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What are Carbon Onions?
Carbon nano-onions are multi-shelled nanoparticles that consist of concentric graphitic layers and a hollow core. The typical diameter of these structures is 3 to 5 nm and they can be regarded as belonging in the giant fullerens family. Carbon nano-onions were discovered in 1980 and their highly symmetric structure gives them extra abilities compared to other carbon nanostructures, e.g nanotubes, nanodiamonds, etc. The number of carbon atoms they consist of can be altered and lead to different physical and chemical properties that may serve various applications.
The synthesis of carbon nano-onions is possible through the following methods: (1) irradiation of carbon black particles, (2) carbon arc plasma, and (3) thermal annealing of nanodiamond particles. Their various properties enable them to serve as nanocapsules for drug delivery or to be used in other applications such as magnetic recording systems, magnetic fluids, etc.
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Ultra-Capacitors or Supercapacitors
Ultracapacitors or supercapacitors (electric double layer capacitors, or EDLCs) are electrochemical systems that store energy within their double-layered structure consisting of opposite charged materials. Capacitor technology offers a good low-cost solution in the attempt to replace rechargeable batteries by providing an indefinite number of discharge cycles, although the power per unit is still low. The charge that is stored in these capacitors depends on the surface area of the electrode. The larger the surface, the greater the charge that can be stored. Electrodes made from activated or porous carbon are used in the production of the highest rated supercapacitors. Although this provides a high storage capacity, it slows down the rate at which charging and discharging occurs.
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The Use of Carbon-onions in Ultracapacitor Technology
The use of onion-like carbons (OLC) in the development of micro-supercapacitors now seems to be a promising venture. This was recently (August 2010) investigated by a team of researchers that described the making of a new capacitor using onion-like shells of graphene for electrodes. (Bystrzejewski, et al.)
The integration of these carbon onions in ultracapacitor devices has led to enhanced energy and power densities. Although their surface area is rather low compared to the surface of the activated carbons, it has qualitative value since it is fully accessible to the electric charges. The team began with creating an exposed electrode out of OLC. The OLC (at 6-7 nm diameter) could adhere onto the electrode without any binding agent or polymer separator making the process easier. They also prepared a capacitor using activated carbon on a similar electrode in order to have a measure of comparison.
The capacitor with the activated carbon had bigger total capacitance, however the rate of charge and discharge couldn't come any near to the OLC performance that reach the 200V/s. The power density was also 100 times higher than that of the activated carbon capacitor. The team even compared the new OLC capacitor to a thin film lithium battery, showing that although the energy per volume is an order of magnitude lower than that of the battery, the power it provides is 10,000 times higher. This means that OLC capacitors may meet the needs of demanding applications. The increased energy density and discharge rates may eventually enable these capacitors to compete with batteries and conventional electrolytic capacitors.
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The integration of carbon onions has opened new horizons in the use of micro-scale energy storage for applications for which conventional electrolytic capacitors are not sufficient. These applications may include portable electronics, biomedical implants, micro-sensors, etc. without excluding the possibility to extend the applications to larger devices when the mass production of large-area electrodes becomes available.
The current goals are set to achieve higher power and energy density by optimizing the structure of carbon nano-onions and the optimization of the device design.
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- “Catalyst-free synthesis of onion-like carbon nanoparticles" M.Bystrzejewski, M.H.Rummeli, T.Gemming, H.Lange, A.Huczko, New Carbon Materials, 2010
- “Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon," Nature Nanotechnology, 2010
- http://www.nanowerk.com : Ultrahigh-power energy storage made with carbon nano-onions
- Carbon "onions" make for rapid-discharge capacitor, J. Trimmer