Electrochemical double layer capacitors capacitance

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

A third category which is primarily outside the scope of Faraday s law, must be considered. The charge is stored in this case in the electrochemical double layer capacitor (ECDLC). But it is bound again to atomic structures, which are also related to the corresponding masses. The capacitance increases with increasing specific surface Ag, which can be well above 1000 m /g in the case of carbon blacks. In addition to Eqs. (4) and (6), a third definition of specific charge is possible ... 

Interesting porous structures are achieved by anodic activation of glassy carbon (GC) in 3 M H2SO4 at -1-1.98 V vs. SCE. The porous layers are well contacted by the pristine GC basis behind this layer [240,434]. Application in electrochemical double layer capacitors seems to be straightforward. However, maximum capacitances are otily about 1 F/cm. This is due to the limitation of the layer thickness. In comparison to Ae maximum in Fig. 24 in the same electrolyte, namely 200 F/cm, this means a layer thickness of only 50 pm. Thicker layers become instable due to scaling. 

The most used materials for electrochemical double layer capacitors are activated carbons, because they are commercially available and cheap, and they can be produced with large specific surface area. In addition to the nanotextural properties, the chemical properties of carbons will determine their efficiency as electrodes. In the present section, an overview of the influence of the porous texture and surface chemistry of carbons on their electrochemical parameters, as capacitance or power density, is presented. 

Pognon, G. T. Brousse, and D. Belanger. 2011. Effect of molecular grafting on the pore size distribution and the double-layer capacitance of activated carbon for electrochemical double-layer capacitors. Carbon, 49,1340-1348. 

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. 

The entire electrochemical double layer can be represented by a series connection of two parallel plate capacitors. It is usually assumed that the structure of the double layer at the solution side does not differ much from that at a metal. The capacitance of the depletion layer is given by... 

Endo, M., T. Maeda, T. Takeda, YJ. Kim, K. Koshiba, H. Hara, and M.S. Dresselhaus, Capacitance and pore-size distribution in aqueous and nonaqueous electrolytes using various activated carbon electrodes. Journal of Electrochemical Society, 2001.148 pp. A910-A914 Qu, D. and H. Shi, Studies of activated carbon used in double-layer capacitors. Journal of Power Sources, 1998. 74 pp. 99-107... 

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