|Fig. 1: Structure of supercapacitor (Source: Wikimedia Commons)|
A supercapacitor (structure shown in Fig. 1) can be viewed as two porous plates immersed in an electrolyte, with a voltage potential applied cross the collectors. A dielectric separator between the two electrodes is used to prevent the change moving between the two electrodes. Supercapacitor fills in the gap between the batteries and conventional capacitors and serves as a high power energy storage device to allow storing the energy from regenerative braking.
There are two types of supercapacitors. One is called electrochemical double layer capacitor (EDLC), based on the capacitance proportional to the electrode surface area. The other type is called pseudocapacitors, based on metal oxides or conducting polymers as electrode materials. In the pseudocapacitors, the charge storage depends on the fast faradaic redox reactions. Carbon is generally used as an electrochemical double layer capacitor.
The electrical energy stored in a supercapacitor increases with its capacitance and operating voltage. The capacitance depends on the materials used while the stability window of the electrolyte determines the operating voltage. The capacitance is proportional to the surface area of the electrode. Hence it is desirable to have a material with high surface area. The current most used material is activated carbon due to its highly porous structure. In addition, low resistance electrode materials will give a high specific power. 
Carbon nanotubes have attracted lots of attention due to its excellent electrical and mechanical properties. They have high surface area and extraordinary conductivity, making them ideal materials for electrodes in supercapacitors. The open network formed by entanglement of nanotubes allows the ions to diffuse easily and it can adapt to the volumetric changes during charge and discharge, hence improving the cycling performance of the supercapacitor.
The morphology of the carbon nanotubes can be tuned further by using different fabrication techniques. For example, Futaba et al. managed to fabricate high densely packed and carbon nanotubes by using the zipping effect of liquid to draw the tubes together.  A wide range of shapes and structures can be fabricated with this method to tune the surface areas of the electrodes.
In addition, carbon nanotubes can be combined with other metal oxides such as RuO2 or conducting polymers such as polyaniline to form hybrid composites to gain the largest capacitance by having both EDLC and pseudocapacitance.
Graphene is also a recently very popular material for energy storage application. It also has high conductivity, large surface area, flexibility and chemical stability. Recently, graphene based material from graphite oxide (GO) can be manufactured in large quantities at low cost and it can then be reduced thermally or chemically to achieve large specific surface area.  Graphene can also be directly grown by chemical vapour deposition but the effective cost is much higher. 
To enable the best performance, metal oxide or conducting polymer can also be coated on graphene to provide pseudocapacitance. As with carbon nanotubes, metal oxide such as MnO2 has been used to enhance the capacitance while conductive polymers such as PEDOT have been used to increase the conduction in the graphene materials. 
In summary, carbon nanotubes and graphene both provide highly porous structure and excellent conduction required for supercapacitors. Their performance can further enhanced further by improving the quality of the materials, creating more porous structure or making composite with other metal oxides or conductive polymers.
© Huiliang Wang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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