Understanding the inter-site distance effect in single-atom catalysts for oxygen electroreduction

Single atoms supported on carbon have gained prominence as oxygen reduction catalysts, while their intrinsically individual and collective properties remain elusive. Here, we demonstrate the effects of active site density and cooperativity between neighboring atomic sites at well-defined distances.

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    Fossil fuel emissions have long been considered a major contributor to global warming and atmospheric pollution. In addressing these challenges, replacing fossil fuels with clean, renewable energy sources is the most promising solution. As we look toward the future, our society is focusing on addressing the pressing energy demands and environmental concerns, which require clean, sustainable technologies such as proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, these devices are hampered by the sluggish reaction kinetics and insufficient stability of the cathodic oxygen reduction reaction (ORR).

    Over the past decade, considerable efforts have been devoted to synthetic methodologies of densely populated single-atom catalysts (SACs).[1],[2] Particularly, atomic-dispersed Fe-N4 catalysts have shown a powerful capability to catalyze various important reactions, such as the ORR.[3],[4] Given the Fe(II) site that is highly active for the ORR, the catalytic performance has been significantly promoted with high-loading SACs.[5] Despite this, there is a notable paucity of evidence-based literature specifically focusing on the correlation between overall activity and single-entity reactivity. Obviously, this evolves the complexity of physical and chemical processes instead of a simple summation over a number of single sites. In fact, subject to prevalent ensemble measurements, few previous reports have noticed or had the opportunity to acquire insightful information of catalytic atoms with the site proximity.

    In our recent work published in Nature Catalysis, we demonstrate that Fe–N4 SACs for the ORR can be comprehensively studied by means of an integrated experimental and theoretical approach. An important opportunity presented by this study is the ability to determine the kinetic behavior of individual active sites in conjunction with the proximity effect of neighboring metal atoms at well-defined distances. Specifically, interactions between adjacent Fe-N4 atomic sites cause the electronic structure to be altered when the inter-site distance is less than 1.2 nm, leading to increased intrinsic ORR activity. Site performance remains markedly improved until about 0.7 nm from neighboring Fe atoms, at which point the intrinsic activity decreases slightly. Thus, this contribution of Fe–N4 catalysts identifies the inter-site distance effect to be the fundamental mechanism supporting the ORR, which may help to maximize the efficiency of densely populated SACs.

Fig. 1. a. Scanning transmission electron microscopy images of representative Fe-N4 catalysts with different inter-site distance (dsite). b. Calibration of dsite with statistical distributions from electron microscopy against the corresponding Fe loading. c. Turnover frequency (TOF) for the ORR varying with dsite at given potentials.

    Complementing prior literature studies focused on the metal-substrate interaction in catalyzed reactions, this study stresses the existence and importance of communicating from site to site. Analysis of the site information presented in this paper can also be used to develop characterization techniques and evaluation of benchmarks for single-atom electrocatalysts. Besides ORR, latest publications on atomically dispersed Fe-Nx sites have showed they can catalyze numerous important reactions, including CO2 reduction and N2 reduction. The inter-site distance effect in SACs can provide important guidelines for designing highly efficient catalysts for a variety of important catalytic reactions based on this fundamental investigation.

    More details could be found through the link: https://doi.org/10.1038/s41929-021-00650-w 

References:

[1] Kaiser, S.K., Chen, Z., Faust Akl, D., Mitchell, S. & Pérez-Ramírez, J. Single-Atom Catalysts across the Periodic Table. Chem. Rev. 120, 11703-11809 (2020).

[2] Ji, S. et al. Chemical Synthesis of Single Atomic Site Catalysts. Chem. Rev. 120, 11900-11955 (2020).

[3] Martinez, U. et al. Progress in the Development of Fe-Based PGM-Free Electrocatalysts for the Oxygen Reduction Reaction. Adv. Mater. 31, 1806545 (2019).

[4] Wan, C., Duan, X. & Huang, Y. Molecular Design of Single-Atom Catalysts for Oxygen Reduction Reaction. Adv. Energy Mater. 10, 1903815 (2020).

[5] Wang, J., Li, Z., Wu, Y. & Li, Y. Fabrication of Single-Atom Catalysts with Precise Structure and High Metal Loading. Adv. Mater. 30, 1801649 (2018).

Zhaoyu Jin

Postdoc, The University of Texas at Austin