Electrochemical double -layers capacitors

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.). 

Michael MS, Prabaharan SRS. High voltage electrochemical double layer capacitors using conductive carbons as additives. Journal of Power Sources 2004 136 250-256. 

Hahn M, Kotz R, Gallay R, Siggel A. Pressure evolution in propylene carbonate based electrochemical double layer capacitors. Electrochimica Acta 2006 52 1709-1712. 

Recently, there has been an increasing interest in the synthesis of ordered mesoporous carbons, since such materials are very promising as adsorbents, catalyst supports, and electrochemical double-layer capacitors. Ordered mesoporous silicas have been shown as suitable templates to prepare periodic mesoporous carbons with various pore shapes and connectivity. The synthesis procedure involves impregnation of the mesoporous silica with an appropriate carbon precursor, carbonization of carbon source, and subsequent removal of silica using an aqueous solution of HF or NaOH. ... 

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 ... 

Another method to improve the structural order of CMs is the conversion of the precursors to fibers prior to the pyrolysis step [377]. The precursor polymer may be stretched in addition. Carbon fibers are manufactured in large quantities as reinforcements in composite materials, after Bowen [403] and Fitzer [404]. Surface and bulk activation can be accomplished by anodic oxidation in dilute aqueous electrolytes (cf. Besenhard et al. [405, 406]). But carbon fibers with various degrees of graphitization have also been employed recently in rechargeable batteries [407-411] and in electrochemical double layer capacitors [18, 412-416]. This takes advantage of two fiber specific effects, namely... 

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. 

Fig. 38. Symmetrical combinations of positive and negative active materials, PA and NA, in (1) conventional accumulators, (2) swing accumulators and (3) electrochemical double layer capacitors. The relative electrolyte (E) volume is shown schematically. The thick, vertical lines represent the current collector (schematically).

Lee, J., Yoon, S., Hyeon, T., et al. (1999). Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors. Chem. Commun., 2177-8. 

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. 

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