Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•-) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O-2(center dot-)) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h(-1) g(catalyst)(-1). Experimental and theoretical studies revealed the mechanism of O-2(center dot-) generation via oxygen activation, including one- and three-electron transfer pathways, and found that O-2(center dot-) mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.