How CO ₂ Self-Consumption Distorts the Apparent Tafel Slope

The Tafel slope is a critical descriptor of electrocatalytic performance; yet, its interpretation is frequently complicated by mass transport limitations. This is particularly true for the electrochemical reduction of CO 2 (eCO 2 R), where the consumption of CO 2 near the catalyst surface can obscure the intrinsic reaction kinetics. Here, we combine theoretical modeling with a reanalysis of experimental data to separate the contributions of mass transport and intrinsic kinetics to the measured Tafel slope for CO 2 reduction to CO on Au catalysts. Our analysis reveals that the “self-consumption” of CO 2, where it reacts with hydroxide produced during the reaction and significantly depletes the local CO 2 concentration. This depletion leads to an apparent Tafel slope that is considerably larger than the intrinsic value, particularly at higher current densities. By correcting for this local concentration change, we find that the previously reported Tafel slopes for eCO 2 R on Au are likely overestimated. Our findings highlight the critical need to account for mass transport effects in both theoretical and experimental studies to accurately determine the intrinsic activity of electrocatalysts. We recommend that a detailed analysis of the local reaction environment should precede any mechanistic claims based on measured Tafel slopes.