Growing designed imidazole-like active sites on graphitic catalysts from selected sources towards efficient oxygen reduction.

A novel method was developed to investigate the growth of N-doped nanoporous graphitic structures via thermal pyrolysis. N-doped catalytic active centers were optimized by inheriting the structure of 4-methylimidazole. Among the three kinds of methylimidazoles, the bond between the methyl group and the heterocyclic structure is the weakest, according to calculations of bond energies using density functional theory. Thus, the methyl group was first broken in the pyrolysis, making the methylimidazoles prone to incomplete thermal pyrolysis, because the reaction time in the floating catalytic pyrolysis was only a few seconds; the rest of the methylimidazoles formed clusters, and their free radicals then formed clusters linked to a graphitic structure, which led to the formation of 4-imidazole-like (MIL) catalytic active centers. X-ray photoelectron spectroscopy analysis revealed that more than 50% of the N-doped structures were imidazole-like structures in the catalysts prepared from methylimidazoles. According to rotating ring-disk electrode tests and DFT calculations, the 4-MIL catalytic structure exhibited the highest catalytic activity among the three catalysts prepared from methylimidazoles: the onset potential of the 4MIL-GNCs was close to that of commercial 20% Pt/C (0.94 V vs. 0.96 V (RHE)). The 4-MIL-GNCs had a high electron transfer number (n = 3.73) and high kinetic current density (Jk = 7.12 mA cm-2 at 0.6 V); the rate-determining step using the 4-MIL-GNCs had the lowest energy barrier in oxygen reduction among the catalysts prepared from methylimidazoles (0.67 vs. 0.79 and 1.16 eV).