Operation and energy characteristics of EDLCs

Activated-caibon-based supercapacitors are composed of two casings, on which a fine layer of powdered carbon is deposited. This layer of carbon is then activated, making its surface extremely porous. The activated carbon therefore has a very high specific surface area. Two types of electrolyte are used aqueous electrolytes and organic electrolytes. 

Aqueous electrolytes are characterized by high ion conductivity and consequently have a very low equivalent series resistance. The major drawback to these electrolytes is their voltage, which is hmited to around 1.2 V. Organic electrolytes have low ion conductivity and therefore a relatively high equivalent series resistance in comparison with aqueous electrolytes. Conversely, their voltage capacitance is relatively high, at aroimd 3 V. Activated-carbon supercapacitors are the most fully developed industrially. 

Metal-oxide-based supercapacitors are characterized by a chemical reaction at the surface of the electrodes. This causes a charge transfer (pseudo-capacitance). The most widely-used metal oxide is ruthenium dioxide (RUO2). This technology uses H2SO4 as the electrolyte and presents a very low internal resistance. However, the very high cost of metal oxides restricts their use to mihtary or space apphcations. 

Conductive-polymer-based supercapacitors store energy by polymer-doping processes. They have capacitances ranging from 200 to 300 F/g. Once again, this is pseudo-capacitance. These supercapacitor systems are not yet at an industrial development level. 

In this chapter, only supercapacitors with activated carbon and organic electrolyte are presented. 

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