A. Fundamentals of ultra-capacitors
The ultra-capacitor is an electrochemical capacitor, which is
composed of two porous conducting electrodes separated by a
separator, Fig. 1 (a). The electrodes are separated by a porous
membrane, so-called a separator, and impregnated by a solvent
electrolyte. Each electrode forms a capacitor with a layer of
the electrolyte's ions. The capacitance depends on size of the
ions and surface of the conducting electrode. The electrodes
are made of porous conducting material such as activated
carbon. Specific surface area of the electrode is as high as
2000m2/g [1]-[5]. Such a large surface area and very thin layer
of the charges (in order of nm) gives specific capacitance up to
250F/g. Rated voltage of the ultra-capacitor cell is determined
by the decomposition voltage of the electrolyte. Typical cell
voltage is 1 to 2.8V, depending on the electrolyte technology.
To obtain higher working voltage, which is determined by the
application, elementary cells are series-connected into one
module [4]. [5].
Ultra-capacitors differ from the other type of capacitors
mainly because their specific capacitance [F/dm3] and energy
density [kJ/dm3] are several orders of magnitudes larger than
that of electrolytic capacitors. In comparison to
electrochemical batteries, the energy density is lower while the
power density is higher than that of conventional batteries.