Evolution Pathway from Iron Compounds to Fe ₁ (II)–N ₄ Sites through Gas-Phase Iron during Pyrolysis

Abstract

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.