Supercapacitor or Ultracapacitor is technically an electrochemical double layer capacitor (EDLC). This technology is already gaining ground in challenging the Battery as the leading and preferred energy storage device. It exemplifies a large improvement from the common electrolytic capacitors, which are quick and powerful but energy poor. Meanwhile, supercapacitors are energy-rich storage devices whose first applications are likely to be in hybrid electric vehicles and backup power supplies.
Supercapacitors or ultracapacitors have a higher energy density, approximately on the order of thousands of times, when compared to its predecessors - the electrolytic capacitors. To illustrate, a D-cell sized electrolytic capacitor will have a capacitance in the range of tens of millifarads. On the other hand, the same size electric double-layer capacitor would have a capacitance of several farads, an improvement of about two or three orders of magnitude in capacitance, but usually at a lower working voltage of around 2 to 3 Volts. Larger, commercial electrochemical double layer capacitors have capacities as high as 5,000 Farads.
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| Supercapacitor or Ultracapacitor |
Design and Construction
A supercapacitor is composed of these three basic components:
Electrodes
The two electrodes are made of activated carbon granules that are in contact with each other to provide a huge surface area, defining the energy density of the component. In addition, current collectors with a high conducting part assure the interface between the electrodes and the connections of the supercapacitor.
Separator
The two electrodes are separated by a membrane, which allows the mobility of charged ions and forbids no electronic contact but is permeable to the electrolyte.
Electrolyte
It supplies and conducts the ions from one electrode to the other.
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| Inside a Supercapacitor |
Also, there are three main factors that determine how much electrical energy a capacitor can store: Electrode surface area, Electrode separation distance and the Properties of the insulating layer separating the electrodes.
In the case of supercapacitors, the charge-separation distance has been reduced to a few nanometers. This is because unlike conventional capacitors, ultracapacitors do not have a dielectric. Instead, they use plates that are actually two layers of the same substrate, and this electrochemical double layer, lead to the effective separation of charge even with the thin physical separation of the layers. Subsequently, this allows the packing of plates with much larger surface area into a given size, yielding very high capacitances in practical sized packages.
As a result of these enhancements, the ratio between electrode surface area to the charge-separation distance has increased exponentially to about 1012 times. It is this ratio, in fact, that makes capacitors "super or ultra." The ability to hold opposite electrical charges in static equilibrium across molecular spacing is the key feature.
However, this double layer design can withstand only a low voltage (2 to 3 V), which means that supercapacitors rated for higher voltages must be made of matched series-connected individual electrochemical double-layer capacitors similar to series-connected cells in higher-voltage batteries.
Recently, research in supercapacitors has focused on improved materials that offer even higher usable surface areas. There are ongoing experiments that explore the possibility of using carbon nanotubes as electrodes. The advantage of carbon nanotubes lies in their uniform nanoscopic pores (about 0.8 nanometers in diameter), which could store much more charge than the nanogate capacitors if the nanotubes could be properly assembled into macroscale units.
Applications
Ultracapacitors find their purpose in the following areas, but not limited to:
Automotive Systems
· Hybrid Fuel Cell Vehicles
· Electric Cars
· Complementary to fuel cells, particularly for stop and go mobility applications
· Heavy Duty Machineries such as forklifts
· Regenerative braking applications
Power Industry
· Replacement for battery banks as backup supply for short-term interruptions on the power grid.
· As an energy storage device for UPS systems located on the premises of critical and sensitive load customers such as hospitals, factories, offices, airport control towers, cell phone towers and residences.
· Energy storage devices for solar or wind power generators.
· Motor startup capacitors for large engines in tanks, submarines, railroad locomotives and trucks.
Supercapacitors are also used in automated meter reading, flash photography devices in digital cameras, portable media players and PC cards.
Advantages and Disadvantages
Supercapacitors have the following advantages and disadvantages as compared to batteries:
Advantages
· Higher power density, and charge and discharge rates
· Longer economic life
· Energy efficient
· Environment friendly
· Better throttle responsiveness
· Little degradation (over hundreds or thousands of cycles)
· Lesser in weight
· Good reversibility
· Higher recovery of energy from braking
Disadvantages
· Energy stored per unit weight is lower than that of an electrochemical battery
· Highest dielectric absorption of all types of capacitors
· Requires complicated electronic control and switching equipment to effectively store and recover energy
· Voltage varies with the energy stored
Manufacturers
The leading manufacturers of supercapacitors today are:
1. Maxwell Technologies - United States
2. EPCOS - Europe
3. VinaTech - Asia
4. Panasonic - Asia
5. Okamura Laboratory – Japan
6. NESS Capacitor Company - South Korea
References:
Miller, J. (2004). Ultracapacitors Challenge the Battery
Miller, J. and Simon, P. (2008). Fundamentals of Electrochemical Capacitor Design and Operation
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