Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp 2 -bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

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Carbon-based Supercapacitors Produced by Activation of Graphene

Rechargeable lithium-sulfur batteries.

Overview of current development in electrical energy storage technologies and the application potential in power system operation

Electrochemical capacitors (i.e. supercapacitors) include electrochemical double-layer capacitors that depend on the charge storage of ion adsorption and pseudo-capacitors that are based on charge storage involving fast surface redox reactions. The energy storage capacities of supercapacitors are several orders of magnitude higher than those of conventional dielectric capacitors, but are much lower than those of secondary batteries. They typically have high power density, long cyclic stability and high safety, and thus can be considered as an alternative or complement to rechargeable batteries in applications that require high power delivery or fast energy harvesting. This article reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, systems, characterization methods, and applications. In particular, the newly developed charge storage mechanism for intercalative pseudocapacitive behaviour, which bridges the gap between battery behaviour and conventional pseudocapacitive behaviour, is also clarified for comparison. Finally, the prospects and challenges associated with supercapacitors in practical applications are also discussed.

Electrochemical capacitors: mechanism, materials, systems, characterization and applications

Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

A review of electrolyte materials and compositions for electrochemical supercapacitors

Carbon materials have attracted intense interests as electrode materials for electrochemical capacitors, because of their high surface area, electrical conductivity, chemical stability and low cost. Activated carbons produced by different activation processes from various precursors are the most widely used electrodes. Recently, with the rapid growth of nanotechnology, nanostructured electrode materials, such as carbon nanotubes and template-synthesized porous carbons have been developed. Their unique electrical properties and well controlled pore sizes and structures facilitate fast ion and electron transportation. In order to further improve the power and energy densities of the capacitors, carbon-based composites combining electrical double layer capacitors (EDLC)-capacitance and pseudo-capacitance have been explored. They show not only enhanced capacitance, but as well good cyclability. In this review, recent progresses on carbon-based electrode materials are summarized, including activated carbons, carbon nanotubes, and template-synthesized porous carbons, in particular mesoporous carbons. Their advantages and disadvantages as electrochemical capacitors are discussed. At the end of this review, the future trends of electrochemical capacitors with high energy and power are proposed.

Carbon Materials for Chemical Capacitive Energy Storage

Activation of ordered mesoporous carbon orientates the development and application of new carbonaceous supercapacitor materials with high energy density and power density. Ordered mesoporous carbons FDU-15 are synthesized in large scale via a soft template method through evaporation induced self-assembly of mesostructure on the sacrificed polyurethane foam. Common activating agent potassium hydroxide (KOH) is utilized to improve the surface area and tailor the pore texture of the ordered mesoporous carbon by adjusting KOH/carbon mass ratio as well as activation time. At low KOH/carbon ratio, the generated micropores increase in volume and either connect to other micropores or eventually become mesopores. At high KOH/carbon ratio, an excess amount of micropores would be generated. Meanwhile, the continuous shrinkage of carbon framework is carried through as prolonged time at high activation temperature. Competition between KOH etching and shrinkage of mesopores is existed during the activation. The latter obviously preponderates over the former at low KOH/carbon ratio, which is reversed at high KOH/carbon ratio. Thus, an optimized micro-mesostructure is achieved under certain activation conditions: maintained ordered mesostructure, suitable microporosity, high surface area (1410 m2 g−1) and large pore volume (0.73 cm3 g−1). The activated sample exhibits improved electrochemical behavior with a gravimetric capacitance of 200 F/g, excellent rate performance and good cycling stability with capacitance retention of ∼98% over 300 cycles.

A comprehensive study on KOH activation of ordered mesoporous carbons and their supercapacitor application

Ordered mesoporous carbon/sulfur nanocomposite of high performances as cathode for lithium–sulfur battery

Soft-template synthesis of ordered mesoporous carbon/nanoparticle nickel composites with a high surface area

Ordered mesoporous carbon materials with magnetic frameworks have been synthesized via a “one-pot” block-copolymer self-assembly strategy associated with a direct carbonization process from resol, ferric citrate and triblock copolymer F127. The effects of iron loading on framework, pore features and magnetic properties of the resultant mesostructured maghemite/carbon composites were investigated by SAXS, WXRD, TEM, N2 sorption, TG and magnetometer measurements. The results show that the mesoporous nanocomposites with a low γ-Fe2O3 content (such as 9.0 wt%) possess an ordered 2-D hexagonal (p6mm) structure, uniform mesopores (∼4.0 nm), high surface areas (up to 590 m2/g) and pore volumes (up to 0.48 cm3/g). Maghemite nanocrystals with a small particle size (∼9.3 nm) are confined in the matrix of amorphous carbon frameworks. With the increase in γ-Fe2O3 content, the surface area and pore volume of the nanocomposites decrease. The particle size of the γ-Fe2O3nanocrystals increases up to 13.1 nm. The iron oxide particles can extend from the carbon walls into mesopore channels, and hence bring a rough pore surface and gradually break down the mesoscopic regularity. The maghemite/carbon nanocomposites exhibit excellent superparamagnetic behaviors. The saturation magnetization strength can be easily adjusted from 2.5 to 12.1 emu/g by increasing the content of γ-Fe2O3. Further H2O2oxidation treatment of the magnetic nanocomposites endows plenty of oxygen-containing functional groups on the carbon surface, which improves their hydrophilic properties efficiently. The γ-Fe2O3 particles, embedding into the carbon matrix, show high stability during the H2O2oxidation process. Such modified nanocomposites with hydrophilic and magnetic framework show evidently improved adsorption properties of water and fuchsin base dye molecules in water and an easy separation procedure.

Yunpu Zhai

Papers

Carbon Materials for Chemical Capacitive Energy Storage

A comprehensive study on KOH activation of ordered mesoporous carbons and their supercapacitor application

Ordered mesoporous carbon/sulfur nanocomposite of high performances as cathode for lithium–sulfur battery

Soft-template synthesis of ordered mesoporous carbon/nanoparticle nickel composites with a high surface area

One-pot synthesis of magnetically separable ordered mesoporous carbon