Oxygen Reduction\nReaction on Pyridinic Nitrogen-Functionalized\nCarbon: Active Site Quantification and Effects of Lewis Basicity

Metal-free carbon materials functionalized\nwith pyridinic\nnitrogen\ngroups exhibit promising electrocatalytic activity for the oxygen\nreduction reaction (ORR). However, not all pyridinic nitrogen groups\nare equally active for the ORR, which remains ambiguous and requires\nrigorous quantification and differentiation by their basicity. Here,\nwe introduce the potentiometric titration method for identifying and\nquantifying nitrogen-containing groups on carbon materials by their\nLewis basicity and reactivity in characteristic tests. Various carbon\nmaterials are functionalized with nitrogen heteroatoms. Potentiometric\ntitration, X-ray photoelectron spectroscopy (XPS), and elemental analysis\nsuggest that a significant amount of pyridinic nitrogen groups are\nburied within the bulk structures and cannot be accessed by protons\nand oxygen molecules. Besides, pyridinic nitrogen functions located\nadjacent to other nitrogen atoms exhibit weaker basicity due to strong\ninductive or resonance effects, resulting in a negligible contribution\nto the ORR activity. ORR measurements under alkaline conditions suggest\nthat the titratable pyridinic nitrogen groups are essential for the\nactive site (or site pair), and kinetic current density is directly\nproportional to the density of titratable pyridinic nitrogen groups.\nFurthermore, the turnover frequency for the ORR increases with the\nLewis basicity of the pyridinic nitrogen groups for all investigated\ncarbon materials in alkaline and acidic conditions. Density functional\ntheory (DFT) calculations suggest that the ORR occurs on the carbon\natoms adjacent to pyridinic nitrogen groups. Pyridinic nitrogen with\na higher Lewis basicity can affect adjacent carbon atoms more efficiently,\nwhich stabilizes the key intermediates for the ORR and decreases the\nactivation barrier. This work provides an informative and convenient\nway for characterizing nitrogen-containing groups on carbon materials,\nespecially in quantifying the active pyridinic nitrogen sites for\nthe ORR.