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

High-performance, low-cost, safe and environmentally friendly batteries are important for portable electronics and electric vehicles. Here, we synthesized NiAlCo-layered double hydroxide (LDH) nanoplates attached to few-walled carbon nanotubes (NiAlCo LDH/CNT) as the cathode material of a rechargeable NiZn battery in aqueous alkaline electrolytes. The α-phase nickel hydroxide with ultrathin morphology and strong coupling to nanotubes afforded a cathode with a high capacity of ∼354 mA h g−1 and ∼278 mA h g−1 at current densities of 6.7 A g−1 and 66.7 A g−1, respectively. Al and Co co-doping is unique for stabilizing α-phase nickel hydroxide with only a small capacity loss of ∼6% over 2000 charge and discharge cycles at 66.7 A g−1. Rechargeable ultrafast NiZn batteries with NiAlCo LDH/CNT cathode and a zinc anode can deliver a cell voltage of ∼1.75 V, energy density of ∼274 W h kg−1 and power density of ∼16 kW kg−1 (based on active materials) with a charging time of <1 minute. The results open the possibility of ultrafast and safe batteries with high energy density.

Ultrafast high-capacity NiZn battery with NiAlCo-layered double hydroxide

Rational Design of Nanostructured Electrode Materials toward Multifunctional Supercapacitors

Fabricating hierarchical core-shell nanostructures is currently the subject of intensive research in the electrochemical field owing to the hopes it raises for making efficient electrodes for high-performance supercapacitors. Here, we develop a simple and cost-effective approach to prepare CuO@MnO2 core-shell nanostructures without any surfactants and report their applications as electrodes for supercapacitors. An asymmetric supercapacitor with CuO@MnO2 core-shell nanostructure as the positive electrode and activated microwave exfoliated graphite oxide (MEGO) as the negative electrode yields an energy density of 22.1 Wh kg−1 and a maximum power density of 85.6 kW kg−1; the device shows a long-term cycling stability which retains 101.5% of its initial capacitance even after 10000 cycles. Such a facile strategy to fabricate the hierarchical CuO@MnO2 core-shell nanostructure with significantly improved functionalities opens up a novel avenue to design electrode materials on demand for high-performance supercapacitor applications.

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Merging of Kirkendall Growth and Ostwald Ripening: CuO@MnO2 Core-shell Architectures for Asymmetric Supercapacitors

One-pot hydrothermal synthesis of reduced graphene oxide/Ni(OH)2 films on nickel foam for high performance supercapacitors

Homogeneous growth of nano-sized β-Ni(OH)2 on reduced graphene oxide for high-performance supercapacitors

Synthesis of a Co–Ni doped LiMn2O4 spinel cathode material for high-power Li-ion batteries by a sol–gel mediated solid-state route

A reduced graphene oxide/Ni(OH)2 composite with excellent supercapacitive performance was synthesized by a facile hydrothermal route without organic solvents or templates used. XRD and SEM results reveal that the nickel hydroxide, which crystallizes into hexagonal β-Ni(OH)2 nanoflakes with a diameter less than 200 nm and a thickness of about 10 nm, is well combined with the reduced graphene oxide sheets. Electrochemical performance of the synthesized composite as an electrode material was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. Its specific capacitance is determined to be 1672 F/g at a scan rate of 2 mV/s, and 696 F/g at a high scan rate of 50 mV/s. After 2000 cycles at a current density of 10 A/g, the composite exhibits a specific capacitance of 969 F/g, retaining about 86% of its initial capacitance. The composite delivers a high energy density of 83.6 W·h/kg at a power density of 1.0 kW/kg. The excellent supercapacitive performance along with the easy synthesis method allows the synthesized composite to be promising for supercapacitor applications.

Xin Liu

Papers

Homogeneous growth of nano-sized β-Ni(OH)2 on reduced graphene oxide for high-performance supercapacitors

Synthesis of a Co–Ni doped LiMn2O4 spinel cathode material for high-power Li-ion batteries by a sol–gel mediated solid-state route

Excellent supercapacitive performance of a reduced graphene oxide/Ni(OH)2 composite synthesized by a facile hydrothermal route

Excellent electrochemical performance of porous nanoparticles-constructed granule LiMn2O4 derived from a highly reactive Mn3O4

Synthesis and electrochemical performance of a LiMn1.83Co0.17O4 shell/LiMn2O4 core cathode material