In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal–organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.

Hollow carbon nanocages (HCNCs) consisting of sp2 carbon shells featured by a hollow interior cavity with defective microchannels (or customized mesopores) across the carbon shells, high specific surface area, and tunable electronic structure, are quilt different from the other nanocarbons such as carbon nanotubes and graphene. These structural and morphological characteristics make HCNCs a new platform for advanced electrochemical energy storage and conversion. This review focuses on the controllable preparation, structural regulation, and modification of HCNCs, as well as their electrochemical functions and applications as energy storage materials and electrocatalytic conversion materials. The metal single atoms‐functionalized structures and electrochemical properties of HCNCs are summarized systematically and deeply. The research challenges and trends are also envisaged for deepening and extending the study and application of this hollow carbon material. The development of multifunctional carbon‐based composite nanocages provides a new idea and method for improving the energy density, power density, and volume performance of electrochemical energy storage and conversion devices.

Due to the unique geometric characteristics and electronic structure of hierarchical 3D nanosheets, they show excellent electron migration rate, ultra‐high specific surface area, and reliable structural stability. Therefore, 3D nanosheets have great application prospects in the field of electrochemical energy storage. Supercapacitor has attracted extensive attention in recent years due to its merits of fast charge and discharge, long cycle life, safety, and stability. Flexibility, miniaturization, and intelligent integration are the development direction of supercapacitor energy storage devices. The emerging 3D printing technology, especially the ink direct writing mode, has greatly improved the design ability and control accuracy of device microstructures. Based on the research progress of 3D graphene nanosheets and 3D MXene nanosheets in the early stage of the authors’ or other teams, this paper proposes to use advanced 3D printing technology to realize the design of flexible all solid‐state supercapacitors by using 3D nanosheets active materials with high specific capacitance. The design methods of interdigital electrodes, multilayer skeleton electrodes and fiber electrodes by 3D printing technology and the performance evaluation of flexible supercapacitor are systematically analyzed. This perspective review aims to provide new conception and theoretical guidance for the design, preparation, and performance optimization of 3D nanosheets‐built materials by 3D printing for the practical application of flexible all‐solid‐state supercapacitor in the future.

metal catalyst not only has a relatively low metal loading but also greatly improves the utilization efficiency of metal atoms. It can change the adsorption/desorption selectivity of the active components on the catalyst to different molecules, thus affecting the reaction kinetics. [4][5][6] With the development of advanced characterization technologies (synchrotron-radiation X-ray absorption fine structure (XAFS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS); special aberration-corrected transmission electron microscopy (AC-TEM), such as high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), etc.), it has become a reality to elucidate the structure-activity relationship of SACs at the angstroms atomic scale, which also provides an opportunity to link heterogeneous catalysis with homogeneous catalysis. [7,8] In particular, SACs provide a perfect operational platform for the study of molecular level catalytic reaction mechanism. The applications of SACs also become more and more wide, including the versatile heterogeneous catalytic fields (thermocatalysis, electrocatalysis, and photocatalysis) or noncatalytic fields (energy storage battery, electromagnetic wave absorption, and so on).Aerogel is a form of solid matter, one of the least dense solids in the world. The most common aerogel is made of silica; but there are also aerogels made of graphene, metal oxides, polymers, and so on. [9] The lightest silica aerogel is only 3 mg cm −3 , which is 3 times heavier than air, so it is also called "frozen smoke" or "solid smoke." Traditional aerogel material refers to a kind of porous solid material of nanometer or micrometer level formed by the sol-gel method, using certain drying methods (such as freeze drying) to make gas replace liquid phase in gel. [10] Aerogels are highly porous materials (more than 90% porosity), whose internal structure is formed by a network of interconnected basic units (nanosheets, nanofibers, or nanoparticles) separated by open pores. The combination of small basic units, high surface area, and open structure makes aerogels have many unique properties in thermal, mechanical, optical, electrical, and acoustic aspects, which can be used as excellent adsorbents, catalysts, heat insulation materials, and soundThe concept of "single-atom catalysis" is first proposed by Tao Zhang, Jun Li, and Jingyue Liu in 2011. Single-atom catalysts (SACs) have a very high catalytic activity and greatly improved atom utilization ratio. At present, SACs have become frontier materials in the field of catalysis. Aerogels are highly porous materials with extremely low density and extremely high porosity. These pores play a key role in determining their surface reactivity and mechanical stability. The alliance of SACs and aerogels can fully reflect their structural advantages and lead to new enhancement effects. Herein, a general concept of "atomic aerogel materials" (AAMs) (or single-atom aerogels (SAAs)) is proposed to descr...

and conventional transition metals SACs supported on metal or non-metal carriers) have been developed and upgraded greatly. [3][4][5] Theoretically speaking, the dispersion limit of supported metal catalysts is the uniform distribution of the metal in the form of single atoms on the support: this is not only the ideal state of supported metal catalysts, but also brings the science of catalysis from the "nano age" into the "atomic age". [6][7][8] SACs usually refer to the active sites where isolated metal atoms are loaded onto the surface of the carriers to reach the maximum atomic utilization. These active sites are anchored by adjacent coordination atoms on the carrier (generally through covalent metal-nonmetal bonding) to prevent the thermodynamic diffusion and irreversible aggregation of single metal atoms. However, SACs all along face an unavoidable physicochemical problem: when the metal particles are reduced to the level of single atomic dispersions, the specific surface area increases sharply (reaching the maximum theoretical value), also leading to the maximum value of the metal surface free energy. [9] Therefore, in the actual application environment and catalytic reaction conditions, especially at high temperature or in a reducing atmosphere, the single atomic metal active sites are extremely prone to agglomeration and coupling to form large atomic clusters (even nanoparticles), resulting in the degradation of catalyst performance or even complete inactivation. [10] Recently, however, scientists have found that fully exposed cluster catalysts with low coordination metal bonds (the coordination number of the metal bond is ≈4.4) [11] and local single-atomic agglomeration [12] show more satisfactory performances in specific complex catalytic systems. These works provide a basic understanding of the dynamic stability of SACs under working conditions and call for a reassessment of the reported stability of SACs by considering realistic reaction conditions.Admittedly, it is quite necessary to reevaluate the realistic catalytic stability and dynamic catalysis mechanism of SACs; nevertheless, we hold the opinion that it is more important to design fundamentally a kind of efficient SACs with high activity and high stability at the same time (such as the "moving but not aggregating" design philosophy for metal active sites). On October 26, 2022, in collaboration with Yong Wang's group at Washington State University, Bruce C. Gates's group at the University of California, and Jingyue Liu's group at Arizona Academician Tao Zhang from China and co-workers designed the first Pt 1 / FeO x single-atom catalysts (SACs) in 2011, and they proposed the concept of "single-atom catalysis" in the field of heterogeneous catalysis. Generally, it is easy for active metal single-atom sites on a carrier to migrate and aggregate, which results in poor performance; or the chemical bond between the metal atom and carrier is too strong (immovable), which results in passivation of the active site. Recently, "nano-island" type SACs were...