Supercapacitors

Electric Double-Layer Capacitors (Supercapacitors)

Despite their apparent similarity to batteries,capacitors are actually designed and used in markedly different ways. A capacitor is an energy storage device that, unlike a battery, generates an electrical field between two parallel conductor plates. As electrons move from one plate to the other, they build potential energy that can be channeled for use in an associated circuit. The accumulation of energy is known as “charging,” and capacitors are generally measured by the quantity, density, and rate of their charge.

An electric double-layer capacitor, or supercapacitor, is capable of charging and storing energy at an exponentially higher density than standard capacitors. For comparison, a typical capacitor’s energy storage is measured in nano- or micro-farads, while a supercapacitor can be rated in farads. To understand the resulting differences in design, application, and cost, it may be helpful to look at some of the distinctive characteristics of an electric double-layer capacitor.

Supercapacitor

A capacitor’s energy capacity is determined by its amount of stored charges and the potential for charging between its plates. The charge potential is greatly influenced by the quality of the material through which the electric field can be sustained, otherwise known as the “dielectric.” In an electric double-layer capacitor, the dielectric is typically suspended in a high surface area carbon material, rendering the dielectric medium exceptionally thin. The large surface area, combined with a narrow medium, results in very high charge potential, or “capacitance,” in a relatively small-sized device; hence the term “supercapacitor.”

While the layers in a double-layer capacitor are electrically conductive, they have a somewhat low tolerance for voltage (usually no more than one volt). Inclusion of an organic electrolyte can increase voltage reception, as can connecting multiple supercapacitors in a serial array. The material used in the dielectric can also affect capacitor efficiency. Activated carbon, for instance, has a much greater surface area than aluminum, which is traditionally used in standard capacitors. Research to develop newer and more effective dielectric substances is continuously underway.

Are Supercapacitors Your Solution?

Manufacturers evaluating various electrical sourcing options should examine the strengths and weaknesses unique to the double-layer format. A supercapacitor’s energy density ratio typically ranges between 0.5 and 10Wh/kg (nominal voltage over weight), which is considerably higher than that of a standard capacitor. While this energy density is still relatively low compared to mainline batteries, such as the lithium-ion model, the supercapacitor’s power density far exceeds the level offered by its counterparts. Power density is contingent on a device’s rate of electrical charging and discharging, meaning that supercapacitors can both generate and distribute energy more quickly than most batteries.

In addition, supercapacitors stop charging when their capacity limit is reached, eliminating the need for detection units to prevent overcharging. Aside from its excellent power density, a supercapacitor also has high cycle efficiency and can undergo millions of charging sequences in its lifespan.

However, low energy density and low voltage tolerance limit the effectiveness of an individual double-layer capacitor as a storage unit, unless it is serially linked to a group of capacitors. Furthermore, the supercapacitor’s linear discharge method often prevents the full charge from being delivered, resulting in small but detrimental energy waste. The high rate of self-discharge (energy loss due to internal chemical reactions) is a similar concern. Supercapacitor controls and electronic switching equipment can also be complex, and typically necessitate workers with specialized operational skills.

Styles of supercapacitors with activated carbon electrodes

Schematic construction of a wound supercapacitor
1.Terminals, 2.Safety vent, 3.Sealing disc, 4.Aluminum can, 5.Positive pole, 6.Separator, 7.Carbon electrode, 8.Collector, 9.Carbon electrode, 10.Negative pole

Schematic construction of a supercapacitor with stacked electrodes
1.Positive electrode, 2.Negative electrode,
3.Separator

Each EDLC cell consists of two electrodes, a separator and an electrolyte. The two electrodes are often electrically connected to their terminals via a metallic collector foil. The electrodes are usually made from activated carbon since this material is electrically conductive and has a very large surface area to increase the capacitance. The electrodes are separated by an ion permeable membrane (separator) used as an insulator to prevent short circuits between the electrodes. This composite is rolled or folded into a cylindrical or rectangular shape and can be stacked in an aluminium can or a rectangular housing. The cell is typically impregnated with a liquid or viscous electrolyte, either organic or aqueous, although some are solid state. The electrolyte depends on the application, the power requirement or peak current demand, the operating voltage and the allowable temperature range. The outer housing is hermetically sealed

Industries That Use Supercapacitors

Although initially used as starter devices for tank and railroad engines, most supercapacitors are currently found in appliances and handheld devices. However, there is a growing market for the product in the transportation industry. Many automotive companies use double-layer capacitors to shield certain electrical engine parts from voltage fluctuations. The supercapacitors rapid charging rate also makes it effective in mass transit braking mechanisms and portable fuel cells for electric/hybrid vehicles.

Supercapacitors also serve as backups to primary batteries in order to bridge brief power interruptions or to smooth electrical flow. If installed parallel to a battery terminal, a supercapacitor can augment an operating battery’s power supply. This enhancement can raise performance during periods of elevated demand and help maintain a steady level of electrical output.

The Future of Supercapacitors

While today’s supercapacitor has a limited range of applications, advances in design might eventually expand the product’s utility. For example, researchers continue to develop and experiment with newer forms of dielectric materials, such as carbon nanotubes, polypyrrole, and barium titanate, which may improve capacitance and energy density. The concept of combining supercapacitors with alternative energy sources to replace car batteries has gained appeal within the current "green" movement, and several public transportation systems have created pilot trials for capacitor-run buses and trains. If these and other developments yield successful results, the electric double-layer capacitor may achieve greater functionality and gain a larger role within the energy industry.