A Combined Probe-Molecule, Mössbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst

Jared L. Kneebone, Stephanie L. Daifuku, Jeffrey A. Kehl, Gang Wu, Hoon T. Chung, Michael Y. Hu, E. Ercan Alp, Karren L. More, Piotr Zelenay, Edward F. Holby, Michael L. Neidig

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76 Scopus citations

Abstract

Nonprecious metal M-N-C (M = Fe or Co) catalysts that are effective for the oxygen-reduction reaction in polymer-electrolyte fuel cells have been developed, but no consensus has yet been reached regarding the nature of the M sites in these heterogeneous catalysts that are responsible for the reaction with dioxygen (O2). While multiple studies have developed correlations between Fe distributions in as-prepared catalysts and ORR activity, the direct identification of sites reactive toward O2 or O2-analogue molecules remains a significant challenge. In the present study, we demonstrate a new approach to identifying and characterizing potential Fe active sites in complex ORR catalysts that combines an effective probe molecule (NO(g)), Mössbauer spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) with density functional theory (DFT) calculations. Mössbauer spectroscopic studies demonstrate that NO(g) treatment of electrochemically reduced PANI-57Fe-C leads to a selective reaction with only a subset of the Fe species present. Nuclear resonance vibrational spectroscopic studies identified new Fe-ligand vibrations associated with the site reactive toward NO(g). DFT calculations of the vibrational properties of a selection of previously proposed active-site structures suggest that graphene zigzag edge-hosted Fe-N structures may be responsible for the observed vibrational behavior with NO(g) probe molecules. Furthermore, such sites are likely also reactive to O2, possibly serving as the ORR active sites in the synthesized materials. (Graph Presented).

Original languageEnglish
Pages (from-to)16283-16290
Number of pages8
JournalJournal of Physical Chemistry C
Volume121
Issue number30
DOIs
StatePublished - Aug 3 2017

Funding

Financial support from the DOE-EERE Fuel Cell Technologies Office is gratefully acknowledged. Work at the Advanced Photon Source was supported by the Department of Energy, Office of Science, under contract DE-AC-02-06CH11357. E.F.H. would like to thank Los Alamos National Laboratory Institutional Computing for computational support. Microscopy was performed as part of a user project at ORNL's Center for Nanophase Materials Sciences (CNMS), which is an Office of Science User Facility.

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