Graphitic Nitrogen Is Responsible for Oxygen Electroreduction on Nitrogen-Doped Carbons in Alkaline Electrolytes: Insights from Activity Attenuation Studies and Theoretical Calculations

To date, controversies remain in the unambiguous identification of the active sites in N-doped carbons for oxygen reduction reaction (ORR). In the present study, prolonged potential cycling was conducted on three N-doped carbons in O₂-saturated 0.1 M KOH aqueous solution, where apparent attenuation of the ORR activity was observed, within the context of limiting current and onset potential. The attenuation trend of the limiting current was closely correlated with the diminishing content of graphitic N, as manifested in X-ray photoelectron spectroscopy measurements and Mott-Schottky analysis. In addition, the specific activity per graphitic N was found to be almost invariant within a wide range of potentials during prolonged potential cycling for all three model catalysts, in good agreement with theoretical prediction, whereas no such a correlation was observed with pyrrolic or pyridinic N. Density functional theory calculations showed that the first-electron reduction, which is a rate-determining step for the 4e^- ORR process, on carbon atoms adjacent to graphitic N, exhibited a much smaller Gibbs free-energy change than that on carbons neighboring pyrrolic or pyridinic N. These results strongly suggest that graphitic N is responsible for the ORR activity of N-doped carbons in alkaline electrolytes. Results in the present work may offer a generic, effective paradigm in the determination of catalytic active sites in heteroatom-doped carbons and be exploited as a fundamental framework for the rational design and engineering of effective carbon catalysts.