Oxygen Reduction Reaction on Pyridinic Nitrogen-Functionalized Carbon: Active Site Quantification and Effects of Lewis Basicity

Abstract

Metal-free carbon materials functionalized with pyridinic nitrogen groups exhibit promising electrocatalytic activity for the oxygen reduction reaction (ORR). However, not all pyridinic nitrogen groups are equally active for the ORR, which remains ambiguous and requires rigorous quantification and differentiation by their basicity. Here, we introduce the potentiometric titration method for identifying and quantifying nitrogen-containing groups on carbon materials by their Lewis basicity and reactivity in characteristic tests. Various carbon materials are functionalized with nitrogen heteroatoms. Potentiometric titration, X-ray photoelectron spectroscopy (XPS), and elemental analysis suggest that a significant amount of pyridinic nitrogen groups are buried within the bulk structures and cannot be accessed by protons and oxygen molecules. Besides, pyridinic nitrogen functions located adjacent to other nitrogen atoms exhibit weaker basicity due to strong inductive or resonance effects, resulting in a negligible contribution to the ORR activity. ORR measurements under alkaline conditions suggest that the titratable pyridinic nitrogen groups are essential for the active site (or site pair), and kinetic current density is directly proportional to the density of titratable pyridinic nitrogen groups. Furthermore, the turnover frequency for the ORR increases with the Lewis basicity of the pyridinic nitrogen groups for all investigated carbon materials in alkaline and acidic conditions. Density functional theory (DFT) calculations suggest that the ORR occurs on the carbon atoms adjacent to pyridinic nitrogen groups. Pyridinic nitrogen with a higher Lewis basicity can affect adjacent carbon atoms more efficiently, which stabilizes the key intermediates for the ORR and decreases the activation barrier. This work provides an informative and convenient way for characterizing nitrogen-containing groups on carbon materials, especially in quantifying the active pyridinic nitrogen sites for the ORR.