On the Origin of Electrocatalytic Selectivity during the Oxygen Reduction Reaction on Au(111)

A puzzling observation during the oxygen reduction reaction (ORR) on weak- binding electrodes such as Au is the preference to form hydrogen peroxide (H2O2), instead of the thermodynamically favored water product. This selectivity cannot be explained on the basis of thermodynamic reaction models that simply assume a series of proton-coupled electron transfers (PCETs). Here, we use ab initio molecular dynamics along with umbrella sampling to obtain free energy profiles for competing key ORR steps on Au(111). Our comparison includes not only PCETs, but also “chemical” reaction steps that do not include an explicit faradaic charge transfer, such as desorption or surface dissociation. This allows to explore favorable reaction paths, while varying the capacitive charging to represent realistic ORR potentials. Our results show that all reaction steps competing with H2O2 formation have sizeable kinetic barriers and are thus prohibited, even though they may be thermodynamically favored. We find that this situation does not change under more reducing conditions and specifically determine the “nobleness” of Au as playing a decisive role in preventing O-O bond scission. It is thus not the applied potential, but the underlying chemistry that drives the ORR selectivity. Our study overall further highlights the kinetic competition between PCET and non-PCET steps that cannot be resolved via simple Brønsted-Evans-Polanyi scaling relations.