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Appendix – Electrochemical Capacitor Technology
Traditional Capacitor Types
There are three distinct types of capacitors: electrostatic, electrolytic, and
electrochemical.
The electrostatic capacitor was invented first. It is referred to historically as a Leyden jar
capacitor and is very similar to the simple parallel-plate capacitor. An electrostatic
capacitor is created by two conductors (metals) separated by an insulator (air or paper, for
instance). Modern electrostatic capacitors use materials other than paper between the
plates, for instance, different types of polymeric films, like Mylar or polypropylene.
These films can be made quite thin so the metal plates can be spaced very close together.
In fact, instead of metal plates, modern electrostatic capacitors consist of a polymeric
film that has been vacuum coated with a thin metal coating on each face, forming a very
thin structure that can be spiral wound. The thickness of the film dictates the separation
between the plates. The dielectric constant of the film establishes the multiplicative
factor previously described. High dielectric constants, very thin films, and high
breakdown voltages are desired in such devices. Electrostatic capacitors are available in
voltage ratings above 10 kV. Electrostatic capacitors are commonly used in high voltage
utility applications, such as flexible AC transmission system (FACTS) devices.
An electrolytic capacitor is more complicated than an electrostatic capacitor. It is
comprised of two electrostatic capacitors in series, a cathode capacitor and an anode
capacitor, separated by a liquid electrolyte. This electrolyte is an ion conductor but an
electron insulator. The motivation for the development of an electrolytic capacitor was to
achieve thinner plate separation thus higher energy than can be achieved by using paper
or film dielectrics as with electrostatic capacitors.
Electrolytic capacitors store energy across an oxide dielectric layer on a metal surface, an
etched aluminum, for instance. A second material sometimes used for electrolytic
capacitors is tantalum in the so-called wet-slug capacitor. The tantalum devices are
expensive and commonly used only in high reliability applications.
The dielectric of an electrolytic capacitor is anodically formed on the surface of a
roughened substrate, for instance, aluminum foil. The dielectric thickness is dependent
on the voltage used for its formation. Aluminum, for example, forms a dielectric film
approximately 14 angstroms thick per volt applied. Thus the dielectric would be ~140
angstroms thick for a 10 V capacitor, clearly much thinner than possible for a polymer
film.
With such a thin dielectric film, it is not practical to apply the second electrode directly
onto it. The approach taken is to use an ion-conducting electrolyte to provide this contact
so the second capacitor becomes series-connected with the first. This second capacitor is
typically created in a similar manner but with a thinner dielectric layer and, therefore,
with much higher capacitance. Thus, an electrolytic capacitor consists of two capacitors
in series with one having substantially higher capacitance than the other. This makes the
capacitance of the device very close to the smaller of the series-connected capacitors, the
one formed at higher (positive) voltage.
Electrochemical Capacitors 56