Ultracapacitors - Review of Literature

Capacitors, the independent variable, are the only other device other than a battery that can store electrical energy (Mottershead, 2003, para.1). They are very simple devices that can not only store electrical charge, but also give off huge amounts of energy in a very short period of time. The basic capacitor consists of two metal plates parallel to each other separated by a small space between them. The metal plates are the electrodes and the space between contains the dielectric medium. The dielectric is any form of electrical insulator, such as oil, paper, glass, ceramics, mica, plastic, or even air. The electrodes store the energy, in the form of static electricity and the dielectric prevents current from directly passing through the electrodes by binding the electrons tightly together. The amount of energy stored in the capacitor is the Capacitance(C). The unit of Capacitance is Farad. Capacitance(C) = Coulomb (Q) / Volt (V). High current is very harmful for the capacitor and can rupture a hole in it, rendering it useless.

There are many categories of capacitors: electrolytic, paper and foil, mica, and ceramic. Electrolytic capacitors provide the highest capacitance for their size and weight than any other type. They are designed like any other capacitor, and consist of two aluminum plates that are separated by a fine absorbent gauze. Once assembled, they are rolled up and placed in an aluminum tin. The gauze is coated with chemical solution and acts like the electrode. When charged the capacitor forms aluminum oxide between the aluminum foil and the gauze, and acts like an insulator. Paper and film, mica, and ceramic capacitors are much simpler and the only variation is with their dielectric (Flower, 2003, p. 235). There are also many methods in which capacitors are made. There are stacked capacitors, which can store a large amount of charge in a small area. Wound capacitors are made up of an electrode dielectric pattern, stacked alternately and then rolled up. Wound capacitors can store the most charge in a given amount of volume. Electric double layer capacitor (EDLC) consists of two electrodes. Thus two series capacitors formed between the two electrodes and the electrolyte surrounding them. As the distance separating the charge is very small these capacitors have a very high capacitance. Electric double layer capacitors are termed as ultracapacitors or supercapacitors. All these advanced capacitors originated off the Lynden Jar, the first capacitor invented, which was a simple water filled glass jar surrounded by aluminum foil.

Ultracapacitors and supercapacitors use activated carbon electrodes, which have plenty of micro pores that can hold charges. These pores offer a very large surface area and separation distance ranging to nanometer (10 -9) magnitudes, thus providing a very high capacitance. Recent researches in the area of ultracapacitors concentrate on using carbon nanotubes for ultracapacitor electrodes. The carbon nanotubes provide uniform nanoscopic pores of about 0.8 nanometers in diameter (Miller, 2004, para. 31). Furthermore, advances in automation of fabrication technology are enabling mass production of ultracapacitors.