Evolution Pathway from Iron Compounds to Fe1(II)–N4 Sites through Gas-Phase Iron during Pyrolysis

Pyrolysis is indispensable for synthesizing highly active Fe–N–C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe–N–C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe1(II)–N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)–O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)–O4 releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)–N4 above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)–N4 sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.