Untangling Cooperative Effects of Pyridinic and Graphitic Nitrogen Sites at Metal-Free N-Doped Carbon Electrocatalysts for the Oxygen Reduction Reaction

Metal-free carbon electrodes with well-defined composition and smooth topography are prepared via sputter deposition followed by thermal treatment with inert and reactive gases. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that three carbons of similar N/C content that differ in N-site composition are thus prepared: an electrode consisting of almost exclusively graphitic-N (N-G), an electrode with predominantly pyridinic-N (N-P), and one with approximate to 1:1 N-G:N-P composition. These materials are used as model systems to investigate the activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity toward 4e-reduction of O-2 is strongly influenced by the N-G/N-P site composition, with the material possessing nearly uniform N-G/N-P composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters are carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure N-G-doping or N-P-doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak, thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed N-G/N-P sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.