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Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells

Nature Catalysis volume 4, pages 10–19 (2021)Cite this article

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Abstract

While Fe–N–C materials are a promising alternative to platinum for catalysing the oxygen reduction reaction in acidic polymer fuel cells, limited understanding of their operando degradation restricts rational approaches towards improved durability. Here we show that Fe–N–C catalysts initially comprising two distinct FeNx sites (S1 and S2) degrade via the transformation of S1 into iron oxides while the structure and number of S2 were unmodified. Structure–activity correlations drawn from end-of-test 57Fe Mössbauer spectroscopy reveal that both sites initially contribute to the oxygen reduction reaction activity but only S2 substantially contributes after 50 h of operation. From in situ 57Fe Mössbauer spectroscopy in inert gas coupled to calculations of the Mössbauer signature of FeNx moieties in different electronic states, we identify S1 to be a high-spin FeN4C12 moiety and S2 a low- or intermediate-spin FeN4C10 moiety. These insights lay the groundwork for rational approaches towards Fe–N–C cathodes with improved durability in acidic fuel cells.

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Fig. 1: Initial activity and reversible spectral changes of Fe with PEMFC potential.
Fig. 2: Effect of PEMFC potential on doublets from fitted in situ Mössbauer spectra.
Fig. 3: Experimental and calculated values of hyperfine parameters versus potential.
Fig. 4: Characterization of Fe0.5 cathode after operation at 0.5 V in PEMFC.
Fig. 5: Correlations between FeNx site amount in Fe0.5 cathode and activity over time.
Fig. 6: Coordination or structural changes of the sites S1 and S2 under in situ or operando conditions.

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Data availability

The raw data that support the findings of this study are available from the corresponding authors upon request. In addition to being available upon request, the XAS raw data associated with this work is permanently stored at Synchrotron SOLEIL; the raw data related to electron microscopy images are permanently stored at LEPMI; the raw data related to X-ray radiographs are permanently stored at the APS synchrotron; the 57Fe Mössbauer spectroscopy data and the raw and reconstructed tomography data are available at Institut Charles Gerhardt Montpellier. Source data are provided with this paper.

Code availability

The source code used for DFT calculation with deMon2k is available at http://www.demon-software.com/public_html/download.html, upon request for academic purposes. VASP is a proprietary software available for purchase at https://www.vasp.at/.

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Acknowledgements

The research leading to these results has received partial funding from the French National Research Agency under the CAT2CAT contract (no. ANR-16-CE05-0007), the FCH Joint Undertaking (CRESCENDO project, grant agreement no. 779366) and the Centre of Excellence of Multifunctional Architectured Materials ‘CEMAM’ (grant no. ANR-10-LABX-44-01). We acknowledge Synchrotron SOLEIL (Gif-sur Yvette, France) for provision of synchrotron radiation facilities at beamline GALAXIES (proposal no. 20170390) and at beamline SAMBA (proposal no. 99190122). I.Z. acknowledges the resources of the APS, a US Department of Energy (DOE) Office of Science User Facility operated for the US DOE Office of Science by the ANL under contract no. DE-AC02-06CH11357. I.M. gratefully acknowledges the computational resources of the National Energy Research Scientific Computing Center (NERSC), a US DOE Office of Science User Facility operated under contract no. DE-AC02-05CH11231. This paper has been assigned LA-UR-19-31453. The computational work of T.M and I.C.O. was granted access to the HPC resources of IDRIS/TGCC under the allocation no. 2019-A0050807369 made by the Grand équipement national de calcul intensif and supported by the LabExCheMISyst ANR-10-LABX-05-01. G.D. acknowledges the financial support from Slovenian Research Agency (P2-0393).

Author information

Author notes

  1. Jingkun Li

    Present address: School of Chemical Engineering and Technology, Tianjin University, Tianjin, China

Authors and Affiliations

  1. Institut Charles Gerhardt Montpellier, UMR 5253, CNRS, Université Montpellier, ENSCM, Place Eugène Bataillon, Montpellier, France

    Jingkun Li, Moulay Tahar Sougrati, Ismail Can Oğuz, Tzonka Mineva & Frédéric Jaouen

  2. Synchrotron SOLEIL, L’orme des Merisiers, BP 48 Saint Aubin, Gif-sur-Yvette, France

    Andrea Zitolo & James M. Ablett

  3. The Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, NM, USA

    Ivana Matanovic

  4. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM, USA

    Ivana Matanovic

  5. Department of Chemical and Biomolecular Engineering, National Fuel Cell Research Center, University of California Irvine, Irvine, CA, USA

    Plamen Atanassov, Ying Huang & Iryna Zenyuk

  6. School of Science and Technology, Physics Division, University of Camerino, Camerino, MC, Italy

    Andrea Di Cicco

  7. University of Grenoble Alpes, University of Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, Grenoble, France

    Kavita Kumar, Laetitia Dubau & Frédéric Maillard

  8. Department of Materials Chemistry, National Institute of Chemistry, Ljubljana, Slovenia

    Goran Dražić

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Contributions

J.L. and F.J. designed and synthesized the materials, and conducted the electrochemical and physical characterizations. M.T.S. and J.L. designed and conducted the in situ and ex situ Mössbauer spectroscopy measurements. M.T.S. conducted Mössbauer data analysis. A.Z. and J.L. conducted the operando and ex situ XAS measurements. A.D.C. designed the operando fuel cell for XAS. F.J., A.Z., J.L. and J.M.A. conducted the in situ XES experiments. I.C.O., T.M., I.M. and P.A. conducted the DFT computation. K.K., L.D. and F.M. performed TEM and STEM–EDX analyses, G.D. performed atomic-scale STEM analyses, I.Z. and Y.H. performed tomography and TEM analyses. J.L., M.T.S. and F.J. wrote and edited the manuscript with input from all authors. F.J. supervised the project.

Corresponding author

Correspondence to Frédéric Jaouen.

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The authors declare no competing interests.

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Peer review information Nature Catalysis thanks Esen Alp, Gang Wu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Ex situ characterization of pristine powder catalysts.

SEM images of a, Fe0.5 and b, Fe0.5-950(10), TEM images of c, Fe0.5 and d, Fe0.5-950(10), XRD patterns of e, Fe0.5 and f, Fe0.5-950(10), STEM images of Fe0.5 in g, high-angle annular dark-field (HAADF) mode, h, fast Fourier transform filtered HAADF and i, annular bright field mode. Fe0.5-950(10) powder was stored in glove box before any characterisation, to avoid formation of Fe particles in ambient conditions. Fe0.5 catalyst does not lead to the formation of Fe particles even after months of storage in ambient conditions.

Source data

Extended Data Fig. 2 Ex situ 57Fe Mössbauer spectra of Fe0.5.

Acquired in ambient air at 300 K for Fe0.5 powder a, at 300 K for Fe0.5-cathode b, at 5 K for Fe0.5 powder c, and at 5 K for Fe0.5-cathode d.

Source data

Extended Data Fig. 3 Operando X-ray absorption spectroscopy of Fe0.5.

Operando Fe K-edge XANES a, and FT-EXAFS b, spectra of Fe0.5-cathode as a function of electrochemical potential, c, position of redox peak in the CV of Fe0.5 compared to ΔE (right handside y-axis), the threshold energy of the XANES spectrum relative to a metallic Fe foil. The spectra were measured in a PEMFC (Cell 2). The cell temperature was 80 °C, the flow rates for O2 and H2 gases were 60 sccm with 100 % relative humidity, no backpressure. The cathode loading was 4 mgFeNC·cm-2, the anode loading was 0.5 mgPt·cm-2 and the membrane was Nafion 211. The operando XAS acquisition duration was ca 4 min at each potential. For c), the CV was measured with a rotating disk electrode (RDE) in N2-saturated 0.1 M HClO4 at 20 mV·s-1 and the loading of Fe0.5 was 0.8 mg·cm−2. The reference and counter electrodes were a RHE and graphite rod.

Source data

Extended Data Fig. 4 Potential holds applied vs. time for the in situ 57Fe Mössbauer spectroscopy study.

The duration of each potential hold was 36 h, and each hold is labelled according to the potential value and the cycle number (the number in brackets).

Source data

Extended Data Fig. 5 Scheme of the formation of iron oxides and the corresponding Mössbauer signal.

A fraction of S1 sites demetallate during 0.2 V (1) a, leading to the formation of high-spin Fe2+ (possibly complexed with sulfonic acid groups in Nafion ionomer) with an associate D3 signal at room temperature b. Upon exposure to air ex situ, such high-spin Fe2+ is mainly transformed into ferric oxides, then contributing with a doublet component in EoT Mössbauer spectra at 300 K c, with IS and QS values similar to those of D1 (compare a and c). When the EoT Mössbauer spectra are recorded at low temperatures, the ferric oxide particles become magnetically ordered at T ≤ 80 K, then contributing with a sextet component d.

Source data

Extended Data Fig. 6 End-of-Test characterisation of Fe0.5 cathode after a durability test at 0.2 V.

Mössbauer spectrum at 5 K of a, pristine Fe0.5-cathode and c, Fe0.5-cathode after 72 h operation in cell 3 at 0.2 V under O2 at room temperature. TEM micrograph of b, pristine Fe0.5-cathode and d, Fe0.5-cathode corresponding to c. The low amount of Fe oxide in the pristine Fe0.5-cathode identified in a) and/or non-uniform distribution of Fe oxide particles challenged their identification with TEM. e, XRD spectra of the Fe0.5-cathode before and after 72 h operation in cell 3 at 0.2 V under O2 at room temperature. The XRD pattern of graphite (00-041-1478) is shown as vertical lines. f, Tafel plots of Fe0.5-cathode before and after 72 h operation in cell 3 at 0.2 V under O2 at room temperature, measured at 80 °C in the commercial PEMFC (Cell 1). An iron-free N-C cathode (synthesized identically as Fe0.5 except that no iron salt was added during ball-milling) is shown as a reference. For (d), the cell temperature was 80 °C, the flow rates for O2 and H2 gases were 60 sccm with 100 % relative humidity, the gauge pressure was 1 bar and the cathode loading was 4 mg·cm−2.

Source data

Extended Data Fig. 7 End-of-Test characterisation of Fe0.5-950(10) cathode after durability test at 0.5 V.

Mössbauer spectra of Fe0.5-950(10) cathode at 5 K before a, and after b, potential hold at 0.5 V in PEMFC for 50 hours. c, The corresponding Tafel plots of Fe0.5-950(10) before and after the durability test. The cell temperature was 80 °C, 60 sccm O2 and H2 gases with 100% relative humidity were fed at cathode and anode respectively, the gauge pressure was 1 bar, and the cathode loading was 4 mg·cm-2. TEM images of Fe0.5-950(10) cathode before d, and after 50-hour durability test at 0.5 V in PEMFC e,f. The durability tests were performed at 0.5 V for 50 h in the same conditions as described above.

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Supplementary Notes 1–6, Figs. 1–20, Tables 1–8 and references.

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Description: atomic coordinates of optimized models

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Li, J., Sougrati, M.T., Zitolo, A. et al. Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells. Nat Catal 4, 10–19 (2021). https://doi.org/10.1038/s41929-020-00545-2

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