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 HO. In contrast to the production of HO 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.