 |
| |
What is an Electric Double Layer Capacitor? |
|
| Batteries store electricity by converting it to chemical energy. Capacitors, which have been used for long time in radios or TV-sets, store electricity as it is without carrying out any conversion. Although the storage capabilities of capacitors used to be much smaller than that of batteries, recently very large ones have been developed. Electric Double Layer Capacitors (EDLCs) are often called super- or ultra-capacitors. However, while those popular names include other capacitors different than EDLCs, we will stick to EDLC rather than super- or ultra-capacitors. |
|
|
++ Electric Double Layer ++ |
|
| When two carbon rods are immersed in a thin sulfuric acid solution, separated each other and applied a slowly raising voltage from zero toward 1.5 volts, almost nothing happens up to 1 V, then at a little over 1.2 V, small bubbles would appear on the surface of the both electrodes. By raising voltage more, you should get vigorous generation of bubbles. Those bubbles at the voltage above 1 V indicate electrical decomposition of water.
|
| Figure 1: Principle of electric double layer capacitor |
|
Below the decomposition voltage, while the current does not flow, there an "electric double layer" does occur at the boundary of electrode and electrolyte. The electrons are charged across the double layer and form a capacitor. When bubbles are coming up at voltages above 1 V, this indicates the capacitor is breaking down by over-voltage causing decomposition of the electrolyte. EDLCs using water-soluble or aqueous electrolytes can be used at a withstanding voltage of about 1 V.
Why is this called a "double layer"? Every morning at rush hour, at the doors of commuting trains in Tokyo, one layer of passengers who can't move at all is pushed against the glass of the train doors . You can see this by watching from outside. Behind them is a second layer where people can move around a little, and can move more easily to get off the train. Similar to this analogy of door glass and passengers, one layer of electrolyte molecules and second layer of diffusion were detected and named electric double layer by Helmhortz in 1879.
Electrical double layer works as an insulator only below the decomposing voltage. When the usable voltage is V and the capacity C then the stored energy U will be,
U = CV2/2 ................(1.1)
Hence, the higher rated voltage V is desirable for larger energy density capacitors.
Up to now, capacitor rated voltage with an aqueous electrolyte is about 0.9V per cell and with non-aqueous electrolyte is 2.3 to 3.3 V for each cell. Figure 1 illustrates the structure of a typical EDLC.
Small EDLCs up to 100 farads are now quite popular and you can buy at electronic parts shops. Figure 2 shows EDLCs on the market on 1999, except the 18-kF 2.7V cell, which was produced in a national project.
|
 |
|
| Figure 2: Appearances of several EDLCs |
| There is a big merit in using an electric double layer in place of plastic or aluminum oxide films in a capacitor. Since the double layer is very thin, as thin as one molecule with no pin holes, the capacity per area is quite large at 2.5~5μF/cm2. |
|
|
++ Structure of Electrode ++ |
|
Even if a few microfarads/cm2 are obtainable, the energy density of capacitors is not too large by using aluminum foil or such. For increased capacitance, electrodes are made from special materials that have a very large surface area, such as activated carbon.
Activated carbons are famous for their surface areas of 1000 to 3000m2/g. How could it be possible to accommodate such a large area within one gram of carbon? Figure 3 shows an observation with a TEM (Transmission Electron Microscope) magnified to 2,000,000 times using phase-contrast method. In the upper photo, each black line identifies a graphite layer with the space between two adjacent lines measuring 0.34 nano-meters. After activation as shown in the lower picture, the space has swollen to make the surface area for double layer.
|
 |
|
| Figure 3: Pores before and after activation of carbon as observed by TEM
[ZOOM] |
To those surface ions are "adsorbed" and results; (uF stands for micro-farad)
1000m2/g * 5μF/cm2 * 10000 = 50F/g
Assuming the same weight of electrolyte is added, 25F/g is a quite a large capacity density. However, by covering 1000m2 with an insulator, or equivalently setting 10,000,000 cells of 5μF capacitors in parallel, are they safe and reliable enough against ruptures or pinholes? It would be impossible if it were a man-made insulation layer, but an electric double layer is generated inherently and completely uncontrollably. The double layer will never break as long as the voltage stays within the rated value.
|
 |
|
| Figure 4: Cutaway picture of electrode of EDLC by SEM
[ZOOM] |
| The carbon electrode of EDLC is made from activated carbon particles painted or rolled on a metal foil collector as illustrated in Figure 1. A cutaway view at 10,000 times magnification using scanning electron microscope is shown in Figure 4. The large rock-like pieces are activated carbon, the binding wires are PTFE (Teflon) and small pebbles are carbon black to facilitate conductivity between activated carbon particles. |
|
|
++ Comparing with Secondary Batteries ++ |
|
Energy Density, or storable energy amount per volume or weight, is one of the most important characters for any storage devices. When the author started this research, those coin-type EDLCs in Figure 2 were already on the market for use in memory backup applications. The large cylindrical 250 ml cell in front row was already made to order.
Nevertheless, the energy density of these capacitors is far smaller than secondary batteries. EDLCs of 1 to 1.5 Wh/kg from those days were only 1/20 of 25 Wh/kg, which is the available value from typical lead-acid batteries. For an electric car that runs on 400 kg of lead-acid batteries, using equivalent capacitors would require 8 tons. Such small energy density prevented success in marketing capacitors for power storage applications.
There was strong belief that capacitors were good at large pulse power but not at high energy density, and to show this feature, capacitor should have very low resistance. In the US, the Department of Energy promoted a project including four national laboratories and 13 industries from 1992. Unfortunately, the project was terminated in 1998 with a conclusion of little prospects. This greatly discouraged the industries of this field not only in US but also in Japan. Promising customers such as car manufacturers and electric power companies accordingly lost their belief in the potential of capacitor storage
|
|
(By: Michio Okamura)
|
|
