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

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Pyrolysis is crucial for synthesizing iron-nitrogen-carbon (Fe-N-C) catalysts for the oxygen reduction reaction (ORR) in acidic media.
• The precise transformation pathways of Fe, N, and C precursors into active ORR sites during pyrolysis are not well understood.
• This knowledge gap hinders the rational design and optimization of Fe-N-C catalysts.

Purpose of the Study:

• To elucidate the evolution pathway of precursors into ORR-active sites during the pyrolysis of Fe-N-C catalysts.
• To identify the specific atomic structure of the active sites formed.
• To understand the mechanism of Fe incorporation into the N-doped carbon matrix.

Main Methods:

• In-situ temperature-resolved X-ray absorption spectroscopy (XAS) was employed to monitor the transformations during pyrolysis.
• Characterization of the intermediate and final species formed at different temperatures.
• Verification of the proposed mechanism using a noncontact pyrolysis approach.

Main Results:

• Iron precursors initially form iron oxides below 300 °C.
• A crystal-to-melt-like transformation leads to tetrahedral Fe₁(II)-O₄ sites below 600 °C.
• Above 600 °C, Fe₁(II)-O₄ releases single Fe atoms that diffuse into N-doped carbon defects, forming Fe₁(II)-N₄ sites via a vapor-phase transport mechanism.

Conclusions:

• The study reveals a distinct vapor-phase single Fe atom transport mechanism for the formation of Fe₁(II)-N₄ active sites.
• This understanding clarifies the precursor-to-product relationship in Fe-N-C catalyst synthesis.
• The findings provide a pathway for the rational design of highly active and selective Fe-N-C catalysts for ORR.