Graphene-based supercapacitors
A capacitor is an energy storage medium similar to an electrochemical battery. Most batteries, while able to store a large amount of energy are relatively inefficient in comparison to other energy solutions such as fossil fuels. It is often said that a 1kg electrochemical battery is able to produce much less energy than 1 litre of gasoline; but this kind of comparison is extremely vague, mathematically illogical, and should be ignored. In fact, some electrochemical batteries can be relatively efficient, but that doesn’t get around the primary limiting factor in batteries replacing fossil fuels in commercial and industrial applications (for example, transportation); charge time.
High capacity batteries take a long time to charge. This is why electrically powered vehicles have not taken-off as well as we expected twenty or thirty years ago. While you are now able to travel 250 miles or more on one single charge in a car such as the Tesla Model S, it could take you over 43 hours to charge the vehicle using a standard 120v wall socket in order to drive back home. This is not acceptable for many car users. Capacitors, on the other hand, are able to be charged at a much higher rate, but store (as already mentioned) somewhat less energy.
Supercapacitors, also known as ultracapacitors, are able to hold hundreds of times the amount of electrical charge as standard capacitors, and are therefore suitable as a replacement for electrochemical batteries in many industrial and commercial applications. Supercapacitors also work in very low temperatures; a situation that can prevent many types of electrochemical batteries from working. For these reasons, supercapacitors are already being used in emergency radios and flashlights, where energy can be produced kinetically (by winding a handle, for example) and then stored in a supercapacitor for the device to use.
A conventional capacitor is made up of two layers of conductive materials (eventually becoming positively and negatively charged) separated by an insulator. What dictates the amount of charge a capacitor can hold is the surface area of the conductors, the distance between the two conductors and also the dielectric constant of the insulator. Supercapacitors are slightly different in the fact that they do not contain a solid insulator.
Instead the two conductive plates in a cell are coated with a porous material, most commonly activated carbon, and the cells are immersed in an electrolyte solution. The porous material ideally will have an extremely high surface area (1 gram of activated carbon can have an estimated surface area equal to that of a tennis court), and because the capacitance of a supercapacitor is dictated by the distance between the two layers and the surface area of the porous material, very high levels of charge can be achieved.
While supercapacitors are able to store much more energy than standard capacitors, they are limited in their ability to withstand high voltage. Electrolytic capacitors are able to run at hundreds of volts, but supercapacitors are generally limited to around 5 volts. However, it is possible to engineer a chain of supercapacitors to run at high voltages as long as the series is properly designed and controlled.
