 Principles Behind Electrical Double-layer Capacitors,
and
Their Features
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[1] Principles
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Electrical 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 electrical
double-layer. When an external electric
field is applied, the electrical 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.
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[2] Benefits
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The electrical 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:
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<1> |
Benefits
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(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.)
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(2) |
There is no need for special
charging circuits or for control during discharge.
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(3) |
Overcharging or overdischarging
does not have a negative effect on the lifespan, as
it does with batteries.
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(4) |
This technology is extremely "clean
energy" in terms of environmental friendliness.
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(5) |
Because the electronic
parts can be soldered, there are no problems with unstable
contacts as there are with batteries.
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<2> |
Weaknesses
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(1) |
The lifespan is limited
because of the use of electrolyte.
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(2) |
The electrolyte may leak
if the capacitor is used incorrectly.
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(3) |
When compared to aluminum
electrolytic capacitors, these capacitors have high
internal resistances and thus cannot be used in AC
circuits.
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<3> |
Examples of Application
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Taking advantage of the
benefits described above, electrical double-layer capacitors
are used broadly in applications such as the following:
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(1) |
Memory backup for timers,
programs, etc., in video and audio equipment.
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(2) |
Backup power sources when
changing batteries for portable electronic equipment.
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(3) |
Power sources for equipment
that uses solar cells, such as watches and display
lights.
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(4) |
Starters for small motors
and cell motors.
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Figure 1: The Operating
Principle in Electrical Double-layer Capacitors
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[3] A Comparison of Electrical Double-layer Capacitors,
Batteries, and Aluminum Electrolytic Capacitors
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(1)
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The table below shows a comparison of the performance
and the basic materials in electrical double-layer
capacitors.
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Electrical
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 electrical 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 |
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(2)
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A comparison of the capacitance of the electrical double-layer
capacitor is shown in Figure 2.
Capacitance (Farad)
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