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Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

Abstract

While platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe–N–C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe–N–C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe–N–C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe–N–C materials. These new insights open the path to bottom-up synthesis approaches and studies on site–support interactions.

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Figure 1: 57Fe Mössbauer spectroscopy characterization.
Figure 2: Fourier transforms of the experimental EXAFS spectra of Fe1.0, Fe0.5, Fe0.5-900 and Fe0.5-950.
Figure 3: Electrochemical characterization of Fe–N–C catalysts and a Pt/C benchmark catalyst.
Figure 4: Fe K-edge EXAFS analysis of Fe0.5 with an FeN4 moiety having one (left) or two (right) oxygen atoms in the axial direction.
Figure 5: Comparison between the K-edge XANES experimental spectrum of Fe0.5 (black dashed lines) and the theoretical spectrum calculated with the depicted structures (solid red lines).

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Acknowledgements

We acknowledge funding by ANR under contract 2011 CHEX 004 01 and Synchrotron SOLEIL (Gif-sur Yvette, France) for provision of synchrotron radiation facilities at beamline SAMBA (proposal number 20120701).

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A.Z. and E.F. conducted the XAS analysis and computation. V.G. and V.A. synthesized and electrochemically characterized the materials. M.-T.S. and L.S. conducted the Mössbauer spectroscopy measurement and analysis. T.M. conducted the DFT-D computation. A.Z. and F.J. wrote and edited the manuscript with input from all authors. The project was supervised by F.J. and E.F.

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Correspondence to Andrea Zitolo or Frédéric Jaouen.

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Zitolo, A., Goellner, V., Armel, V. et al. Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature Mater 14, 937–942 (2015). https://doi.org/10.1038/nmat4367

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