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Title: Spectroscopic Characterization of Highly Active Fe–N–C Oxygen Reduction Catalysts and Discovery of Strong Interaction with Nafion Ionomer

Journal Article · · ACS Applied Energy Materials

Scaling up clean-energy applications necessitates the development of platinum group metal (PGM)-free fuel cell electrocatalysts with high activity, stability, and low cost. Here, X-ray absorption (XAS) at the Fe K-edge and Fe Kβ X-ray emission (XES) spectroscopies were used to study the electronic structure of Fe centers in highly active Fe–N–C oxygen reduction catalysts with significant commercial potential. X-ray absorption near-edge structure (XANES) analysis has shown that the majority (>95%) of Fe centers are in the Fe3+ oxidation state, while extended X-ray absorption fine structure (EXAFS) detected a mixture of single site Fe–N4 centers (>95%) and centers with short (~2.5 Å) Fe–Fe interactions of Fe metal and/or Fe-carbide nanoparticles (<5%) featuring the Feo oxidation state. Surprisingly, addition of Nafion, the most widely used ionomer, resulted in pronounced changes in the XAS spectra, consistent with a strong catalyst–ionomer interaction where long Fe–Fe interactions at ~3.1 Å were shown to be a feature of Fe3+ ions bound with the Nafion. We conclude that exposure to Nafion during the device formulation has a different effect from the aggressive acid leaching typically used in the preparation of Fe–N–C catalysts. Furthermore, it was hypothesized that the polymer interacts with single sites’ Fe3+ centers, as well as with graphene layers protecting the Feo nanoparticles, and extracts some Fe ions into the Nafion matrix.

Research Organization:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Brookhaven National Laboratory (BNL), Upton, NY (United States); Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Energy Efficiency and Renewable Energy (EERE), Office of Sustainable Transportation. Hydrogen Fuel Cell Technologies Office (HFTO); National Science Foundation (NSF)
Grant/Contract Number:
AC05-00OR22725; SC0012704; AC02-06CH11357; CHE-2155060
OSTI ID:
2282945
Alternate ID(s):
OSTI ID: 2310278
Report Number(s):
BNL-225307-2024-JAAM
Journal Information:
ACS Applied Energy Materials, Vol. 7, Issue 2; ISSN 2574-0962
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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