The Electrochemical Double Layer Capacitor (EDLC) or Supercapacitor is a form of hybrid between conventional capacitors and the battery, providing both high power and high density, and based on a similar operating principle.
Film capacitors store electrical charge by means of two layers of conductive film separated by a dielectric material. The charge accumulates on both conductive film layers, yet remains separated due to the dielectric between the conductive films.
Electrolytic capacitors are composed of metal, with a thin layer of non-conductive metal oxide added to serve as the dielectric. These capacitors have an inherently larger capacitance than standard film capacitors.

The EDLC is based on the double layer phenomenon occurring between a conductive solid and a liquid solution interface. The double layer capacitance is the result of charge separation in the interface. Electronic charge is accumulated on the solid (conductive carbon) electrode, and an ionic countercharge is accumulated in the liquid, with the pairs of electrodes separated by an ionic conductive, yet electrically insulating, separator.

The distance between the electronic and the ionic charges is very small, roughly 1 nanometer, yet electronic tunneling does not occur. Between charging and discharging, ions and electrons shift locations. In the charged state, a high concentration of ions will be located along the electronically charged carbon surface electrodes. As the electrons flow through an external discharge circuit, slower moving ions will shift away from the double layer. During EDLC cycling, electrons and ions constantly move in the capacitor, yet no chemical reaction occurs.
As a result, electrochemical capacitors can undergo millions of charge and discharge cycles. This phenomenon, which occurs with carbon electrodes of very high surface area and a three-dimensional structure, leads to incredibly high capacitance as compared to standard capacitors. The EDLC can be viewed as two capacitors formed by the solid (carbon)/ liquid (electrolyte) interface, separated by a conductive ionic membrane, the equivalent of two capacitors in a series connection.