Supercapacitors

 

A common misconception that students may develop reading high school books that it is impossible to make a capacitor of even one farad as the size of plates will be prohibitively large. The fact is supercapacitors can have very high capacitance, Ironically, the concept of supercapacitors dates way back to 1957 when GE created the first such device,  around 1980 these devices were being used as memory in computers.

Batteries and capacitors both are sources of energy. The major difference between the two is a cell or battery relies on chemical reaction whereas capacitors stores energy associated with the electric field associated with charges. A battery with time loses its capacity to deliver the same potential difference but the capacitor can steadily give the same voltage in its working life span. Supercapacitors, also called Ultracapacitors can store a very high amount of energy compared to normal capacitors.

A basic capacitor usually consists of two metal plates, separated by an insulator (like air or a plastic film). During charging, electrons are pulled from one plate making it positively charged, and are given to another plate accumulating negative charge. As a consequence, an electric field is created between two plates. The energy is stored in the region between the plates.

The two metal plates in supercapacitors are coated with a porous activated carbon and are immersed in an electrolyte made of positive and negative ions dissolved in a solvent.  During charging, ions from the electrolyte accumulate on the surface of each carbon-coated plate.  Thus, during charging, each electrode ends up having two layers of charge coating conventionally called electric double-layer.

In modern supercapacitors activated carbon is now being replaced by Graphene as it provides for the high relative surface area achieved by packing a highly porous form of graphene powder into a coin-shaped cell which leads to better electrostatic charge storage.  A Graphene supercapacitor can store almost as much energy as a lithium-ion battery. A battery takes hours to charge but a capacitor can charge in seconds, and maintain all this over tens of thousands of charging cycles.  The flipside though is their size for the energy compared with battery can be large besides these capacitors lose charge quickly to the environment. 

Currently, supercapacitors are mostly used in electric vehicles to accelerate utilizing regenerative braking that uses energy in braking in charging up the capacitors. In hybrid vehicles, they are being used in conjunction with batteries to enhance battery life. In Switzerland, the buses have them in their boot which has large space to accommodate them the beauty is they can charge up in just 15 seconds at a bus stop. They are also used in aircraft doors in the event of power failures and to provide burst power to assist in lifting operations in cranes.

With the rapid progress in nanotechnology, it is expected that these devices will have much better specific energy density and  and may provide for an alternative source of cleaner energy.