energy sources to alleviate and coordinate the energy demand. Furthermore, excessive carbon emissions have also broken the original carbon balance in nature and destroyed the harmonious coexistence between humans and nature. [1] It is necessary to utilize surplus electric energy from renewable energy sources such as wind and solar power, to achieve the effective use of energy through oxygen reduction reaction (ORR), [2] carbon dioxide reduction reaction (CO 2 RR), [3] oxygen evolution reaction (OER), [4] hydrogen evolution reaction (HER), [4b,5] nitrogen reduction reaction (NRR), [6] and other processes to complete significantly the effective use of energy, which is an effective solution to alleviate environmental and energy issues. [7] Therefore, it is critical to speed up the development of electrocatalysts with high catalytic activity, stability, and selectivity. So far, most of the commercial heterogeneous nanostructure catalysts are well developed to accelerate the advent of the nano era, which have the unique electronic properties and chemical surface properties due to their structure effects. [8] However, it is still challenging for catalysts to exert the utilization rate of individual atoms. Fortunately, the single-atom catalyst (SAC) that can achieve 100% atomic utilization theoretically and reduce the cost simultaneously is undoubtedly an effective way to maximize the catalytic activity. The structural properties of the SACs provide a distinct advantage in researching the catalytic mechanism. On the one hand, it can be used as a model to explore the electron structure of the single-atom centers and the interaction between the electron cloud of the catalyzed substance in the heterogeneous catalytic process. Besides, it can also be used to study how the same transition metal single-atom catalyst optimizes its intrinsic charge characteristics to have affinity in different catalytic fields. [9] Beyond that, the study of SACs is basic research to promote progress from the era of nanomaterials to the period of single-atom materials. At present, the deep understanding of the structure and property of SACs is an important research direction. Among SACs, non-noble metal-nitrogencarbon (M-N-C, M stands for the first transient elements) with unique electronic properties are well known for their low cost and outstanding electrocatalytic performance. [10] Therefore, the M-N-C catalysts have gradually attracted researchers' attention. Atomically dispersed metal-nitrogen-carbon catalysts (M-N-C) have been widely used inthe field of energy conversion, which has already attracted a huge amount of attention. Due to their unsaturated d-band electronic structure of the center atoms, M-N-C catalysts can be applied in different electrocatalytic reactions by adjusting their own microscopic electronic structures to achieve the optimization of the structure-activity relationship. Consequently, it is of great significance for the revelation of electrocatalytic mechanism and structureactivity relationship of M-N-C catalysts....

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