Ultracapacitors are also called supercapacitors or Electrochemical Double-Layer Capacitors (EDLC). Supercapacitor (ultracapacitor) electrodes are made of porous carbon (activated carbon powder or activated carbon fibers), with pores in the nanometer range and with a very high active surface area (1000-2000 square meters per gram).
A million times higher capacitance
The active surface can not be accessed mechanically, as in ordinary capacitors, where the surface consists of two metallic plates with an insulator inbetween. Instead, a non-aqueous (water-free) electrolyte has to be used as a mediator. Capacitor action arises from the extremely thin electrical double layer, which forms between the electrolyte and the carbon surface. While ordinary capacitors are in the microFarad or milliFarad range, supercapacitors can have capacitances of thousands of Farads. The capacitance of supercapacitors/ultracapacitors is thus millions of times higher than that of the ordinary capacitors used in electronic circuits.
Low voltage
In terms of energy storage, the superiority of supercapacitors is not quite as overwhelming. The energy stored in a capacitor is given by E = 1/2(C*U^2). This implies that the energy of a given capacitor increases by the square of the voltage applied. Ordinary capacitors can work at several hundred volts, but the supercapacitor electrolyte decomposes if it is subjected to more than a few volts - the present limit is about 3 V.
To make a supercapacitor package suitable for say 300 V, one hundred individual units (with say 4000 F each) have to be connected in series. Series connection of one hundred capacitors brings the capacitance of the entire package down to a one-hundredth of each unit, i.e. to 40 F in our example. This still represents an impressive edge over ordinary 300 V capacitors, which have capacitances in the microFarad or milliFarad range.
Electric vehicles
Supercapacitors may be used to supplement rechargeable batteries in certain applications, e.g. in electric and hybrid automobiles. Batteries actually store 5 to 10 times more energy per kilogram than corresponding supercapacitors/ultracapacitors, but this seemingly huge advantage is offset by two important disadvantages.
Rechargeable battery - high energy, low power, short cycle life
The charging and discharging of a battery depends on chemical reactions, which are time-dependent. The stored energy can only be delivered at a low rate, i.e. with low power.
The chemical reactions in the battery during charge/discharge "wear down" the battery after a few thousand cycles, which most owners of cars or mobile phones have experienced.
Supercapacitor - medium energy, high power, long cycle life
The energy stored in a supercapacitor can be delivered almost instantly, giving the device up to a hundred times higher power than that of a rechargeable battery. In addition, supercapacitor function does not depend on chemical reactions. The cycle life of a supercapacitor is hence much longer, up to a million charge/discharge cycles.
Buffering power surges
In electric vehicles supercapacitors are used to meet the need for occasional power surges, e.g. when going uphill. One such surge may empty the supercapacitor, but when the vehicle subsequently travels on level ground, the supercapacitor is recharged from the battery at the battery's own slow rate and is then ready for a new surge.
Supercapacitors are also essential for "regenerative braking", i.e. for utilizing braking energy to recharge the batteries. Regenerative braking produces far more power than a battery can accept in one go. Supercapacitors in contrast can easily "swallow" a braking surge. They are used as "buffers", delivering the received surge energy to the battery at a rate that the battery can accept.
The terms ultracapacitor and supercapacitor are derived from brand names. There is unfortunately no generally accepted terminology in the field - both designations are used with approximately the same frequency.
Reference:
B. E. Conway: "Electrochemical Supercapacitors", New York 1999.