In Situ Production of Hydroxyl Radicals via Three-Electron Oxygen Reduction: Opportunities for Water Treatment.

The electro-Fenton (EF) process is an advanced oxidation technology with significant potential; however, it is limited by two steps: generation and activation of H 2 O 2 . In contrast to the production of H 2 O 2 via the electrochemical two-electron oxygen reduction reaction (ORR), the electrochemical three-electron (3e - ) ORR can directly activate molecular oxygen to yield the hydroxyl radical (⋅OH), thus breaking through the conceptual and operational limitations of the traditional EF reaction. Therefore, the 3e - ORR is a vital process for efficiently producing ⋅OH in situ, thus charting a new path toward the development of green water-treatment technologies. This review summarizes the characteristics and mechanisms of the 3e - ORR, focusing on the basic principles and latest progress in the in situ generation and efficient utilization of ⋅OH through the modulation of the reaction pathway, shedding light on the rational design of 3e - ORR catalysts, mechanistic exploration, and practical applications for water treatment. Finally, the future developments and challenges of efficient, stable, and large-scale utilization of ⋅OH are discussed based on achieving optimal 3e - ORR regulation and the potential to combine it with other technologies.