Origin\nof Selective Production of Hydrogen Peroxide\nby Electrochemical Oxygen Reduction

Abstract

Oxygen reduction reaction (ORR) is\none of the most important electrochemical\nreactions. Starting from a common reaction intermediate *–O–OH,\nthe ORR splits into two pathways, either producing hydrogen peroxide\n(H₂O₂) by breaking the *–O bond or leading\nto water formation by breaking the O–OH bond. However, it is\npuzzling why many catalysts, despite the strong thermodynamic preference\nfor the O–OH breaking, exhibit high selectivity for hydrogen\nperoxide. Moreover, the selectivity is dependent on the potential\nand pH, which remain not understood. Here we develop an advanced first-principles\nmodel for effective calculation of the electrochemical reaction kinetics\nat the solid–water interface, which were not accessible by\nconventional models. Using this model to study representative catalysts\nfor H₂O₂ production, we find that breaking the\nO–OH bond can have a higher energy barrier than breaking *–O,\ndue to the rigidity of the O–OH bond. Importantly, we reveal\nthat the selectivity dependence on potential and pH is rooted into\nthe proton affinity to the former/later O in *–O–OH.\nFor single cobalt atom catalyst, decreasing potential promotes proton\nadsorption to the former O, thereby increasing the H₂O₂ selectivity. In contrast, for the carbon catalyst, the proton\nprefers the latter O, resulting in a lower H₂O₂ selectivity in acid condition. These findings explain the experiments\nand highlight the kinetic origins of the selectivity. Our work improves\nthe understanding of ORR by uncovering the proton affinity as a new\nfactor and provides a new model to effectively simulate the atomic-level\nkinetics of heterogeneous electrochemistry.