Electric double layer capacitors (EDLC), like rechargeable batteries and aluminum electrolytic capacitors, are used in a variety of electrical products as capacitors that charge and discharge electricity. In recent years, there has been much exploration of new uses for EDLCs, and it is expected that they will become even more commonly used in the future.
EDLCs store electrical energy using the electric double layer that is created at the interface between the conductive solid material and the electrolyte. Normal capacitors such as aluminum electrolytic capacitors comprise a dielectric sandwiched between two electrodes facing each other, but EDLCs are different in that they do not contain a dielectric. EDLCs use the electric double layer to function as the dielectric.
The conductive solid material commonly used in EDLCs is activated carbon, which has many minute holes made in the surface to expand the surface area. The space between the two activated carbon electrodes is filled with electrolyte, and electricity is charged and discharged simply by the adsorption and desorption of ions to and from the electrode surfaces.
Figure 1: Electric Double Layer Capacitor Principle
|The basic structure of an EDLC, like that of a cylindrical aluminum electrolytic capacitor, is a coiled structure comprised of an electrode foil, made of activated carbon electrodes formed onto aluminum foil, coiled with a separator.||
Figure 2: Basic Structure of Cylindrical EDLC
As explained above, EDLCs do not use a chemical reaction such as a redox reaction but rather store electricity by means of the physical adsorption of ions to the large specific surface area of activated carbon. Compared to aluminum electrolytic capacitors, EDLCs offer larger capacity, but their larger internal resistance means that their use as ripple absorption for alternating current circuits is not appropriate. For this reason, EDLCs are mainly used like rechargeable batteries, to provide energy backup in direct current circuits. The advantages of EDLCs over rechargeable batteries are listed below.
・High current (fast) charging / discharging is possible
・Long cycle life, allowing for charging / discharging tens of thousands of times
・Can be used in a wide range of temperatures, offer stable operation even at low temperatures
・Very safe, with no risk of explosion or ignition
・Environmentally-friendly, with no heavy metals used
Properties data is provided below.
The service life of EDLCs is affected by the operating temperature. Generally, the rule of double the life for every 10°C of temperature decrease applies, as it does for aluminum electrolytic capacitors.
The life expectancy can be roughly calculated using the equation below.
The operating temperature is not the ambient temperature of the device; it is necessary to check the temperature of the capacitor including the heat generated by charging / discharging electricity.
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