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

Roman Ezhov; Olga Maximova; Xiang Lyu; Denis Leshchev; Eli Stavitski; Alexey Serov; Yulia Pushkar

doi:10.1021/acsaem.3c02522

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

Roman Ezhov, Olga Maximova, Xiang Lyu, Denis Leshchev, Eli Stavitski, Alexey Serov, Yulia Pushkar

Energy Storage and Conversion Manufactur

Research output: Contribution to journal › Article › peer-review

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Abstract

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 K_β 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 Fe⁰ 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. It was hypothesized that the polymer interacts with single sites' Fe³⁺ centers, as well as with graphene layers protecting the Fe⁰ nanoparticles, and extracts some Fe ions into the Nafion matrix.

Original language	English
Pages (from-to)	604-613
Number of pages	10
Journal	ACS Applied Energy Materials
Volume	7
Issue number	2
DOIs	https://doi.org/10.1021/acsaem.3c02522
State	Published - Jan 22 2024

Funding

This research was supported by NSF, CHE-2155060 (Y.P.) and by the US DOE Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under the ElectroCat Consortium, DOE technology managers D. Peterson and D. Papageorgopoulos. The use of the Advanced Photon Source, an Office of Science User Facility operated by the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract DE-AC02-06CH11357. The PNC/XSD (Sector 20) facilities at the Advanced Photon Source and research at these facilities were supported by the US Department of Energy, Basic Energy Science and the Canadian Light Source. This research used beamline 8-ID (ISS) of the National Synchrotron Light Source II, a US 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. This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). All authors acknowledge samples of Fe−N−C catalysts from Pajarito Powder, LLC. This research was supported by NSF, CHE-2155060 (Y.P.) and by the US DOE Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, under the ElectroCat Consortium, DOE technology managers D. Peterson and D. Papageorgopoulos. The use of the Advanced Photon Source, an Office of Science User Facility operated by the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract DE-AC02- 06CH11357. The PNC/XSD (Sector 20) facilities at the Advanced Photon Source and research at these facilities were supported by the US Department of Energy, Basic Energy Science and the Canadian Light Source. This research used beamline 8-ID (ISS) of the National Synchrotron Light Source II, a US 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. This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ). All authors acknowledge samples of Fe–N–C catalysts from Pajarito Powder, LLC.

Funders	Funder number
DOE Public Access Plan
ElectroCat Consortium
National Science Foundation	CHE-2155060
U.S. Department of Energy	DE-AC02- 06CH11357
Office of Science
Basic Energy Sciences
Argonne National Laboratory
Brookhaven National Laboratory	DE-AC05-00OR22725, DE-SC0012704
Hydrogen and Fuel Cell Technologies Office

Keywords

Fe−N−C catalyst
X-ray absorption spectroscopy
X-ray emission spectroscopy
earth-abundant fuel cell catalyst
oxygen reduction reaction

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

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10.1021/acsaem.3c02522

Cite this

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title = "Spectroscopic Characterization of Highly Active Fe−N−C Oxygen Reduction Catalysts and Discovery of Strong Interaction with Nafion Ionomer",

abstract = "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 {\AA}) Fe−Fe interactions of Fe metal and/or Fe-carbide nanoparticles (<5%) featuring the Fe0 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 {\AA} 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. It was hypothesized that the polymer interacts with single sites' Fe3+ centers, as well as with graphene layers protecting the Fe0 nanoparticles, and extracts some Fe ions into the Nafion matrix.",

keywords = "Fe−N−C catalyst, X-ray absorption spectroscopy, X-ray emission spectroscopy, earth-abundant fuel cell catalyst, oxygen reduction reaction",

author = "Roman Ezhov and Olga Maximova and Xiang Lyu and Denis Leshchev and Eli Stavitski and Alexey Serov and Yulia Pushkar",

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AU - Ezhov, Roman

AU - Maximova, Olga

AU - Lyu, Xiang

AU - Leshchev, Denis

AU - Stavitski, Eli

AU - Serov, Alexey

AU - Pushkar, Yulia

PY - 2024/1/22

Y1 - 2024/1/22

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AB - 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 Fe0 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. It was hypothesized that the polymer interacts with single sites' Fe3+ centers, as well as with graphene layers protecting the Fe0 nanoparticles, and extracts some Fe ions into the Nafion matrix.

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KW - oxygen reduction reaction

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

Abstract

Funding

Keywords

UN SDGs

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