Graphene is a thin layer of pure carbon, tightly packed and bonded together in a hexagonal honeycomb lattice. It is widely regarded as a “wonder material” because it is endowed with an abundance of astonishing traits – it is the thinnest compound known to man at one atom thick, as well as the best known conductor. It also has amazing strength and light absorption traits, and is even considered ecologically friendly and sustainable as carbon is widespread in nature and part of the human body.
While there are certain types of batteries that are able to store a large amount of energy, they are very large, heavy and release energy slowly. Capacitors, on the other hand, are able to charge and discharge quickly but hold much less energy than a battery. The use of graphene in this area, though, presents exciting new possibilities for energy storage, with high charge and discharge rates and even economical affordability. Graphene improved performance thereby blurs the conventional line of distinction between supercapacitors and batteries.
Supercapacitors, also known as EDLC (electric double-layer capacitor), differ from regular capacitors in that they can store tremendous amounts of energy. Similar to the capacitors, they contain two metal plates, only coated with a porous material known as activated carbon. They are immersed in an electrolyte made of positive and negative ions dissolved in a solvent. One plate is positive and the other is negative. During charging, ions from the electrolyte accumulate on the surface of each carbon-coated plate. Supercapacitors also store energy in an electric field that is formed between two oppositely charged particles, only they have the electrolyte in which an equal number of positive and negative ions is uniformly dispersed. Thus, during charging, each electrode ends up having two layers of charge coating (electric double layer).
Graphene is often suggested as a replacement for activated carbon in supercapacitors, in part due to its high relative surface area (which is even more substantial than that of activated carbon). The surface area is one of the limitations of capacitance and a higher surface area means better electrostatic charge storage. In addition, graphene based supercapacitors will utilise its lightweight nature, elastic properties and mechanical strength.
Graphene based supercapacitors showed potential to store almost as much energy as lithium-ion batteries, charge and discharge in seconds and maintain all this over tens of thousands of charging cycles. One of the ways to achieve this is by using a highly porous form of graphene with a large internal surface area (made by packing graphene powder into a coin-shaped cell, and then dry and press it).
Unlike capacitors and supercapacitors, batteries store energy in a chemical reaction. This way, ions are inserted into the atomic structure of an electrode, instead of just clinging to it like in supercapacitors. This makes supercapacitors able to charge and discharge much faster than batteries. Due to the fact that a supercapacitor does not suffer the same wear and tear as a chemical reaction based battery, it can survive hundreds of thousands more charge and discharge cycles.
Supercapacitors boast a high energy storage capacity compared to regular capacitors, but they still lag behind batteries in that area. Supercapacitors are also usually more expensive per unit than batteries. Technically, it is possible to replace the battery of a cell phone with a supercapacitor, and it will charge much faster although it will not stay charged for long. Supercapacitors are very effective, however, at accepting or delivering a sudden surge of energy, which makes them a fitting partner for batteries in the aerospace and transportation sectors.
Primary energy sources such as internal combustion engines, fuel cells and batteries work well as a continuous source of low power, but cannot efficiently handle peak power demands or recapture energy because they discharge and recharge slowly. Supercapacitors deliver quick bursts of energy during peak power demands, and then quickly store energy and capture excess power that's otherwise lost. In the example of an electric vehicle, a supercapacitor can provide needed power for acceleration, while a battery provides range and recharges the supercapacitor between surges.
Graphene supercapacitors are already on the market, and several companies, including Skeleton Technology, Ionic Industries, ZapGoCharger, Angstron Materials, Sunvault Energy, and the CRRC are developing such solutions.
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