Elementary Mechanisms in Electrocatalysis: Revisiting the ORR Tafel Slope

We use microkinetic modeling to demonstrate that deviations from ideal Tafel kinetics, which assume a linear relationship between overpotential and log-current, are an inherent property of multi-step heterogeneous electrocatalytic reactions. We show that in general, deviations from ideal Tafel behavior can often be attributed to a simultaneous increase in the rate of the rate-limiting elementary step and a change in the number of available active sites on the electrode as overpotential is induced. Our analysis shows that in the oxygen reduction reaction (ORR) on Pt electrodes, which exhibits nonlinear Tafel behavior, changing electrode potential affects not only the rate-limiting step (the initial electron transfer to molecular oxygen), but also the concentration of surface intermediates—mainly OH and H2O. Based on comparison of measured and predicted changes in Tafel slope (as well as pH dependence), we show that alternative interpretations of the non-ideal Tafel behavior of ORR on Pt, such as changes in rate-limiting step or adsorbate repulsion effects, are inconsistent with the observed ORR kinetics.