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Active carbon supercondenser

March 03, 2003

CIDETEC has just completed the project, subsidised by the Basque Government and CEGASA, "The development of a active carbon supercondenser for high-tension applications", aimed at developing home-grown technology which would allow the construction of an active carbon-based, double-layer prototype.

Supercondensers (SC's) are made up of two porous electrodes insulated from possible electrical contact by a separator paper impregnated with an ion-conducting medium or electrolyte. Energy storage is produced by separation of negative and positive charges on the electrode/electrolyte interface as a result of a potential difference applied externally to these electrodes. The combination of high specific area electrodes with an electrolyte of a high concentration of ions allows the creation of SC's of high potential and great ability to withstand charge-discharge cycles. The reason for the choice of active carbon technology as the electrode material - as opposed to other possibilities such as metal oxides or conducting polymers - is justified on the basis of the better perspectives for the future that it offers given, fundamentally, its low cost and easy accessibility. The integration of the SC's with energy storage systems - batteries and fuel cells - allows a more rational design for electricity supply systems in that base consumption is supplied by a primary energy source while peak-time demand is provided by the SC's.

A study of the available literature has facilitated a selection of optimal values for those parameters which define SC behaviour. Taking the value of capacity (electric charge per unit of voltage), the typical value is in the order of 100 faradays per gram of electrode. The choosing of different carbon-based materials was targeted at achieving of this capacity and its enhancement.

The preparation of the electrodes requires a bonding agent, typically Teflon, which guarantees the connectivity between the particles in the active carbon. The active carbon/bonding agent combination can be used in pellet form in order to set up an electrode or, otherwise, deposited on a carbon-based support and used in the same way. As regards the kind of electrolyte employed as an ion-conducting medium, both aqueous and organic electrolytes were tested. These electrolytes impregnate the separator paper, which can be of two types, cellulose or hydrophilised polypropylene.

The SC made up of the electrodes, the separator and the electrolyte was tested in an experimental cell. The recording of the properties of the SC's was carried out with a Voltalab 41 potentiostat in such a way that a number of different methods were used to measure, amongst other parameters, the capacity of the SC. Also charge-discharge cycles were applied at constant intensity and impedance measurements were made, thus determining the equivalent resistance in series of the SC.

The standard recording of properties of the SC was carried out on electrodes with an average active area of 2-3 cm2 and with an electrode mass of approximately 50 mg. The values for absolute capacities obtained under these conditions for SC's with aqueous electrolytes came to 3 F, which meant an average value for specific capacity of the active materials of approximately 120 F/g. As regards the organic SC's, slightly lower values (2 F or 100 F/g) have been obtained, for the moment. In both cases more than 2000 charge-discharge cycles were applied. The electrode/separator/electrolyte combination with improved yields in the experimental cell was used as the basis for the building of the first prototypes designed at CIDETEC.

The next target in this research field, apart from the improvement of the prototypes designed to date regarding their specific capacity and their ability to withstand cycles, is the testing of new carbon-based materials synthesised using nanotechnology (as with the carbon nanotubes), which allow an enhancement of the already exceptional yields for this type of supercondenser.

Elhuyar Fundazioa