Ultracapacitors

Ultracapacitors

Ultracapacitor is one of the names given to a type of capacitor known as a electric double layer capacitor (EDLC). Ultracapacitors work in a similar way to capacitors, but they have a much higher energy density and can have capacitances of up to 5000 Farads. In a capacitor, a dielectic is used to increase the capacitance by allowing the electrodes of the capacitor to be very close. However, there are practical limitations on the size of a capacitor using a dielectric to get a high capacitance due to the limitations in the size of the surface area of the electrodes and the distance between the plates (charge separation tends to be from millimeters to micrometers). Ultracapacitors, on the other hand, do not make use of a dielectric in the sense that typical capacitors do, but make use of new technology that allows for a massive surface area relative to the charge separation (which can get down to a few nanometers).

It was discovered in the 1960s that when two pieces of activated carbon were immersed in a liquid electrolyte, an excellent capacitor was formed. This is mainly due to the activated carbon's microscopic nodules, which have enormous surface area per unit volume. Ultracapacitors today are essentially made by coating two metal foil electrodes with activating carbon and separating them with a very thin piece of paper and immersing the carbon coated electrodes into a liquid electrolyte. Electrons accumulate in the carbon on the electrode connected to a negative terminal, which attract positive ions from the electrolyte into the pores of the carbon. Positive charges accumulate in the carbon on the other electrode, which attract negative ions form the electrolyte. The thin paper separator stops current from flowing between the two electrodes by preventing them from touching, and allows the two types of ions to move freely. This actually creates two layers of charge, which looks like two capacitors in series, and is also why ultracapacitors are often called electric double layer capacitors. This technology has allowed the charges on ultracapacitors to get extremely close to each other and the surface area of the electrodes to be massive (for typical electrodes weighing 250 grams, the total area would be 375000 square meters).