Reliable high-capacity energy storage is one of the major challenges we face in developing new electrical technology. From long-range electric cars to backups for variable-output alternative energy sources such as wind turbines, the need for inexpensive energy storage—both long- and short-term—will only increase. While chemical batteries are the primary way this is accomplished today, capacitors are also very common, although current models are hampered by their inability to store a lot energy without being prohibitively large in size.
A new type of electrochemical capacitor based on graphene promises to break those limitations. As described in a Science paper by Maher F. El-Kady, Veronica Strong, Sergey Dubin, and Richard B. Kaner, these "supercapacitors" are capable of holding a lot of charge on a relatively small surface area and are robust under stress. In addition, the researchers constructed their capacitor using an ordinary DVD writer to reduce a graphite oxide film into graphene, demonstrating how simple manufacturing would be. The electrochemical graphene capacitor is a potential prototype for a new class of flexible electronic devices with high power capabilities.
Capacitors are usually constructed from two parallel pieces of material (often a metal foil in smaller capacitors), with an insulator in between, or in the case of electrochemical capacitors, an electrolyte. The business ends of the capacitor are its electrodes; larger electrodes offer more surface area on which to accumulate electric charge. This charge polarizes the insulating material, intensifying the electric field, which is how energy is stored.
Capacitors have several advantages over batteries: they can charge and discharge rapidly with little degradation over many charging cycles. Where they typically fall behind batteries is in the energy density: batteries are able to store a lot of energy in a relatively small volume, as much as 30 times the amount standard capacitors can handle.
One way to get around this limitation of capacitors is by increasing the effective surface area of the electrodes. This in turn allows more charge to accumulate, and a larger amount of stored energy. For this purpose, graphene is a great candidate: a two-dimensional hexagonal lattice of carbon, graphene has an exceptionally high effective surface area, is an excellent conductor, and is mechanically flexible.
To construct a graphene capacitor, El-Kady et al. took an ordinary DVD disc and covered it in a graphite oxide (GO) film. (Graphite oxide is extremely inexpensive and can be produced in large quantities, so it is an ideal raw ingredient for manufacturing.) The researchers then used a LightScribe DVD drive to irradiate the GO film with its laser, reducing the film into graphene a few layers thick. They peeled the graphene off and applied it to a flexible substrate (though the graphene itself is robust enough to work as an electrode on its own). Sandwiching a similarly malleable insulating layer between two identical electrodes completes the device.
The resulting capacitor is thin (less than 100 microns) and continues to function with no degrdation even after bending and straightening multiple times. Due to the special electronic properties of graphene, the capacitors exhibited power output much greater than similarly sized lithium-ion batteries, without sacrificing energy storage capacity.
With these advantages—as well as the much cheaper cost of manufacturing compared with conventional rechargeable batteries—El-Kady et al. propose combining multiple graphene capacitors to make battery replacements for small electronics such as laptops. In addition, the flexibility of the capacitors allows them to be used in thin bendable gadgets, potentially opening up an entirely new class of devices unconstrained by the need to contain bulky battery packs.
Science, 2012. DOI: 10.1126/science.1216744 (About DOIs).
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