Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors This review provides comprehensive knowledge to this field We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors

Design and Mechanisms of Asymmetric Supercapacitors.

5. In Situ Polymerization 3907 5.1. General Polymerization 3907 5.2. Photopolymerization 3910 5.3. Surface-Initiated Polymerization 3912 5.4. Other Methods 3913 6. Colloidal Nanocomposites 3913 6.1. Sol-Gel Process 3914 6.2. In Situ Polymerization 3916 6.2.1. Emulsion Polymerization 3917 6.2.2. Emulsifier-Free Emulsion Polymerization 3919 6.2.3. Miniemulsion Polymerization 3920 6.2.4. Dispersion Polymerization 3921 6.2.5. Other Polymerization Methods 3923 6.2.6. Conducting Nanocomposites 3924 6.3. Self Assembly 3926 7. Other Preparative Methods 3926 8. Characterization and Properties 3928 8.1. Chemical Structure 3928 8.2. Microstructure and Morphology 3929 8.3. Mechanical Properties 3933 8.3.1. Tensile, Impact, and Flexural Properties 3933 8.3.2. Hardness 3936 8.3.3. Fracture Toughness 3937 8.3.4. Friction and Wear Properties 3937 8.4. Thermal Properties 3938 8.5. Flame-Retardant Properties 3941 8.6. Optical Properties 3942 8.7. Gas Transport Properties 3943 8.8. Rheological Properties 3945 8.9. Electrical Properties 3945 8.10. Other Characterization Techniques 3946 9. Applications 3947 9.1. Coatings 3947 9.2. Proton Exchange Membranes 3948 9.3. Pervaporation Membranes 3948 9.4. Encapsulation of Organic Light-Emitting Devices 3948

Polymer/Silica Nanocomposites: Preparation, Characterization, Properties, and Applications

A facile and novel one-step method of growing nickel-cobalt layered double hydroxide (Ni-Co LDH) hybrid films with ultrathin nanosheets and porous nanostructures on nickel foam is presented using cetyltrimethylammonium bromide as nanostructure growth assisting agent but without any adscititious alkali sources and oxidants. As pseudocapacitors, the as-obtained Ni-Co LDH hybrid film-based electrodes display a significantly enhanced specific capacitance (2682 F g−1 at 3 A g−1, based on active materials) and energy density (77.3 Wh kg−1 at 623 W kg−1), compared to most previously reported electrodes based on nickel-cobalt oxides/hydroxides. Moreover, the asymmetric supercapacitor, with the Ni-Co LDH hybrid film as the positive electrode material and porous freeze-dried reduced graphene oxide (RGO) as the negative electrode material, exhibits an ultrahigh energy density (188 Wh kg−1) at an average power density of 1499 W kg−1 based on the mass of active material, which greatly exceeds the energy densities of most previously reported nickel or cobalt oxide/hydroxide-based asymmetric supercapacitors.

Nickel–Cobalt Layered Double Hydroxide Nanosheets for High-performance Supercapacitor Electrode Materials

Supercapacitors have attracted much interest in the past decades owing to their important applications, but most of them are focused on solitary or simple metal oxides. Here, a novel supercapacitor electrode composed of multicomponent MnCo2O4@Ni(OH)2 belt-based core–shell nanoflowers is reported by a facile and cost-effective method. This hybrid electrode exhibits a significantly enhanced specific capacitance. An asymmetric supercapacitor based on this unique hybrid nanoflowers as anode and an activated carbon film as cathode demonstrates high energy density, high power density, and long cycling lifespan.

Preparation of MnCo2O4@Ni(OH)2 Core–Shell Flowers for Asymmetric Supercapacitor Materials with Ultrahigh Specific Capacitance

A facile one-step hydrothermal co-deposition method for growth of ultrathin Ni(OH)2-MnO2 hybrid nanosheet arrays on three dimensional (3D) macroporous nickel foam is presented. Due to the highly hydrophilic and ultrathin nature of hybrid nanosheets, as well as the synergetic effects of Ni(OH)2 and MnO2, the as-fabricated Ni(OH)2-MnO2 hybrid electrode exhibits an ultrahigh specific capacitance of 2628 F g−1. Moreover, the asymmetric supercapacitor with the as-obtained Ni(OH)2-MnO2 hybrid film as the positive electrode and the reduced graphene oxide as the negative electrode has a high energy density (186 Wh kg−1 at 778 W kg−1), based on the total mass of active materials.

One-Step Fabrication of Ultrathin Porous Nickel Hydroxide-Manganese Dioxide Hybrid Nanosheets for Supercapacitor Electrodes with Excellent Capacitive Performance

The change of the properties of acrylic-based polyurethane via addition of nano-silica

This paper reports the first nickel hydroxide–manganese dioxide–reduced graphene oxide (Ni(OH)2-MnO2-RGO) ternary hybrid sphere powders as supercapacitor electrode materials. Due to the abundant porous nanostructure, relatively high specific surface area, well-defined spherical morphology, and the synergetic effect of Ni(OH)2, MnO2, and RGO, the electrodes with the as-obtained Ni(OH)2-MnO2-RGO ternary hybrid spheres as active materials exhibited significantly enhanced specific capacitance (1985 F·g–1) and energy density (54.0 Wh·kg–1), based on the total mass of active materials. In addition, the Ni(OH)2-MnO2-RGO hybrid spheres-based asymmetric supercapacitor also showed satisfying energy density and electrochemical cycling stability.

Limin Wu

Papers

Nickel–Cobalt Layered Double Hydroxide Nanosheets for High-performance Supercapacitor Electrode Materials

Preparation of MnCo2O4@Ni(OH)2 Core–Shell Flowers for Asymmetric Supercapacitor Materials with Ultrahigh Specific Capacitance

One-Step Fabrication of Ultrathin Porous Nickel Hydroxide-Manganese Dioxide Hybrid Nanosheets for Supercapacitor Electrodes with Excellent Capacitive Performance

The change of the properties of acrylic-based polyurethane via addition of nano-silica

Porous Nickel Hydroxide–Manganese Dioxide-Reduced Graphene Oxide Ternary Hybrid Spheres as Excellent Supercapacitor Electrode Materials