First step of the oxygen reduction reaction on Au(111): A computational study of O2 adsorption at the electrified metal/water interface

Local field effects at the electrical double layer change the energies of reaction intermediates in heterogeneous electrocatalysis. The resulting dependence on (absolute) electrode potential can be pivotal to a catalyst's performance in acid or alkaline media. And yet, such local field effects are very difficult to describe theoretically and are often ignored. In this work, we focus on O2 adsorption as the first step of the oxygen reduction reaction (ORR) on Au(111). Different physical effects of the local field are elucidated and compared by systematically improving the model of the double layer: from an applied saw-tooth potential in vacuum, to an implicit solvent model, and explicitly modeled water via ab initio molecular dynamics (AIMD). We find all models predict a dominant dipole-field type interaction that significantly strengthens O2 binding at increasingly reducing conditions. However, only an atomically-resolved solvent model such as provided by AIMD can properly capture the additional stabilization due to explicit H-bonding from the water network. This contribution comes with the formation of a peroxo-like surface species and a more dramatic field response around the ORR onset. Our results overall demonstrate the importance of including local electric field effects in models of the electrochemical interface, while assessing the level of detail that is required to be accounted for.