Atomically dispersed and nitrogen coordinated single metal site (MNx, M = Fe, Co, or Mn) moieties embedded in partially graphitized carbon (denoted as M–N–C) are recognized as the most promising platinum group metal‐free catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. However, simply regulating their coordination environments and local structures of single metal sites cannot fundamentally change active site structure, which leads to insufficient activity and stability. A second transition metal can be incorporated to design dual‐metal sites, offering a new opportunity to modulate the electronic and geometric structures of M–N–C catalysts. Therefore, exploring optimal atomically dispersed dual‐metal‐site is essential to designing new active sites with enhanced ORR activity, and stability, especially breaking the activity‐stability trade‐off. This review provides a comprehensive analysis of the advances in developing atomically dispersed dual‐metal site catalysts for the ORR, including innovative synthesis methods, primary structural configurations, and the mechanisms to promote catalytic performance. We aim to elucidate the crucial structure–property correlation, emphasizing the inherent electronic and geometric effects of dual metal sites. Finally, we discuss the current challenges of dual‐metal site catalysts concerning rational design, precise synthesis, and high‐fidelity structural characterization.
- Award ID(s):
- 1900401
- NSF-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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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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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.
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The urgent need to address the high-cost issue of proton-exchange membrane fuel cell (PEMFC) technologies, particularly for transportation applications, drives the development of simultaneously highly active and durable platinum group metal-free (PGM-free) catalysts and electrodes. The past decade has witnessed remarkable progress in exploring PGM-free cathode catalysts for the oxygen reduction reaction (ORR) to overcome sluggish kinetics and catalyst instability in acids. Among others, scientists have identified the newly emerging atomically dispersed transition metal (M: Fe, Co, or/and Mn) and nitrogen co-doped carbon (M–N–C) catalysts as the most promising alternative to PGM catalysts. Here, we provide a comprehensive review of significant breakthroughs, remaining challenges, and perspectives regarding the M–N–C catalysts in terms of catalyst activity, stability, and membrane electrode assembly (MEA) performance. A variety of novel synthetic strategies demonstrated effectiveness in improving intrinsic activity, increasing active site density, and attaining optimal porous structures of catalysts. Rationally designing and engineering the coordination environment of single metal MN x sites and their local structures are crucial for enhancing intrinsic activity. Increasing the site density relies on the innovative strategies of restricting the migration and agglomeration of single metal sites into metallic clusters. Relevant understandings provide the correlations among the nature of active sites, nanostructures, and catalytic activity of M–N–C catalysts at the atomic scale through a combination of experimentation and theory. Current knowledge of the transferring catalytic properties of M–N–C catalysts to MEA performance is limited. Rationally designing morphologic features of M–N–C catalysts play a vital role in boosting electrode performance through exposing more accessible active sites, realizing uniform ionomer distribution, and facilitating mass/proton transports. We outline future research directions concerning the comprehensive evaluation of M–N–C catalysts in MEAs. The most considerable challenge of current M–N–C catalysts is the unsatisfied stability and rapid performance degradation in MEAs. Therefore, we further discuss practical methods and strategies to mitigate catalyst and electrode degradation, which is fundamentally essential to make M–N–C catalysts viable in PEMFC technologies.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.1c11331
