Journey of Fe-N4 Sites from Birth in Furnace till Death in Cell
Authors: Jia Qingying , Jiao Li , Myers Deborah J. 
Published: July 2022

One central topic in the Fe-N-C electrocatalysis for the oxygen reduction reaction (ORR) is the nature of the active site(s) responsible for the superior ORR activity of Fe-N-C electrocatalysts in acidic media. Significant progress has been made in this regard in the past decade, thanks to the employment of advanced in situ/operando characterization techniques including Mossbauer and X-ray absorption spectroscopy (XAS). The key findings are that the local structure of at least one type of Fe-N4 sites changes dramatically from produced in the furnace, exposed to air, and immersed in electrochemical media in aqueous solution or proton exchange membrane fuel cells (PEMFCs) with applied potentials. Despite these new findings, what the local structure of Fe-N4 sites is when they are catalyzing the ORR in acidic media, assuming Fe-N4 sites as the primary ORR-active centers of Fe-N-C electrocatalysts, remains unclear. Another mystery is the cause(s) of the rapid degradation of Fe-N4 sites during PEMFC operation.
Herein, we walk through a possible journey of Fe-N4 sites from being produced by pyrolysis till decomposition in PEMFCs. In particular, we propose a new local structure of the exposed Fe-N4 site at lower potentials in acidic media: (H2O)2-Fe(II)-N4 with an octahedral structure subject to Jahn-Teller distortions. Specifically, in-plane Fe(II)-N4 sites are formed upon high-temperature (> 600 ℃) pyrolysis. Those Fe(II)-N4 sites accessible by air get spontaneously oxidized forming (O)2-Fe(III)-N4 ((O)2 represents the two axial oxygen ligands rather than one oxygen molecule) when exposed to air. The (O)2-Fe(III)-N4 structure is most likely also the form of the exposed Fe-N4 sites in acidic media at elevated potentials ( > 0.8 V versus reversible hydrogen electrode (RHE). As the potential scans cathodically across the Fe(III)/Fe(II) redox potential, the (O)2-Fe(III)-N4 structure transforms to the (H2O)2-Fe(II)-N4 structure with the axial ligands of (O)2 replaced by water molecules with elongated Fe-OH2 bonds due to the Jahn-Teller effect on high-spin Fe(II) octahedral complexes. This high-spin (O)2-Fe(III)-N4 structure was inferred from the combination of the presence of two Fourier-Transform of extended X-ray absorption fine structure (FT-EXAFS) peaks of several Fe-N-C catalysts1,2 at lower potentials; the low intensity of the pre-edge peak in the X-ray absorption near edge structure (XANES) spectra; with in situ Mossbauer results.3 In contrast, the Jahn-Teller effect is absent on high-spin Fe(III) octahedral complexes such as (O)2-Fe(III)-N4, correspondingly only one Fe-N/O FT-EXAFS peak was observed at elevated potentials or ex situ. The H2O molecule(s) in the (H2O)2-Fe(II)-N4 is replaceable by O2 molecules during the ORR. The exposed Fe-N4 sites transform to inorganic iron species such as iron oxides during PEMFC operation according to the recent operando Mossbauer results,4 which accounts partly for the degradation of Fe-N-C catalysts during the ORR.

Acknowledgements

This work was supported by the US Department of Energy under award number DE-EE0008416. We acknowledge the support from the DOE, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis Consortium (ElectroCat) and the DOE programme and technology managers, D. Papageorgopoulos, D. Peterson and N. Garland. This research used beamline 7-BM and 8-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. In-temperature XAS experiments were performed at the MRCAT in Advanced Photon Source (APS), a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

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