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High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells

Nature Catalysis volume 5, pages 311–323 (2022)Cite this article

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Abstract

Non-precious iron-based catalysts (Fe–NCs) require high active site density to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells. Site density is generally limited to that achieved at a 1–3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by preforming a carbon–nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single-atom Fe–N4 sites, as identified by 57Fe cryogenic Mössbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 × 1019 and 7.8 × 1019 sites g−1, with a turnover frequency of 5.4 electrons sites−1 s−1 at 0.80 V in a 0.5 M H2SO4 electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm−2 at 0.90 ViR-free using H2–O2 and 145 mA cm−2 at 0.80 V (199 mA cm−2 at 0.80 ViR-free) using H2–air.

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Fig. 1: Calculation of the Fe–NC SD and effect on performance.
Fig. 2: Structural analysis of Fe–NC catalysts.
Fig. 3: Analysis of Fe coordination environments before and after activation.
Fig. 4: SD and TOF measurements of Fe–NCs in a pH 5.2 electrolyte.
Fig. 5: ORR activity and correlation with SD and kinetic activity in a pH 0.3 electrolyte.
Fig. 6: Performance tests with Fe–NCΔ-DCDA as the cathode catalyst in a single-cell PEMFC.

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Data availability

The data used in the production of the figures in this paper are available for download at https://doi.org/10.5281/zenodo.6411262. Additional data can be available from the authors upon reasonable request.

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Acknowledgements

This work was funded by the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement no. 779366. This Joint Undertaking receives support from the European Union’s Horizon 2020 research and innovation programme, Hydrogen Europe and Hydrogen Europe research. The work was supported by the UK Engineering and Physical Sciences Research Council under project EP/P024807/1.

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Authors and Affiliations

  1. Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, London, UK

    Asad Mehmood, Mengjun Gong & Anthony Kucernak

  2. ICGM, Université de Montpellier, CNRS, ENSCM, Montpellier, France

    Frédéric Jaouen, Aaron Roy & Moulay-Tahar Sougrati

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

    Andrea Zitolo & Anastassiya Khan

  4. Department of Chemistry, Chemical Engineering Division, Technical University of Berlin, Berlin, Germany

    Mathias Primbs & Peter Strasser

  5. Johnson Matthey Technology Center, Reading, UK

    Alex Martinez Bonastre & Dash Fongalland

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

    Goran Drazic

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  1. Asad Mehmood
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Contributions

A.M. and A.K. conceived the idea and designed the experiments. A.M. and M.G. synthesized the materials and carried out electrochemical tests and physical characterizations. A.K. developed the geometrical models of the catalyst. A.M. and M.G. carried out the small-size single-cell tests. F.J., A.R. and M.-T.S. performed the Mössbauer measurements and data analysis, A.Z. and A.K. performed the XAS measurements, data analysis and fitting, and interpretation of the XANES and EXAFS results, M.P. and P.S. conducted CO chemisorption measurements and data analysis, A.M.B. and D.F. carried out large-size single-cell tests, and G.D. performed energy-dispersive X-ray spectroscopy STEM and atomic-resolution STEM measurements and data analysis. A.M., F.J. and A.K. wrote and edited the manuscript with feedback from all the contributing authors. A.K. acted as the project supervisor.

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Correspondence to Anthony Kucernak.

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Supplementary Method 1, Figs. 1–37, Tables 1–7 and Notes 1–10.

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Mehmood, A., Gong, M., Jaouen, F. et al. High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells. Nat Catal 5, 311–323 (2022). https://doi.org/10.1038/s41929-022-00772-9

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