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.

The main development goals for double-layer capacitor are

• long lifetime
• increase of the rated voltage
• improvement of the range of operating temperature
• increase of the energy and power densities.

According to Montena, the rated voltage will be increased up to 3 V within the next years. To easily access automotive applications a temperatures range from –35 up to 105 °C seems realistic. Montena fixes energy and power density goals for double-layer technology at 10 Wh/kg and 10 kW/kg.

From on-going research and development, Montena expects an increase of the electrolyte decomposition voltage and ionic conductance, an increase of the electrode accessible surface, chemical and mechanical stability as well as electronic conductance and the separator electronic insulation level and ionic conductance. A main activity lies in the development of new electrolytes based on the combination of novel organic solvents and improved conduction salts, permitting not only a higher rated voltage and a higher conductivity but also a larger operating temperature range.

State of development

Double-layer capacitors are only starting to become a mature and reliable technology.

As can be seen from Figure 4 the performance of double-layer capacitors is fitted for the range from light rail vehicles to regional trains.

Figure 4: Ragone diagram and charging times (corresponding to braking times of different trains)

Source: IZT, data mainly from: Hentschel et al. 2000

According to Montena, during the next years the costs of double-layer capacitors will decrease significantly. Several reasons are given:

• First, the nominal voltage of Montena BOOSTCAPs®, currently rated at 2.5 V, will go up to 3 V within the next years. This way, the costs of a module built up from several capacitors units will be reduced by about 25 %.
• Second, an important cost reduction will come from the automation of the whole production process. Especially, the winding technology used for the production of double-layer capacitors offers a big potential for cost reduction.
• Third, cf. Scale effects .

Montena estimates that, thanks to these improvements, future costs per energy content will come down to 10 US$ per 1000 F. In addition, module costs will decrease due to higher production volumes and new low-cost voltage sharing technologies.