Supercapacitors

Supercapacitors have been described in monographs (2-4). The first model for the distribution of ions near the surface of a metal electrode was invented by Helmholtz in 1874 (2). He imagined two 

In this way, the Gouy-Chapman-Stem-Grahame model of the electrical double layer was bom (5). This model is stiU qualitatively accepted, although a number of additional parameters have 

Metal oxide-based materials, carbon materials, and conducting polymers for electrochemical supercapacitor electrodes have been reviewed in detail (6). Two important future research directions have been summarized The development of composite and nano-structured electrochemical supercapacitor materials to overcome the problem of low energy density of electrochemical supercapacitors. 

A supercapacitor composite electrode has been described (7). The composition is listed in Table 2.1. 

Poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) 81 Graphite oxide 16 

For graphene/CP composite films, the goal of combining the materials has been both to obtain a mechanically more robust material and to combine the attractive properties of the individual components to obtain a superior material. As discussed above, graphene/CP composite materials can be S3mthesized by a range of different methods. In this section, electropolymerization of graphene/CP composite electrode materials and the direct use of such electrodes in the field of supercapacitors and electrochromic devices will be briefly summarized. 

Depending on the charge storage mechanism, supercapacitors can be classified into two types electrical double layer capacitors (EDLC) and pseudocapadtors [108]. EDLCs store and release energy based on the accumulation of charges at the interface between a porous electrode, typicalty a carbonaceous material with high surface area, and the electrotyte. In pseudocapadtors, the mechanisms rely on fast and reversible Faradaic redox reactions at the surface and/or in the bulk. 

Fabrication of freestanding thin-film composite electrodes by electrodeposition was first reported by Wang et al. [85]. In this work, freestanding and flexible graphene composite paper was used as the working electrode on which a PANi film was formed by an in situ anodic electropol mierization technique 

A micro-supercapacitor was fabricated by electrodepositing PANi nanorods on the surfaces of rGO patterns [121]. This microsupercapacitor possesses a high specific capacitance of 970 F/g at a relatively high discharge current density of 2.5 A/g, showing retention of 90% after 1700 cycles (Fig. 7.10). 

Recently, Zhang et al. [123] have demonstrated a flexible composite membrane of reduced graphene oxide and pol5T)5n role nanowire [rGO-PPy-NWs] via in situ reduction. A S5mimetric supercapacitor has been fabricated by direct coupling of two membrane electrodes, without the use of any binder or conductive additive. The supercapacitor achieved a large areal capacitance [175 mF/cm ] and excellent cycling stability. The in situ reduction of GO in the composite dispersion with PPy-NWs renders the formation of the rGO-PPy composite foam via self-assembly, as shown in Fig. 7.11. 

Composite materials that combine double-layer and pseudocapacitance 

The amount of charge which can be stored in the electrode/electrolyte double layer is typically of the order of 15-40 pF/cm2. A large capacitance 

A typical configuration of a double-layer supercapacitor involves two metallic collectors which hold in place the carbon powder electrodes, which in turn are separated by an electrolyte, in most of the cases formed by liquid solutions (Fig. 9.28.) A layer of porous, non-conductive material acts as a separator. 

Solid ionic conductors can also be used in the fabrication of solid state double-layer supercapacitors. An example is the device developed in the late 1960s by Gould Ionics which adopted a cell system using a silver-carbon electrode couple separated by the highly ionically conducting solid electrolyte RbAg4I5 (see Section 9.1)  

Company Electrode material Electrolyte Energy density (Wh/kg) Power density (W/kg) 

In the late 1980s, the system was reconsidered by Quadri Electronics who produced an improved supercapacitor under the trade name HYPERCAP . Very high rate and peak power capabilities - current pulses in excess of 10 A with rise times of the order of milliseconds, and 3 kW/kg, respectively - have been reported for these solid state devices. 

Conventional capacitors store energy in the electric charge between two conducting plates that are separated by an insulating or dielectric material. The quantity of energy stored is given by  

Compared with lead-acid batteries, the specific energies of supercapacitors are relatively low ( 10Whkg ). Nevertheless, they can be charged and discharged at very high rates (high-power capability) and can be cycled tens of thousands of times. 

Coupling the ultrasmall separation distance with a relatively large surface area, in ultracapacitors the ratio of available surface area to charge separation distance h makes capacitors ultra. The ability to hold opposite electrical charges in static equilibrium across molecular spacing is the important feature. 

Three main factors determine how much electrical energy a capacitor can store the electrode surface area, the electrode 

Different types of electrode materials have been used for the development of redox supercapacitors. Especially PTh derivatives have received considerable attention due to their applicability in both aqueous and organic electrolyte solutions. 

The value of double-layer capacitance Qi can be determined by electrochemical impedance spectroscopy (EIS], which is a key factor to characterize electrochemical systems as components of doublelayer capacitors. The electrochemical response of a conductive or 

The fundamental basis of supercapacitor design is to have two activated carbon electrodes immersed in an organic electrolyte. The electrodes are separated by a membrane which permits mobility of ions whilst preventing electronic contact. The range of available electrodes now includes metal oxides and 

Amongst Maxwell s recent sales successes has been the order from Enercon for 1.5 million imits to be used in wind power, blade pitch systems. Each of the turbines has three blades and incorporates between 200 and 700 Boostcap supercapacitors to provide back-up power. The potential growth of this market is illustrated by information from the European Wind Energy Association showing that wind power capacity in Europe exceeded 40 GW in 2005, a figure five years ahead of official targets. The capacity rose from 34,372 MW at the end of 2004 to 40,504 MW at the end of 2005. 

Despite its sales successes Maxwell reported a net loss of 7.1 million on 2005 fiscal year revenue of 45.4 million. However the company is investing in additional manufacturing capacity to cater for growing demand for its products. It is also finalising plans for high volume manufacture offshore, and expanding its product range. 

The advantages for the use of intrinsically conducting polymers in electrochemical capacitors rather than carbon-based or mixed metal oxide electrodes may be summarised as follows  

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Conway, B. (1999). Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications. New York Kluwer Academic/Plemim. 

M. Fujiinoto, K. Ueno, T. Nouina, M. Takaha-shi, K. Nishio, T. Saito, Proc. Symp. on New Sealed Rechargeable Batteries and Supercapacitors, 1993, p. 280. 

Without any doubt the microporous polyethylene pocket will meet all requirements of modern starter batteries for the foreseeable future. Whether and to what extent other constructions, such as valve-regulated lead-acid batteries, other battery systems, or even supercapacitors, will find acceptance, depends — besides the technical aspects — on the emphasis which is placed on the ecological or economical factors. 

JT. Takamura, M. Kikuchi, J, Ebana, M. Naga-shima, Y. lkezawa, in New Sealed Rechargeable Batteries and Supercapacitors (Eds. B. M, Barnett, E. Dowgiallo, G, Halpert, Y. Matsuda, Z. Takehara), The Electrochemical So-... 

Any device (battery, supercapacitor, smart mirror, or muscle) stored in a compacted state requires an initial activation-relaxation before use. 

Conducting polymers have found applications in a wide variety of areas,44 45 and many more have been proposed. From an electrochemical perspective, the most important applications46 appear to be in batteries and supercapacitors 47,48 electroanalysis and sensors49-51 electrocatalysis,12,1, 52 display and electrochromic devices,46 and electromechanical actuators.53... 

B.E. Conway, Electrochemical Supercapacitors, Kluwer Academic/Plenum, New York,... 

Another type of supercapacitor has been developed in whieh instead of ideally polarizable electrodes, electrodes consisting of disperse platinum metals are used at which thin oxide films are formed by anodic polarization. Film formation is a faradaic process which in certain cases, such as the further partial oxidation and reduction of these layers, occurs under conditions close to reversibility. 

The supercapacitors described in the literature have an overall specific capacity of about 1 to 5 F/g (i.e., when allowing for the weight of the two electrodes, the leads, the electrolytes, and aU peripheral components). In them, electric energy can be accumulated with a density of 1 to 5 Wh/kg (which is one to two orders of mag-nimde less than in batteries). 

It was seen above that different types of electrochemical supercapacitors exhibit specific capacities many orders of magnitude higher than the film and electrolytic capacitors known before. It must be added at once, however, that the behavior of supercapacitors differs appreciably from that of ideal film capacitors. In contrast to... 

The practical value of supercapacitors has been proven over a certain intermediate range of currents. At lower currents ordinary batteries which have a higher... 

Specific energy reserve are more appropriate. At high currents, losses during supercapacitor discharge may prove to be excessive. 

Ingram, M. D., H. Staesche, and K. S. Ryder, Activated polypyrrole electrodes for high-power supercapacitor apphcations. Solid State Ionics, 169, 51 (2004). 

Winter M, Brodd RJ. 2004. What are batteries, fuel cells, and supercapacitors Chem Rev 104 4245-4269. 

New Carbon Based Materials for Electrochemical Energy Storage Systems Batteries, Supercapacitors and Fuel Cells... 

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