A DFT Study on the Activity Origin of Fe-N-C Sites for Oxygen Reduction Reaction.

Iron-nitrogen-carbon materials have been known as the most promising non-noble metal catalyst for proton-exchange membrane fuel cells (PEMFCs), but the genuine active sites for oxygen reduction reaction (ORR) are still arguable. Herein, by the thorough density functional theory investigations, we unravel that the planar Fe 2 N 6 site exhibits excellent ORR catalytic activity over both FeN 3 and FeN 4 sites, and the potential-determining step is determined to be the *OH hydrogenation step with an overpotential of 0.415 V. The ORR activity of Fe 2 N 6 site originates from the low spin magnetic moment (1.11 μ B ), which leads to high antibonding states and low d-band center of the Fe center, further leads to weak binding strength of *OH species. The density of FeN 4 sites only has little influence on the ORR activity owing to the similar interaction between active site and intermediates in ORR. Our research sheds light on the activity origin of iron-nitrogen-carbon materials for ORR.