A double layer capacitor and its further developed forms, called supercapacitors or ultracapacitors, are used as high-performance energy storage units. They are deployed among other things in hybrid or electric vehicles. Their main advantages are vast power densities of several thousand watts per kilogram and high cycle stability. An ultracapacitor can for instance be charged and discharged more than a million times.
While it is true that a double-layer capacitor only achieves around ten per cent of the energy density of a lithium-ion battery, it can deliver larger currents and has a faster response time, since it can release its stored energy much faster than a battery. It is therefore particularly suitable for storing brake energy (recuperation) and releasing this energy during acceleration. Another typical application area is providing emergency power supply to servers for data backup in the event of a power failure.
A huge surface area ensures power density
The secret of the ultracapacitor's high performance lies, among other things, in the huge surface area of its electrodes made of porous carbon – activated carbon or graphite. Their surface area measures up to 3,000 square metres per gram of material – the size of half a football pitch. A vast number of ions can accumulate on this huge surface during charging. The double-layer capacitor is named after its structure consisting of two graphite and electrolyte layers separated by an ion-permeable membrane, the so-called separator.