Electrochemical Capacitors and Supercapacitors

An ideal (classical) electrostatic capacitor consists of two plane-parallel metal plates having surface areas S and a mutual distance 5, the gap being filled with air or a dielectric layer (the latter variety often is called a film capacitor). When a capacitor is charged (by applying an electrostatic potential difference A / between the two plates), electrical charges +Q m electron deficit) and Q (an efectron excess), which are equal in magnitude but opposite in sign, will accnmulate on the plates. The values of Q are proportional to the potential difference  

Fundamentals of Electrochemistry, Second Edition, By V. S. Bagotsky Copyright 2006 John Wiley Sons, Inc. 

An important parameter of all capacitors, apart from their electrostatic capacity, is the maximum potential difference, between the plates which can be admit- 

Ideal film capacitors have another important special feature (i.e., their operation is not linked to chemical reactions) hence, phenomena of aging and degradation of the active masses are absent. For this reason such a capacitor will sustain an unlimited number (many millions) of charge-discharge cycles. 

The basic defect of film capacitors is tfieir low value of specific electrostatic capacity. Therefore, such capacitors are practically useful only in the pico- and nanofarad range. For this reason, valiant attempts have been made in recent years to increase the specific capacity of capacitors per unit of mass, volume, and plate (electrode) surface area. 

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Electrochemical capacitors and supercapacitors have also received increasing attention both for energy storage and also due to the fact that such carbon-supported electrode capacitors play an important role in PEM fuel cell operation [9]. 

In electrochemical capacitors (or supercapacitors), energy may not be delivered via redox reactions and, thus the use of the terms anode and cathode may not be appropriate but are in common usage. By orientation of electrolyte ions at the electrolyte/electrolyte interface, so-called electrical double layers (EDLs) are formed and released, which results in a parallel movement of electrons in the external wire, that is, in the energy-delivering process. 

A classic definition of electrochemical ultracapacitors or supercapacitors summarizes them as devices, which store electrical energy via charge in the electrical double layer, mainly by electrostatic forces, without phase transformation in the electrode materials. Most commercially available capacitors consist of two high surface area carbon electrodes with graphitic or soot-like material as electrical conductivity enhancement additives. Chapter 1 of this volume contains seven papers with overview presentations, and development reports, as related to new carbon materials for this emerging segment of the energy market. 

Figure 2. Representation of (A, top) an electrochemical capacitor (supercapacitor), illustrating the energy storage in the electric double layers at the electrode—electrolyte interfaces, and (B, bottom) a fuel cell showing the continuous supply of reactants (hydrogen at the anode and oxygen at the cathode) and redox reactions in the cell.

An electrochemical capacitor is a device that stores electrical energy in the electrical double layer that forms at the interface between an electrolytic solution and an electronic conductor. The term applies to charged carbon—carbon systems as well as carbon-battery electrode and conducting polymer electrode combinations sometimes called ultracapacitors, supercapacitors, or hybrid capacitors. 

Types of capacitors and mode of energy storage after Ref.206 Reprinted from B.E. Conway, Electrochemical supercapacitors. Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York (1999). Copyright 1999 with permission from Kluwer Academic Pubhshers. 

K. Kinoshita and X. Chu, Carbon for Supercapacitors in Proceedings of the Symposium on Electrochemical Capacitors, F. M. Denlick and M. Tomkiewicz, Editors, PV 95-29, The Electrochemical Society Proceeding Series, Pennington, NJ, 1995, p. 171-180. 

PRINCIPLE AND PROPERTIES OF ELECTROCHEMICAL CAPACITORS 8.2.1 Terminology and Kinds of Supercapacitors... 

In recent years there has been increasing interest in the power capacitors, ultracapacitors or supercapacitors based on electrochemical systems. These include electric double layer capacitor (EDLC) types based on carbon electrodes with suitable electrolyte systems, and electrochemical capacitors with pseudocapacitance [34,35],... 

Refs. [i] Delnick FM, Tomkiewicz M (eds) (1996) Electrochemical capacitors - Proceedings vol 95-29. The Elecrochemical Society, Pennington [ii] Conway BE (1999) Electrochemical supercapacitors Scientific fundamentals and technological applications. Springer, New York... 

The double-layer capacitor is one of the electrochemical capacitors showing intermediate performances between conventional capacitors and rechargeable batteries from the viewpoint of energy and power densities. Although the terms supercapacitor and ultracapacitor are often used for double-layer capacitors, in a sense that they have higher capacitance than conventional capacitors (ceramic, film, aluminum electrolytic, or tantalum electrolytic capacitors), these terms are not to be used because they are the trademarks of certain companies products. 

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