(Invited) Structural and Mechanistic Basis for the Oxygen Reduction Activity of Pyrolyzed Fe-N-C Electrocatalysts

Oxygen reduction reaction (ORR) constitutes a critical element in the commercialization of electrochemical energy conversion and storage devices. Consequently, the replacement of unsustainable noble metal catalysts with earth-abundant materials constitutes a vital technological strategy towards fixing the twin challenge of energy security and climate change. The success in this regard requires the development of scalable non-precious metal (non-PGM) catalysts with high performance in practical devices. The major bottleneck in developing Fe-N-C materials as the leading non-PGM catalysts lies in the poor understanding of the nature of active sites and reaction mechanisms. Herein, we systematically characterized a series of Fe-N-C catalysts under both ex situ and in situ conditions using combined microscopic and spectroscopic techniques. We show that the structures of active sites under ex situ and working conditions are drastically different. Resultantly, the active site structure proposed here, a non-planar ferrous Fe-N4 moiety embedded in distorted carbon matrix characterized by a high Fe2+/3+ redox potential, is in contrast with those proposed hitherto derived from ex situ characterizations. This site reversibly switches to an in-plane ferric Fe-N4 moiety poisoned by oxygen adsorbates during the redox transition, with the population of active sites controlled by the Fe2+/3+ redox potential. The high inherent activity of the active site is correlated to its near-optimal Fe2+/3+ redox potential, which essentially originates from its favorable biomimetic dynamic nature that balances the site-blocking effect and O2 dissociation. The porous and disordered carbon matrix of the catalyst plays pivotal roles for its measured high ORR activity by hosting high population of reactant-accessible active sites. Acknowledgement: The authors deeply appreciate financial assistance from the U.S. Department of Energy, EERE (DE-EE-0000459). Use of the National Synchrotron Light Source (beamline X3B), Brookhaven National Laboratory (BNL), was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. This publication was made possible by the Center for Synchrotron Biosciences grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NBIB). Support from beamline personnel Dr. Erik Farquhar and Mark Chance (X3B) are gratefully acknowledged. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.