ReviewCoordination anchoring synthesis of high-density single-metal-atom sites for electrocatalysis
Graphical abstract
Introduction
To alleviate the increasing energy crisis and rising environmental concerns in the 21st century, exploring advanced technologies for clean and sustainable energy conversion has become an urgent and necessary task [1], [2]. As one of the most promising approaches, electrocatalytic technologies driven by renewable energy has been developed and rapidly promoted to convert various small molecules like water (H2O) [3], [4], [5], nitrogen (N2) [6], [7], [8], and carbon dioxide (CO2) [9], [10], [11] into fuels and high value-added chemicals (e.g., hydrogen, ammonia, formic acid, etc.). However, the high activation barriers of these thermo-stable molecules result in the kinetic sluggishness of these reactions [12], [13], [14], [15], thus making the electrocatalysts crucial for reducing the activation energy barrier, accelerating the reaction rate, and controlling the direction of electrochemical reactions. Therefore, to further promote the commercial application of electrocatalytic energy conversion technologies, including fuel cells [16], [17], [18], [19], metal-air batteries [20], [21], [22], [23], [24], and water splitting devices [5], [25], [26], [27], [28], the development of highly efficient, durable, and economic electrocatalysts is urgently required.
Recently, single-atom catalysts (SACs) with single-metal-atom sites (SMAS), where single metal atoms surrounded by nonmetal-atoms dispersed on functional supports [29], have been demonstrated as excellent electrocatalysts due to their maximal atomic usage and unique site structures [30], [31], [32]. On account of the uniform coordinate structures, the nitrogen coordinated Fe sites (Fe-Nx) achieve an excellent selectivity toward CO in the electroreduction of CO2 with high current densities [33]. Moreover, with the construction of the well-defined SMAS, the electrocatalytic mechanism and structure–activity relationships are further investigated. Wu’s group recently reported that the compressively strained Co-N4 sites in Co-N-C catalysts exhibited high intrinsic activity in electrocatalytic ORR with a 4e− pathway, which demonstrated a better activity of the strained Co-N coordinate structure than Co-OH or Co-O structures [34]. For CO2 electroreduction, two model Ni-complexed porphyrins were studied, in which isolated Ni sites with specific tetraphenylporphyrin (N4–TPP) and 21-oxatetraphenylporphyrin (N3O − TPP), showing that the broken ligand-field symmetry is the key for superior CO2 performance [35]. However, the high surface free energy of SMAS leads them‘1 to agglomerate into nanoparticles, making it challenging to increase the loading density of SMAS during synthesis and catalytic processes [36].
Among the various approaches for synthesizing stable SMAS, coordinate anchoring is one of the most efficient strategies due to the significant coordinate interactions between SMAS and supports. For example, when choosing dicyandiamide to provide coordinate sites for anchoring transition-metal atoms, the loading of isolated metal-Nx sites could reach up to 20 wt% for enhanced catalytic activities [37]. Constructing coordination matrices such as metal–organic frameworks (MOFs) with the strong coordination between ligands and metal ions is also efficient to prevent the migration of SMAS during the high-temperature pyrolysis [38]. The coordinative anchoring strategy provides ideal methodologies for improving the loading density, tuning the coordination environment, and enhancing the stability of SMAS, which could significantly improve the activity of SACs and thus achieve promising electrocatalytic applications [39], [40], [41], [42]. The porphyrinic MOFs with nitrogen-rich sites are chosen to coordinate transition metal atoms, which can effectively avoid aggregation during the synthetic process. The loading density of SMAS could be significantly increased by controlling the spatial distance between metal centers within porphyrinic MOFs [43]. Therefore, the rational design of the unique anchoring sites by adopting coordinative anchoring synthesis to control the dispersion of SMAS with high loading density has become a hotspot in the advanced development of high-performance SACs [44].
In this review, we summarize the recent development of high-loading SMAS electrocatalysts with an emphasis on the coordinative anchoring synthesis and their electrocatalytic applications. The first section introduces general synthetic methods of SMAS, including impregnation, coprecipitation, atom layer deposition, photochemistry, ball milling, in-situ thermal pyrolysis, ion exchange, and high-temperature migration. Subsequently, the coordination anchoring synthetic strategies are comprehensively summarized from the perspectives of molecular coordination anchoring, building matrix anchoring (i.e., metal–ligand complex matrix, etc.), and surface coordination anchoring with attention to manipulating the metal center and coordination environment of SMAS. The following section is devoted to the electrochemical reaction mechanisms and applications related to SMAS, including hydrogen evolution reaction, oxygen evolution reaction, nitrogen reduction reaction, oxygen reduction reaction, and CO2 reduction reaction. In the final section, the remaining challenges and brief perspectives are presented. We hope that this review could provide in-depth insights into the synthetic chemistry and material science of SMAS and further promote the development of single atom electrocatalysis.
Section snippets
General synthetic methods for SMAS
Since the emergence of SACs in 2011, the synthesis of well-defined SMAS has been attracting intensive attention and lays the foundation of their applications. In the last decade, several general methods have been proposed for synthesizing SACs with various metal centers. Typically, precursors with single or multi- metal cores are firstly adsorbed, then reduced, and finally restricted onto supports. Such processes usually require a high temperature to decompose the metal precursors or break the
Coordination anchoring protocols for building high-density SMAS
Combined with the general synthetic methods discussed above, the facile synthesis of SACs with high-load SMAS still faces severe issues. First, when increasing the metal loading on SACs, the high surface energy of single-atom metal sites would lead the metal atoms to agglomerate into nanocrystals, which demands high-density coordination anchoring sites to stabilize the SMAS for obtaining high-activity and durable catalysts in various electrochemical reactions. Second, the rational design of
High-density SMAS for electrocatalysis
With the aforementioned practical anchoring strategies, the design and manufacture of SACs with high-density and tunable-structured SMAS have enriched new fields in developing high-performance electrocatalysts. The achievement of SACs can maximumly improve the atom economy by exposing every metal atom as an active site. Meanwhile, by employing appropriate anchoring methods, the elaborate coordination environments and the electronic structure of SMAS can be specifically engineered and modulated.
Conclusion and perspectives
Single-atom catalysts have evolved very rapidly in electrocatalysis, and advanced fabrication methods for SMAS are necessary. In this review, we present a systematic summary of coordination anchoring synthetic strategies to prepare desired SMAS with well-designed structure and high loadings, including the molecular coordination anchoring, directly building SMAS matrix, and the surface coordination anchoring. The coordinate structures of SMAS can be delicately regulated by the targeted design of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge the financial support from National Key Research and Development Program of China (2020YFB1505801), the National Natural Science Foundation of China (22025208, 22075300, 21902162 and 22102159), Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM 202202), and the Fundamental Research Funds for the Central Universities (2652020018).
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These authors contributed equally to this work.