Origin of Selective Production of Hydrogen Peroxide by Electrochemical Oxygen Reduction.

Abstract

Oxygen reduction reaction (ORR) is one of the most important electrochemical reactions. Starting from a common reaction intermediate *-O-OH, the ORR splits into two pathways, either producing hydrogen peroxide (H2O2) by breaking the *-O bond or leading to water formation by breaking the O-OH bond. However, it is puzzling why many catalysts, despite the strong thermodynamic preference for the O-OH breaking, exhibit high selectivity for hydrogen peroxide. Moreover, the selectivity is dependent on the potential and pH, which remain not understood. Here we develop an advanced first-principles model for effective calculation of the electrochemical reaction kinetics at the solid-water interface, which were not accessible by conventional models. Using this model to study representative catalysts for H2O2 production, we find that breaking the O-OH bond can have a higher energy barrier than breaking *-O, due to the rigidity of the O-OH bond. Importantly, we reveal that the selectivity dependence on potential and pH is rooted into the proton affinity to the former/later O in *-O-OH. For single cobalt atom catalyst, decreasing potential promotes proton adsorption to the former O, thereby increasing the H2O2 selectivity. In contrast, for the carbon catalyst, the proton prefers the latter O, resulting in a lower H2O2 selectivity in acid condition. These findings explain the experiments and highlight the kinetic origins of the selectivity. Our work improves the understanding of ORR by uncovering the proton affinity as a new factor and provides a new model to effectively simulate the atomic-level kinetics of heterogeneous electrochemistry.