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Principles: Electric double-layer

Principles

Electric double-layer capacitors are based on the operating principle of the electric double-layer that is formed at the interface between activated charcoal and an electrolyte.

The activated charcoal is used as an electrode, and the principle behind the capacitor is shown in Figure 1. Activated charcoal is used in its solid form, and the electrolytic fluid is liquid. When these materials come in contact with each other, the positive and negative poles are distributed relative to each other over an extremely short distance. Such a phenomenon is known as an electric double-layer. When an external electric field is applied, the electric double-layer that is formed in the vicinity of the activated charcoal's surface within the electrolytic fluid is used as the fundamental capacitor structure.

Benefits

The electric double-layer design does not have the solid dielectric that is used in the previous designs, nor does it have the chemical reactions such as are found in batteries during charging and discharging. Rather this design has the following characteristics:

1. Benefits

  1. This design allows farad-order capacitance in a small device.
    (When activated charcoal, with its large surface area, is used the thickness of the dielectric is extremely thin.)
  2. There is no need for special charging circuits or for control during discharge.
  3. Overcharging or overdischarging does not have a negative effect on the lifespan, as it does with batteries.
  4. This technology is extremely "clean energy" in terms of environmental friendliness.
  5. Because the electronic parts can be soldered, there are no problems with unstable contacts as there are with batteries.

2. Weaknesses

  1. The lifespan is limited because of the use of electrolyte.
  2. The electrolyte may leak if the capacitor is used incorrectly.
  3. When compared to aluminum electrolytic capacitors, these capacitors have high internal resistances and thus cannot be used in AC circuits.

3. Examples of Application

Taking advantage of the benefits described above, electric double-layer capacitors are used broadly in applications such as the following:

  1. Memory backup for timers, programs, etc., in video and audio equipment.
  2. Backup power sources when changing batteries for portable electronic equipment.
  3. Power sources for equipment that uses solar cells, such as watches and display lights.
  4. Starters for small motors and cell motors.
製品情報

Figure 1: The Operating Principle in Electric Double-layer Capacitors

A Comparison of Electric Double-layer Capacitors, Batteries, and Aluminum Electrolytic Capacitors

  1. The table below shows a comparison of the performance and the basic materials in electric double-layer capacitors.
    Electric Double-layer Capacitor Aluminum Electrolytic Capacitor Ni-cd Battery Lead Sealed Battery
    Use temperature range −25 to 70°C −55 to 125°C −20 to 60°C −40 to 60°C
    Electrode materials Activated charcoal Aluminum （+）NiOOH
    （-）Cd    		 （+）PbO2
    （-）Pb
    Electrolytic fluid Organic solvent Organic solvent KOH H2SO4
    Method of electromotive force Use of the naturally occurring electric double-layer effect as the dielectric. Use of the aluminum oxide as the dielectric. Use of chemical reaction. Use of chemical reaction.
    Pollution None None Cd Pb
    Number of charge/discharge cycles > 100,000 times > 100,000 times 500 times 200 to 1000 times
    Capacity per unit volume 1 1／1000 100 100
  2. A comparison of the capacitance of the electric double-layer capacitor is shown in Figure 2.

    Capacitance (Farad)

    Figure 2