Recent progress in hollow sphere-based electrodes for high-performance supercapacitors

Vanadate-based Fe-MOFs as promising negative electrode for hybrid supercapacitor device

In the supercapacitor field, negative electrodes are mainly concentrated in carbon-based materials, such as activated carbon, carbon nanotubes, graphene, and so forth. However, materials based on metal-organic frameworks (MOFs) as negative active components are relatively rare. Herein, a series of composite materials based on graphene oxide (GO) and vanadate-based Fe-organic frameworks have been prepared by hydrothermal method namely GO/Fe-VO4-BIPY. The deposition amount of polyoxometalate-based metal-organic frameworks (POMOFs) on the surface of graphene is adjusted by changing the content of POMOFs. Through the deposition, it can effectively reduce the accumulation between graphene, and increase the dispersion of POMOFs. As a result, the charge storage performance of the as-obtained materials is greatly improved. Among these materials, GO/Fe-VO4-BIPY-1 has the most prominent performance, with a specific capacitance of 190 F g-1at 0.5 A g-1, which is attributed to the excellent synergistic effect between the Faraday chemical reaction and electric double-layer capacitance. In comparison with pristine Fe-VO4-BIPY, GO/Fe-VO4-BIPY-1 delivers more excellent surface area and therefore exhibits abundant redox reaction sites, achieving better electrochemical performance the best. After assembly with the positive Ni(OH)2electrode, the maximum energy density of 46.84 W h kg-1at a power density of 850 W kg-1is achieved.

Preparation of Sulfur-doped Carbon for Supercapacitor Applications: A Review

Electrochemical capacitors, also known as supercapacitors (SCs), have lately played an important role in energy storage and conversion systems due to their specific characteristics such as high strength, durability, and environmental friendliness. A wide range of materials is used as electrodes for SC applications because the electrochemical efficiency is primarily determined by the electrode materials used. Carbonaceous materials with unique surface, chemical, electrochemical, and electronic characteristics have become attractive for energy storage research, but they cannot meet the rising need for high specific energy and specific power. Besides, heteroatom-doped carbon materials have shown pseudocapacitance characteristics and improved specific energy, specific power, and conductivity. This makes them more adaptable in SC application. Among different heteroatom doping of carbon, S-doped carbon has gained considerable attention in SC applications due to its unpaired electrons and easily polarizable nature. S-doped carbon materials-based SCs have demonstrated enhanced surface wettability, improved conductivity, and induced pseudocapacitance effect, thereby delivering improved specific energy and specific power. Many reports on S-doped carbon for SC applications have been published, but there is no specific Review on the preparation of S-doped carbon for SC applications. This Review focuses on recent developments in the field of SC electrodes made from S-doped carbon materials. Herein, the preparation methods and applications of S-doped carbon for SCs were summarized following a brief discussion of different electrochemical characterization techniques of SCs. Finally, the challenges of S-doped carbon materials and their potential prospects were discussed to give crucial insights into the favorable factors for future innovations of SC electrodes. This Review aims to provide insight for further research on the preparation of S-doped carbon for electrochemical energy storage applications.

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.

Hollow Micro/Nanostructured Ceria-Based Materials: Synthetic Strategies and Versatile Applications

Hollow micro/nanostructured CeO₂ -based materials (HMNCMs) have triggered intensive attention as a result of their unique structural traits, which arise from their hollowness and the fascinating physicochemical properties of CeO₂ . This attention has led to widespread applications with improved performance. Herein, a comprehensive overview of methodologies applied for the synthesis of various hollow structures, such as hollow spheres, nanotubes, nanoboxes, and multishelled hollow spheres, is provided. The synthetic strategies toward CeO₂ hollow structures are classified into three major categories: 1) well-established template-assisted (hard-, soft-, and in situ template) methods; 2) newly emerging self-template approaches, including selective etching, Ostwald ripening, the Kirkendall effect, galvanic replacement, etc.; 3) bottom-up self-organized formation synthesis (namely, oriented attachment and self-deformation). Their underlying mechanisms are concisely described and discussed in detail, the differences and similarities of which are compared transversely and longitudinally. Niche applications of HMNCMs in a wide range of fields including catalysis, energy conversion and storage, sensors, absorbents, photoluminescence, and biomedicines are reviewed. Finally, an outlook of future opportunities and challenges in the synthesis and application of CeO₂ -based hollow structures is also presented.

Synthesis of NiMn-LDH Nanosheet@Ni3S2 Nanorod Hybrid Structures for Supercapacitor Electrode Materials with Ultrahigh Specific Capacitance

One of the key challenges for pseudocapacitive electrode materials with highly effective capacitance output and future practical applications is how to rationally construct hierarchical and ordered hybrid nanoarchitecture through the simple process. Herein, we design and synthesize a novel NiMn-layered double hydroxide nanosheet@Ni₃S₂ nanorod hybrid array supported on porous nickel foam via a one-pot hydrothermal method. Benefited from the ultrathin and rough nature, the well-defined porous structure of the hybrid array, as well as the synergetic effect between NiMn-layered double hydroxide nanosheets and Ni₃S₂ nanorods, the as-fabricated hybrid array-based electrode exhibits an ultrahigh specific capacitance of 2703 F g⁻¹ at 3 A g⁻¹. Moreover, the asymmetric supercapacitor with this hybrid array as a positive electrode and wood-derived activated carbon as a negative electrode demonstrates high energy density (57 Wh Kg⁻¹ at 738 W Kg⁻¹) and very good electrochemical cycling stability.

Hollow nanostructures of metal oxides as next generation electrode materials for supercapacitors

Hollow nanostructures of copper oxides help to stabilize appreciably higher electrochemical characteristics than their solid counter parts of various morphologies. The specific capacitance values, calculated using cyclic voltammetry (CV) and charge-discharge (CD) studies, are found to be much higher than the values reported in literature for copper oxide particles showing  intriguing morphologies or even composites with trendy systems like CNTs, rGO, graphene, etc. The proposed cost-effective synthesis route makes these materials industrially viable for application in alternative energy storage devices. The improved electrochemical response can be attributed to effective access to the higher number of redox sites that become available on the surface, as well as in the cavity of the hollow particles. The ion transport channels also facilitate efficient de-intercalation, which results in the enhancement of cyclability and Coulombic efficiency. The charge storage mechanism in copper oxide structures is also proposed in the paper.

Hierarchical VOOH hollow spheres for symmetrical and asymmetrical supercapacitor devices

Hierarchical VOOH hollow spheres with low crystallinity composed of nanoparticles were prepared by a facile and template-free method, which involved a precipitation of precursor microspheres in aqueous solution at room temperature and subsequent hydrothermal reaction. Quasi-solid-state symmetric and asymmetric supercapacitor (SSC and ASC) devices were fabricated using hierarchical VOOH hollow spheres as the electrodes, and the electrochemical properties of the VOOH//VOOH SSC device and the VOOH//AC ASC device were studied by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Results demonstrated that the electrochemical performance of the VOOH//AC ASC device was better than that of the VOOH//VOOH SSC device. After 3000 cycles, the specific capacitance of the VOOH//AC ASC device retains 83% of the initial capacitance, while the VOOH//VOOH SSC device retains only 7.7%. Findings in this work proved that hierarchical VOOH hollow spheres could be a promising candidate as an ideal electrode material for supercapacitor devices.

Low-crystalline mesoporous CoFe2O4/C composite with oxygen vacancies for high energy density asymmetric supercapacitors

A low-crystalline mesoporous cobalt ferrite and carbon composite (L-CoFe₂O₄/C) material was prepared via a sol–gel approach and calcination process. An L-CoFe₂O₄/C//AC asymmetric supercapacitor exhibited high energy density and power density.