Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal–Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems

Detailed understanding of the nature of the active centers in non-precious-metal-based electrocatalyst, and their role in oxygen reduction reaction (ORR) mechanistic pathways will have a profound effect on successful commercialization of emission-free energy devices such as fuel cells. Recently, using pyrolyzed model structures of iron porphyrins, we have demonstrated that a covalent integration of the Fe–N_<i>x</i> sites into π-conjugated carbon basal plane modifies electron donating/withdrawing capability of the carbonaceous ligand, consequently improving ORR activity. Here, we employ a combination of <i>in situ</i> X-ray spectroscopy and electrochemical methods to identify the various structural and functional forms of the active centers in non-heme Fe/N/C catalysts. Both methods corroboratively confirm the single site 2e^– × 2e^– mechanism in alkaline media on the primary Fe²⁺–N₄ centers and the dual-site 2e^– × 2e^– mechanism in acid media with the significant role of the surface bound coexisting Fe/Fe_<i>x</i>O_<i>y</i> nanoparticles (NPs) as the secondary active sites