Fluorine enhanced pyridinic-N configuration as an ultra-active site for oxygen reduction reaction in both alkaline and acidic electrolytes

An ultra-active metal-free ORR electrocatalyst (F–N-AC-1000) was developed by fluorine doped N-based activated carbon. The as-prepared F–N-AC-1000 exhibited half-wave potentials of 0.89 V and 0.75 V (vs. RHE) in alkaline and acidic electrolytes, respectively, which are both among the most active metal-free ORR catalysts reported in the literature. F–N-AC-1000 also showed much higher methanol tolerance and stability than the commercial Pt/C catalyst. Experimental and theoretical calculation results demonstrated a novel viewpoint on synergistic effect that the superior ORR performance could be mainly ascribed to the incorporation of fluorine atom into the pyridinic-N configuration, and the charge and spin densities of the adjacent C atoms (Lewis base site) were significantly improved in comparison with the absolute pyridine structure. High ORR performance was attributed to the enhanced Lewis base site rather than the increase of the total content of active sites at the location of the doped fluorine and nitrogen atoms. The hierarchical pore structure with large specific surface area could supply more available active sites and provide fast transport channel for electrons and products. This study will provide useful guidance for the design of superior metal-free ORR catalysts, promoting the application of metal-free carbon-based catalysts in batteries or fuel cells.