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Understanding Active Sites in Pyrolyzed Fe–N–C Catalysts for Fuel Cell Cathodes by Bridging Density Functional Theory Calculations and 57Fe Mössbauer Spectroscopy

Journal Article · · ACS Catalysis
 [1];  [2];  [3];  [1];  [1];  [4];  [4];  [4];  [1]
  1. Institut Charles Gerhardt Montpellier, UMR 5253, CNRS, Université Montpellier, ENSCM, Montpellier 34090, France
  2. The Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, New Mexico 87131, United States; Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
  3. The Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, New Mexico 87131, United States; Chemical &, Biomolecular Engineering and National Fuel Cell Research Center, University of California, Irvine, California 92697-2580, United States
  4. Université Grenoble Alpes CNRS, CEA, DRF/IRIG/LCBM/pmb, 17 rue des Martyrs, Grenoble 38000, France
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Pyrolyzed Fe–N–C materials are promising platinum-group-metal-free catalysts for proton-exchange membrane fuel cell cathodes. However, the detailed structure, oxidation, and spin states of their active sites are still undetermined. As such, 57Fe Mössbauer spectroscopy has identified FeNx moieties as the most active sites, with their fingerprint being a doublet in low-temperature Mössbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (ΔEQS) for a range of FeNx structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe–N–C catalyst, whereas further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device, and 57Fe Mössbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeNx moieties in Fe(II) low-spin and Fe(III) high-spin states, whereas a minor fraction of sites could exist in the Fe(II) S = 1 state. From the analysis of the 57Fe Mössbauer spectrum under the external magnetic field and the comparison of calculated and measured ΔEQS values, we assign the experimental doublet D1 with a mean ΔEQS value of around 0.9 mm·s–1 to Fe(III)N4C12 moieties in high-spin state and the experimental doublet D2 with a mean ΔEQS value of around 2.3 mm·s–1 to Fe(II)N4C10 moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites, whereas D2 may correspond either to bulk sites that are inaccessible to O2 or to surface sites that bind O2 weaker than D1.
Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Sponsoring Organization:
French National Research Agency (ANR); Fuel Cells and Hydrogen Joint Undertaking (FCH JU); USDOE Office of Science (SC)
Grant/Contract Number:
89233218CNA000001; AC02-05CH11231
OSTI ID:
1601412
Report Number(s):
LA-UR--19-27987
Journal Information:
ACS Catalysis, Journal Name: ACS Catalysis Journal Issue: 10 Vol. 9; ISSN 2155-5435
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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Cited By (4)

Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells journal December 2020
Engineering Local Coordination Environments of Atomically Dispersed and Heteroatom‐Coordinated Single Metal Site Electrocatalysts for Clean Energy‐Conversion journal November 2019
On the Influence of Oxygen on the Degradation of Fe‐N‐C Catalysts journal January 2020
On the Influence of Oxygen on the Degradation of Fe‐N‐C Catalysts journal February 2020

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Related Subjects

fuel cell
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
FeN4
Fe−N−C
Mössbauer spectroscopy
cathode
density functional theory
oxygen reduction
quadrupole energy splitting