Engineering Rich-Cation Vacancies in CuCo2O4 Hollow Spheres with a Large Surface Area Derived from a Template-Free Approach for Ultrahigh Capacity and High-Energy Density Supercapacitors

Intriguing cationic defects with hollow nano-/microstructures are a critical challenge but a potential strategy to discover electrochemical energy conversion and storage devices with improved electrochemical performances. Herein, we successfully produced a highly porous, and large surface area of self-templated CuCo₂O₄ hollow spheres (CCOHSs) with cationic defects via a solvothermal route. We hypothesized that the inside-out Ostwald ripening mechanism of the template-free strategy was the framework for forming the CCOHSs. Cationic defects (Cu) within the CCOHSs were identified by employing various analytical techniques, including energy-dispersive X-ray spectroscopy analysis of both scanning and transmission electron microscopy, X-ray photon spectroscopy, and inductively coupled plasma-atomic emission spectroscopy. The resulting CCOHSs had significant properties, such as a high specific surface area of 98.32 m² g^-1, rich porosity, and battery-type electrode behavior in supercapacitor applications. Notably, the CCOHSs demonstrated an outstanding specific capacity of 1003.7 C g^-1 at 1 A g^-1, with excellent structural integrity and cycle stability. Moreover, the fabricated asymmetric CCOHS//activated carbon device exhibited a high energy density of 65.2 Wh kg^-1 at a power density of 777.8 W kg^-1.

Nanohollow Carbon for Rechargeable Batteries: Ongoing Progresses and Challenges

Among the various morphologies of carbon-based materials, hollow carbon nanostructures are of particular interest for energy storage. They have been widely investigated as electrode materials in different types of rechargeable batteries, owing to their high surface areas in association with the high surface-to-volume ratios, controllable pores and pore size distribution, high electrical conductivity, and excellent chemical and mechanical stability, which are beneficial for providing active sites, accelerating electrons/ions transfer, interacting with electrolytes, and giving rise to high specific capacity, rate capability, cycling ability, and overall electrochemical performance. In this overview, we look into the ongoing progresses that are being made with the nanohollow carbon materials, including nanospheres, nanopolyhedrons, and nanofibers, in relation to their applications in the main types of rechargeable batteries. The design and synthesis strategies for them and their electrochemical performance in rechargeable batteries, including lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and lithium-sulfur batteries are comprehensively reviewed and discussed, together with the challenges being faced and perspectives for them.

Design of Hollow Nanostructures for Energy Storage, Conversion and Production

Hollow nanostructures have shown great promise for energy storage, conversion, and production technologies. Significant efforts have been devoted to the design and synthesis of hollow nanostructures with diverse compositional and geometric characteristics in the past decade. However, the correlation between their structure and energy-related performance has not been reviewed thoroughly in the literature. Here, some representative examples of designing hollow nanostructure to effectively solve the problems of energy-related technologies are highlighted, such as lithium-ion batteries, lithium-metal anodes, lithium-sulfur batteries, supercapacitors, dye-sensitized solar cells, electrocatalysis, and photoelectrochemical cells. The great effect of structure engineering on the performance is discussed in depth, which will benefit the better design of hollow nanostructures to fulfill the requirements of specific applications and simultaneously enrich the diversity of the hollow nanostructure family. Finally, future directions of hollow nanostructure design to solve emerging challenges and further improve the performance of energy-related technologies are also provided.

Sequential Templating Approach: A Groundbreaking Strategy to Create Hollow Multishelled Structures

Thanks to their distinguished properties such as optimized specific surface area, low density, high loading capacity, and sequential matter transfer and storage, hollow multishelled structures (HoMSs) have attracted great interest from scientists in broad fields, including catalysis, drug delivery, solar cells, supercapacitors, lithium-ion batteries, electromagnetic wave absorption, and sensors. However, traditional synthesis methods such as soft-templating and hierarchical self-assembly methods can hardly realize the controllable synthesis of HoMSs, thus limiting their development and application. Here, the development process of HoMSs is first succinctly reviewed and the shortcomings of the traditional synthesis method are concluded. Subsequently, the sequential templating approach, which shows great generality for the synthesis of HoMSs with controllable composition and geometry configuration and exhibits remarkable effect on the scientific research field, is introduced. The basic material science and chemical reaction mechanism involved in the synthesis and manipulation of HoMSs using the sequential templating approach are then explained in detail. In addition, the effect of the geometric characteristics of HoMSs on their application properties is highlighted. Finally, the current challenges and future research directions of HoMSs are also suggested.

Hollow multi-shelled structures for energy conversion and storage applications

Materials with hollow multi-shelled structures composed of various compositions are promising candidates for energy conversion and storage applications.

Tailoring porous carbon spheres for supercapacitors

The last decade has witnessed significant breakthroughs in the synthesis of porous carbon spheres (PCSs). This Review provides an updated summarization on the controlled synthesis of PCSs for supercapacitors. The synthetic methodologies can be generally categorized into (i) hard templating, (ii) soft templating, (iii) the modified Stöber method, (iv) hydrothermal carbonization (HTC), and (v) aerosol-assisted methods. The obtained PCSs include microporous/mesoporous/macroporous carbon spheres, single-/multi-shelled hollow carbon spheres, and yolk@shell carbon spheres. The structure-electrochemical performance correlation is discussed. Finally, the future research directions on the rational design of PCSs for supercapacitors are predicted.

Intricate Hollow Structures: Controlled Synthesis and Applications in Energy Storage and Conversion

Intricate hollow structures garner tremendous interest due to their aesthetic beauty, unique structural features, fascinating physicochemical properties, and widespread applications. Here, the recent advances in the controlled synthesis are discussed, as well as applications of intricate hollow structures with regard to energy storage and conversion. The synthetic strategies toward complex multishelled hollow structures are classified into six categories, including well-established hard- and soft-templating methods, as well as newly emerging approaches based on selective etching of "soft@hard" particles, Ostwald ripening, ion exchange, and thermally induced mass relocation. Strategies for constructing structures beyond multishelled hollow structures, such as bubble-within-bubble, tube-in-tube, and wire-in-tube structures, are also covered. Niche applications of intricate hollow structures in lithium-ion batteries, Li-S batteries, supercapacitors, Li-O₂ batteries, dye-sensitized solar cells, photocatalysis, and fuel cells are discussed in detail. Some perspectives on the future research and development of intricate hollow structures are also provided.

A family of microscale 2 × 2 × 2 Pocket Cubes

A family of Pocket Cubes with different chemical compositions but with the same overall mesoscale microstructures was prepared for potential applications in energy storage and water treatment.

High Performance Particle/Polymer Nanofiber Anodes for Li-ion Batteries using Electrospinning

Electrospun nanofiber mats containing carbon nanoparticles in a poly(vinylidene fluoride) binder were prepared and characterized as Li-ion battery anodes. The mats exhibited an initial capacity of 161 mAh g(-1) with 91.7% capacity retention after 510 cycles at 0.1 C (1 C=372 mA gcarbon (-1)). Whereas many nanoscale electrodes are limited to low areal and/or volumetric capacities, the particle/polymer nanofiber anodes can be made thick with a high fiber volume fraction while maintaining good rate capabilities. Thus, a nanofiber anode with a fiber volume fraction of 0.79 exhibits a volumetric capacity of 55 mAh cm(-3) at 2 C, which is twice that of a typical graphite anode. Similarly, thick nanofiber mats with a high areal capacity of 4.3 mAh cm(-2) were prepared and characterized. The excellent performance of electrospun anodes is attributed to electrolyte intrusion throughout the interfiber void space and efficient Li(+) transport between the electrolyte and carbon nanoparticles in the radial fiber direction.