What\nIs the “Other” Site in M–N–C?

Single\nmetal atoms embedded in nitrogen-doped graphene (M–N–C)\nhave emerged as a promising catalyst for a wide variety of reactions.\nIn addition to the pyridinic site, there is another site responsible\nfor the catalytic activity, but its structure is under debate. Here,\nwe resolve its structure using first-principles calculations. Using\nFe–N–C as a representative example, we systematically\nexplore numerous possible structures and discover a new moiety with\ncomparable energy to the pyridinic. This moiety features a hybrid\ncoordination environment between pyridinic and porphyrinic and is\nlocated at the edge of graphene sheets or pores. We further calculate\nits X-ray absorption spectrum, catalytic thermodynamics for oxygen\nreduction reaction (ORR), and stability under ORR conditions, all\nof which support its existence. Lastly, we show that this site also\nexists in other M–N–C with different M elements. This\nstudy uncovers a new and important structure in M–N–C\nand paves a critical step toward site engineering for improved catalytic\nperformance.