News Resource: School of Chemistry and Chemical Engineering

Editor: News Agency of BIT

Translator: Shao Yikang, News Agency of BIT

Beijing Institute of Technology, September 19, 2022: Oxygen reduction reactions (ORR) are important semi-reactions in fuel cells and metal-air batteries, which also remains a cost-effective method for electrosynthesis of H₂O₂. However, the problem of low selectivity and activity still needs to be addressed. The key to rationally optimizing ORR activity and selectivity is to control the electronic structure of the electrocatalyst. Carbon-supported atomically dispersed metal sites (C-ADMSs) are characterized by isolated metal atoms with a well-defined local coordination structure in a carbon substrate, where the coordination bonds, bond lengths, and coordination numbers can be adjusted to alter the selectivity or activity of ORR. Thanks to their metallase-like properties, C-ADMSs have become an ideal tunable platform to control the selectivity and activity of ORR. Therefore, it is of great significance to deeply summarize the regulation of metal centric coordination environment in C-ADMSs for ORR activity and selectivity.

Recently, Professor Zhang Jiatao's team from the School of Chemistry and Chemical Engineering, in collaboration with Professor Chen Chen of Tsinghua University, systematically elaborated on the coordination structure change of the metal site dispersed by carbon atoms caused by coordination environment regulation and coordination structure change, as well as the corresponding oxygen reduction selectivity and activity regulation. In particular, unique insights are proposed on the regulation strategy of the coordination shell, such as the influence of temperature on the coordination environment in material synthesis, the regulation of the coordination environment of different shell layers, the stability problem of the active site and the optimization scheme and so on. The relating results were published in the International Journal of Materials Chemistry A (DOI: 10.1039/D2TA05110A) under the title "Tailoring the Selectivity and Activity of Oxygen Reduction by Regulating the Coordination Environments of Carbon-Supported Atomically Dispersed Metal Sites”. BIT is the first communication unit. Zhu Pan, a graduate student, is the sole author of the paper.

Fig.1: Schematic illustration of coordination environment regulationof C-ADMSs for tailoring the ORR selectivity and activity.

The ORR process is divided into two electron reactions – the generation of OOH-/H₂O₂and the four-electron reaction – of which generate OH-/H₂O. There is a clear competition between these two different response pathways. Since the two-electron process involves only *OOH intermediates, the mechanism is clearly the simplest reaction step. *The binding free energy of the *OOH intermediate (ΔG(*OOH)) determines the catalytic activity and selectivity. Different chemical intermediates such as *OOH, *O, and *OH are involved in the four-electron process through a variety of pathways, including *OOH association, *OOH dissociation, and shaft-end adsorption pathway.

Fig.2 ORR reaction mechanism and pathway

According to coordination chemistry and crystal field theory, atoms that are directly coordinated with the central active metal can have a specific and significant effect on the overall catalytic site. Since the central metallic bit usually coordinates 4 atoms (usually d-p σ-bonds), its bond energy or local electronic structure is determined by the first coordination atom.

Fig.3 The first shell coordination environment controlling

The coordination number of the central active metal has been shown to have an intrinsic relationship with intermediate adsorption and thus with catalytic activity. In order to reveal the mechanism of coordination regulation, a variety of strategies have been explored. Among them, the nitrogen coordination number is precisely regulated by the control strategy based on high-temperature calcination and reduction.

Fig.4 Coordination number regulation of the first shell

Unlike atoms that coordinate directly with the first layer of central metal, singular atoms doped in the second and higher coordination layers can be customized for remote electron interactions for isolated reaction centers.

Fig. 5 Coordination environment regulation of the second shell and higher shell

In recent years, the development of C-ADMSs has become an electron structure with ultra-high atomic utilization and obvious tunable metal sites, which has brought valuable direction to catalysis-related fields and is considered to be the next generation of emerging catalysts. The unique selectivity and activity of C-ADMS is the basis for further improving catalytic performance, expanding the range of catalytic reactions and practical applications. However, the precise characterization of the active center site, the dynamic reconstruction of the catalytic site caused by the reaction in progress, the updated coordination atom, and the practical application in industrial production are still issues worthy of study and challenge.

Fig. 6 Challenges and prospects of ORR catalysts’ high performance

Title of the paper: Tailoring the Selectivity and Activity of Oxygen Reduction by Regulating the Coordination Environments of Carbon-Supported Atomically Dispersed Metal Sites

Paper link:https://pubs.rsc.org/en/content/articlelanding/2022/ta/d2ta05110a#!divAbstract

DOI：doi.org/10.1039/D2TA05110A

Brief introduction of author attached:

Zhao Di received her Ph.D. from BIT in 2017. After being a postdoctoral fellow in the Department of Chemistry of Tsinghua University, she joined BIT in 2020, currently positioning at an associate professor in the School of Chemistry and Chemical Engineering. Her research interests include nano, cluster, and single-atom catalyst synthesis and catalytic properties. Related research results can be found in J. Am. Chem. Soc.、Angew. Chem. Int. Ed.、Chem. Soc. Rev.、Energy Environ. Sci., Nano Energy and other international academic journals with nearly 20 related papers, some of which have been invited to be the cover of the journal and have been featured by C&EN. 3 papers are highly cited by ESI. One international patent was granted. She won the second postdoctoral innovation talent support program. As the project leader, she undertook the project of China Postdoctoral Science Foundation and participated in the projects of the National Natural Science Foundation of China and the horizontal project of enterprises.