Double-layer capacitors also called ‘supercapacitors’ or ‘ultracapacitors’ store energy in the electric field of an electrochemical double-layer. The use of high surface-area electrodes result in an extremely large capacitance.

Principle

Modern double-layer capacitors consist of two activated carbon electrodes, which are immersed into an electrolyte. The electronic contact between the electrodes is inhibited by a separating membrane which at the same time permits the mobility of the charged ions. If a voltage is applied between the two electrodes, the organic electrolyte supplies and conducts the ions from one electrode to the other. When the capacitor is charged, anions and cations are located close to the electrodes and balance the excess charge in the activated carbon. This way, across the boundary between activated carbon and electrolyte two charged layers of opposed polarity are formed.

Figure 1: Double-layer capacitor

Quelle: Schneuwly et al. 2002

Characteristics of modern double-layer capacitors

Currently-marketed ultracapacitors reach values of over 3000 F in the high power range. These high values can be achieved with electrodes covered with specific active coal whose surface density per weight unit can reach 3000 m²/g.

Current ultracapacitors made by Montena obtain energy densities of up to 5 Wh/kg and power densities of up to 10 kW/kg.

Typical charging times range from 300 to 5 sec. For braking energy storage on rail vehicles, the high energy supercaps type with charging time of about 300 sec., and the high power supercaps type with charging time of about 10 sec are of special interest.

Figure 2: Technical data of some Montena double-layer capacitors

Quelle: Schneuwly et al. 2002

Comparison with other storage technologies

Figure 3: Ragone diagram

Source: IZT, data from Schneuwly et al. 2002

Fields of application

Transportation applications: railways (on-board and stationary storage of braking energy in DC systems, diesel-electric vehicles, catenary-free operation of city light rail, starting system for diesel engines), hybrid-electric cars.

Industrial applications: Uninterruptible power supplies, elevators, pallet trucks etc.

Manufacturer:

Alcatel Alsthom, Cooper Electronic Technologies, EPCOS, Montena Components SA and others.