Tuning Three-Electron Oxygen Reduction Pathway for •OH Production from O2: A Critical Review of Fundamental Principles, Catalyst/Electrode Development, and Application

Abstract

The oxygen reduction reaction (ORR) has been widely studied and applied in various fields. Traditionally, the ORR could be classified into a two-electron pathway with hydrogen peroxide (H₂O₂) as the product and a four-electron pathway with water (H₂O) as the product. Recently, a three-electron pathway, namely, catalyzing O₂ directly into a hydroxyl radical (•OH), has attracted growing attention. Given the development prospects of this emerging reaction, this review focuses on the fundamental principles, catalytic material developments, and applications of the three-electron ORR-based Fenton (like) process. New insight into two/three/four-electron ORR based on proton/electron flow is illustrated, and the required features as well as electrode design strategy are summarized. Extensive discussions on the development and application of monometallic, polymetallic, and metal-free three-electron ORR electrodes are provided. The catalytic mechanisms involved in reactive oxygen species (ROSs) formation, structure–function relationship, and key active site transformation are presented. Finally, the challenges and future prospects of the three-electron ORR are discussed.