A critical review on regulating multi-electron pathways of oxygen reduction reaction on modified carbon-catalysts for organic wastewater treatment

The oxygen reduction reaction (ORR) including 2-elelctron, (2 + 1)-electron, and 4-electron pathways is crucial electrochemical reaction in a series of technologies for renewable energy utilization and environmental pollutants treatment. This review summarized different strategies to regulate the ORR process of modified carbon, each considering the characteristic mechanism that promote the design of active sites and techniques for regulating electron transfer. The design of ORR active sites in modified carbon encompasses heteroatom-doping, metal-heteroatom interaction and pore/defects engineering. The strategies for regulating electron transfer to enhance the ORR involve applying carbon catalysts with single-atom or confinement effects, promoting Fe(II)/Fe(III) cycles and introducing other reactive species. Subsequently, the performance and durability of ORR electrocatalysts for resource utilization and wastewater treatment by electro-Fenton, or in-situ electrocatalysis or microbial fuel cells were investigated to explore the efficiency of multi-electron pathway. Finally, its challenges and future prospects are outlined. This review would help to elucidate the multi-electron ORR mechanism in different electrochemical systems and to provide essential insights for the design and application of ORR electrocatalysts for organic wastewater treatment.