A nature-inspired metal-free electrocatalyst towards efficient electron transfer and robust cascade oxygen reduction for wastewater treatment.

The pressing demand for removing high-risk emerging contaminants from wastewater calls for tailored treatment strategies, for which heterogeneous electrocatalysis induced by cascade oxygen reduction reaction (ORR) holds considerable potential. This process, however, suffers from poor interfacial electron transfer and discounted performance in non-acidic conditions. Inspired by the electron respiration chain of cells, a metal-free, quinone-based catalyst (PBth-BQ) was innovatively designed and synthesized. With excellent redox reversibility over 50 cycles and no risk of metal leaching, it boosted the direct electron transfer by 110 % compared to the bare graphite substrate and facilitated cascade ORR to generate ·OH for effective contaminant abatement in the pH range of 3-13. Among them, pH 8 demonstrated the best performance, which is suitable for wastewater treatment. In particular, PBth-BQ performed well as both anodic and cathodic electrodes in azithromycin mineralization with different oxygen donors, verified by the in-situ mass spectrum. Considering the abundance of quinone-like structures in oxidized carbon materials, this biomimetic design may inspire the further exploration of cheap and efficient electrocatalysts for wastewater treatment.