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Abstract Fe‐N‐C single‐atom catalysts (SACs) are emerging as a promising class of electrocatalysts for the oxygen reduction reaction (ORR) to replace Pt‐based catalysts. However, due to the limited loading of Fe for SACs and the inaccessibility of internal active sites, only a small portion of the sites near the external surface are able to contribute to the ORR activity. Here, this work reports a metal–organic framework‐derived Fe‐N‐C SAC with a hierarchically porous and concave nanoarchitecture prepared through a facile but effective strategy, which exhibits superior electrocatalytic ORR activity with a half‐wave potential of 0.926 V (vs RHE) in alkaline media and 0.8 V (vs RHE) in acidic media while maintaining excellent stability. The superior ORR activity of the as‐designed catalyst stems from the unique architecture, where the hierarchically porous architecture contains micropores as Fe SAC anchoring sites, meso‐/macro‐pores as accessible channels, and concave shell for increasing external surface area. The unique architecture has dramatically enhanced the utilization of previously blocked internal active sites, as confirmed by a high turnover frequency of 3.37 s−1and operando X‐ray absorption spectroscopy analysis with a distinct shift of adsorption edge.
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Altering Ligand Fields in Single-Atom Sites through Second-Shell Anion Modulation Boosts the Oxygen Reduction Reaction
Single-atom catalysts based on metal–N4 moieties and anchored on carbon supports (defined as M–N–C) are promising for oxygen reduction reaction (ORR). Among those, M–N–C catalysts with 4d and 5d transition metal (TM4d,5d) centers are much more durable and not susceptible to the undesirable Fenton reaction, especially compared with 3d transition metal based ones. However, the ORR activity of these TM4d,5d–N–C catalysts is still far from satisfactory; thus far, there are few discussions about how to accurately tune the ligand fields of single-atom TM4d,5d sites in order to improve their catalytic properties. Herein, we leverage single-atom Ru–N–C as a model system and report an S-anion coordination strategy to modulate the catalyst’s structure and ORR performance. The S anions are identified to bond with N atoms in the second coordination shell of Ru centers, which allows us to manipulate the electronic configuration of central Ru sites. The S-anion-coordinated Ru–N–C catalyst delivers not only promising ORR activity but also outstanding long-term durability, superior to those of commercial Pt/C and most of the near-term single-atom catalysts. DFT calculations reveal that the high ORR activity is attributed to the lower adsorption energy of ORR intermediates at Ru sites. Metal–air batteries using this catalyst in the cathode side also exhibit fast kinetics and excellent stability.
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- Award ID(s):
- 1900401
- PAR ID:
- 10315077
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- ISSN:
- 0002-7863
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Clusters of nitrogen- and carbon-coordinated transition metals dispersed in a carbon matrix (e. g., Fe−N−C) have emerged as an inexpensive class of electrocatalysts for the oxygen reduction reaction (ORR). Here, it was shown that optimizing the interaction between the nitrogen-coordinated transition metal clusters embedded in a more stable and corrosion-resistant carbide matrix yielded an ORR electrocatalyst with enhanced activity and stability compared to Fe−N−C catalysts. Utilizing first-principles calculations, an electrostatics-based descriptor of catalytic activity was identified, and nitrogen-coordinated iron (FeN4) clusters embedded in a TiC matrix were predicted to be an efficient platinum-group metal (PGM)-free ORR electrocatalyst. Guided by theory, selected catalyst formulations were synthesized, and it was demonstrated that the experimentally observed trends in activity fell exactly in line with the descriptor-derived theoretical predictions. The Fe−N−TiC catalyst exhibited enhanced activity (20 %) and durability (3.5-fold improvement) compared to a traditional Fe−N−C catalyst. It was posited that the electrostatics-based descriptor provides a powerful platform for the design of active and stable PGM-free electrocatalysts and heterogenous single-atom catalysts for other electrochemical reactions.more » « less
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Abstract Clusters of nitrogen‐ and carbon‐coordinated transition metals dispersed in a carbon matrix (e. g., Fe−N−C) have emerged as an inexpensive class of electrocatalysts for the oxygen reduction reaction (ORR). Here, it was shown that optimizing the interaction between the nitrogen‐coordinated transition metal clusters embedded in a more stable and corrosion‐resistant carbide matrix yielded an ORR electrocatalyst with enhanced activity and stability compared to Fe−N−C catalysts. Utilizing first‐principles calculations, an electrostatics‐based descriptor of catalytic activity was identified, and nitrogen‐coordinated iron (FeN4) clusters embedded in a TiC matrix were predicted to be an efficient platinum‐group metal (PGM)‐free ORR electrocatalyst. Guided by theory, selected catalyst formulations were synthesized, and it was demonstrated that the experimentally observed trends in activity fell exactly in line with the descriptor‐derived theoretical predictions. The Fe−N−TiC catalyst exhibited enhanced activity (20 %) and durability (3.5‐fold improvement) compared to a traditional Fe−N−C catalyst. It was posited that the electrostatics‐based descriptor provides a powerful platform for the design of active and stable PGM‐free electrocatalysts and heterogenous single‐atom catalysts for other electrochemical reactions.more » « less
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Abstract Fuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.1c11331
