Electric Field Effects in Oxygen Reduction Kinetics: Rationalizing pH Dependence at the Pt(111), Au(111), and Au(100) Electrodes

In this work, we develop a new microkinetic model for the oxygen reduction reaction (ORR) which incorporates field effects into the established computational hydrogen electrode model. We find that the field can significantly alter the binding energy of ORR adsorbates, particularly *OOH, *O2, and *H2O2. By considering these effects, we find that the model gives polarization curves and rotating ring disk electrode currents consistent with those found experimentally on the Pt(111), Au(111), and Au(100) electrodes, allowing us to predict onset potentials for both overall ORR activity and activity toward hydrogen peroxide as well as reproduce experimental Tafel slopes. In particular, we resolve the peculiar behavior observed on the Au(100) surface, where increasing pH leads to greatly enhanced 4e-ORR activity, without strongly affecting 2e-ORR activity. We then use these predictions to better understand activity across all ORR catalysts, showing that weak-binding catalysts are more influenced by pH because of the strong field effects on the *OOH adsorbate. Finally, we argue that considering field effects can expand the search for improved ORR catalysts by allowing us to deconvolute the activity dependence of catalysts on the standard hydrogen electrode and the reversible hydrogen electrode.