17 August 2013

3D graphene nanoballs could make supercapacitors more super

Graphene has been very hot in scientific circles since it was first produced in 2004. This one-atom thick layer of carbon has been found to have such remarkable properties that its discoverers were awarded a Nobel Prize in 2010. Graphene is incredibly light, and has excellent electrical conductivity. This has led to its use in prototype supercapacitors, but making these devices scalable has proven a challenge. A team of researchers from Ulsan National Institute of Science and Technology in South Korea might have solved that issue by turning graphene into tiny three-dimensional orbs.

The point of a supercapacitor is to take on significant charge quickly, then hold that charge for a very long time before releasing it (also very quickly). Supercapacitors are basically a middle ground between regular capacitors and rechargeable batteries — the speed and power of a capacitor with the larger energy storage of a battery. These devices are also of interest because they can reverse charge in this way many times before wearing out.

While existing supercapacitors have low overall capacities (energy density), their power density is considerably higher than any lithium-ion cell. Graphene is seen as an ideal material for this application because in addition to high conductivity, it has huge surface area relative to its mass. Thus, it can be used to increase the power density and recharge speed while reducing the weight.

The processes used to fabricate graphene sheets are varied, but chemical vapor deposition (CVD) is the most common. CVD gets the job done, but it’s not a terribly efficient process. Supercapacitors based on graphene need to be considerably more economical to be viable, and that’s what the Ulsan National Institute team, led by Prof. Ji-Hyun Jang, has developed. This new approach can synthesize three-dimensional graphene nanonetworks (3D GNs) through a similar chemical vapor deposition process.

Rather than making sheets of graphene and carefully winding it into a compact supercapacitor, the researchers have been able to essentially “grow” tiny spheres of graphene that retain the superb electrical properties of 2D graphene. This technique should be able to produce customizable 3D graphene structures with much more ease (and at lower cost) than current methods.

The team calls the spherical structures “mesoporous graphene balls,” and they enhance graphene superconductors in two important ways. The graphene nanonetworks present in these objects actually reinforces the already high conductivity of graphene, thus increasing conductivity. The mesopores in the graphene balls also form tiny ion transport channels to improve the superconductor’s durability. The superconductors created by Professor Jang’s team showed an incredible level of resilience. After 10,000 charge-discharge cycles, the capacitance was measured at 96% of original. You’re lucky if a rechargeable battery lasts a few hundred cycles before becoming unusable.

The researchers have managed to produce a few grams of mesoporous graphene balls at a time, which is more than it sounds like. If the process proves to be as scalable as Jang suspects, it could finally make supercapacitors viable for much larger applications. Cell phones could use supercapacitors to charge in a few minutes, and recharging an electric vehicle might take less time than filling the tank in a gasoline-powered car.

Now read: Worlds first telescopic contact lens gives you superman like vision.

Research paper: DOI: 10.1021/nn401850z  – “Chemical Vapor Deposition of Mesoporous Graphene Nanoballs for Supercapacitor” (Paywalled)


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