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The Evolution Pathway from Iron Compounds to Fe1(II)-N4 Sites Through Gas-Phase Iron During Pyrolysis

preprint
submitted on 25.10.2019 and posted on 29.10.2019 by Jingkun Li, Li Jiao, Evan Wegener, Lynne K. LaRochelle Richard, Ershuai Liu, andrea Zitolo, Moulay-Tahar Sougrati, Sanjeev Mukerjee, Zipeng Zhao, Yu Huang, Fan Yang, Sichen Zhong, Hui Xu, A. Jeremy Kropf, Frederic Jaouen, Deborah J. Myers, Qingying Jia

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap ob- scures the connections between the input precursors and output products, clouding the pathway toward Fe-N-C catalyst improve- ment. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single atom Fe1(II)- N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C, and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)-O4 releases a single Fe atom that flows into the N-doped carbon defect forming Fe1(II)-N4 above 600 °C. This vapor phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via “non-contact pyrolysis” wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

Funding

US Department of Energy under award number DE-EE0008416 and DE-EE0008075

History

Email Address of Submitting Author

lijingkun1987@gmail.com

Institution

Institut Charles Gerhardt Montpellier, UMR 5253, CNRS, Université Montpellier

Country

France

ORCID For Submitting Author

0000-0003-1699-3089

Declaration of Conflict of Interest

The authors declare no competing financial interests.

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