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Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells

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Abstract

Nitrogen-coordinated single atom iron sites (FeN4) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. However, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface. We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN4 sites into highly stable pyridinic N-coordinated FeN4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mgPt cm−2). The encouraging stability improvement represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications.

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Fig. 1: Catalyst synthesis principles.
Fig. 2: Catalyst structures and catalytic properties.
Fig. 3: MEA performance of the Fe–AC and Fe–AC–CVD cathode catalysts.
Fig. 4: Nano-CT tomography and EIS analysis of PGM-free cathode.
Fig. 5: Carbon structure changes in catalysts after nitrogen-doped carbon layer deposition.
Fig. 6: Possible FeN4 site conversion during the CVD process.
Fig. 7: DFT calculations to elucidate the activity of two types of FeN4 site.
Fig. 8: DFT calculations to study the stability of two types of FeN4 site.

Data availability

The authors declare that all data supporting the findings of this study are available within the paper and Supplementary Information files. Source data are provided with this paper.

Code availability

All the DFT calculations were performed using the commercial software VASP. All the input and output files of the calculations are available from Guofeng Wang per request.

Change history

  • 15 July 2022

    In the version of this article initially published, there was a plotting error in Supplementary Fig. 31b, lower trace, which has now been corrected in the Supplementary Information online.

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Acknowledgements

We acknowledge the support from the US DOE Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (DE-EE0008076 and DE-EE0008417). Electron microscopy research was supported by the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory. The Talos F200X S/TEM tool was provided by US DOE, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science user facilities. XAS measurements were performed at MRCAT at the Advanced Photon Source, a US DOE Office of Science user facility operated for the US DOE by Argonne National Laboratory. The operation of MRCAT is supported both by DOE and the MRCAT member institutions. This work was in part authored by Argonne National Laboratory, which is operated for the US DOE by the University of Chicago Argonne LLC under contract number DE-AC02-06CH11357. G. Wu also acknowledges support from the National Science Foundation (CBET-1604392, 1804326). Z. Feng acknowledges the support from the National Science Foundation (CBET-1949870, 2016192). G. Wang gratefully acknowledges the computational resources provided by the Center for Research Computing at the University of Pittsburgh. We also thank B. Lavina of the Advanced Photon Source for help with the acquisition of Mössbauer spectroscopy data.

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Contributions

G. Wu and S. Liu were the primary writers of the manuscript. S. Liu, Y.Z., Q.S. and G. Wu designed catalyst synthesis and performed the electrochemical experiments, characterized the catalyst and analysed the data. C.L., Q.G. and J.X. carried out fuel cell tests and data analysis. H.X. assisted with MEA design and fabrication. H.M.M. conducted XPS and Auger experiments and data analysis. D.A.C., M.J.Z. and H.Y. together performed electron microscopy imaging and further characterizations. B.L. and G. Wang designed and performed DFT calculations. M.W., M.L., A.J.K., Z.F. and D.J.M. together designed and performed X-ray absorption spectroscopy and data analysis. E.E.A. and D.J.M. designed and performed Mössbauer spectroscopy experiments and data analysis. J.B., J.L. and S. Litster acquired and analysed X-ray nano-CT imaging. G. Wu supervised the execution of the overall project.

Corresponding authors

Correspondence to Guofeng Wang, Deborah J. Myers, Jian Xie, David A. Cullen, Shawn Litster or Gang Wu.

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Competing interests

G. Wu, S. Liu and H. Xu have filed joint patent applications through the University at Buffalo and Giner Inc. (US Patent application number 17/531,461 and PCT application number PCT/US21/60195) on technology related to the Fe2O3 precursor, the NH4Cl treatment and the CVD process. H. Xu is an employee at Giner Inc., who assisted with MEA design and fabrication. The remaining authors declare no competing interests.

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Supplementary Figs. 1–43, Notes 1–9, Tables 1–18 and References.

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Source Data Fig. 2

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Source Data Fig. 3

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Liu, S., Li, C., Zachman, M.J. et al. Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells. Nat Energy 7, 652–663 (2022). https://doi.org/10.1038/s41560-022-01062-1

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