in the output; when an output is negatively sensitive to an input, an increase in the input results in a decrease in the output. The sensitivity analysis revealed that the CO partial current density is highly negatively sensitive to mass transfer (i.e., 𝐿 () ) due to low availability of CO2 in the aqueous electrolyte; however, the H2 partial current density is less sensitive to mass-transfer effects because of the high availability of solvent water, which was assumed to be the proton source for HER (Figure , S6). We also observed that the CO partial current density was positively sensitive to 𝑖 ",+, and 𝛼 +, , but more interestingly, negatively sensitive to the HER kinetics (𝑖 ",. ! and 𝛼 . ! ) (Figures ). As the HER current increases, the pH rises due to concomitant generation of OH -, and the generated OH -anions consume reactant CO2 via homogeneous buffer reactions thereby reducing the COER partial current.