Researchers from the University of California, Los Angeles (U.S.A.) used a common computer optical drive to convert thin layers of graphite oxide into graphene, which was then incorporated into flexible electrochemical capacitors.
(a, b) A graphite oxide film on a flexible substrate is attached to a DVD and inserted into a LightScribe drive. (c, d) The IR laser in the LightScribe drive removes oxygen from the film, converting it to a thin layer of graphene with a large surface area. The color changes from brown to black. (e, f) The graphene film is used as an electrode in a flexible supercapacitor with power and energy densities comparable to batteries.
Researchers from the University of California, Los Angeles (U.S.A.) used a common computer optical drive to convert thin layers of graphite oxide into graphene, which was then incorporated into flexible electrochemical capacitors (ECs, also called supercapacitors or ultracapacitors). The ECs with these laser-scribed graphene electrodes offer better—possibly breakthrough—performance that could make them competitive with batteries. Researchers Maher F. El-Kady, Veronica Strong, Sergey Dubin and Richard B. Kaner used a standard LightScribe DVD optical drive to make electrodes for ECs that demonstrated high energy densities, high conductivity and good physical and electrical stability (Science 335, 1326).
Plenty of desktop computers have LightScribe DVD drives, which incorporate a laser for printing labels directly onto optical disks. The drives are commercially available for roughly $20 USD. The labeling laser (as opposed to the data-writing laser) provides an IR 788-nm beam with 5 mW power. In its conventional use, the LightScribe laser traces a computer-generated image onto the label side of the disk, which is equipped with a dye that changes color when the laser hits it. The researchers used this laser to reduce graphite oxide (an electrical insulator) to graphene (an electrical conductor).
After coating a flexible substrate with a thin film of graphite oxide and affixing it to an optical disk, the laser beam drives oxygen out of the film and rearranges the bonds. Patterning the entire area took about 20 minutes. Each pass by the laser over the surface changes the conductivity. After six passes, the material had changed from graphite oxide to graphene with an excellent conductivity of 1738 S/m—an order of magnitude better than the conductivity of the activated carbon materials in commercially available ECs. Also, the low oxygen content (3.5 percent) contributes to the high-cycling stability of the ECs.
ECs store more charge than conventional capacitors and charge faster than batteries. In the past, however, they have not been able to match the energy density of batteries. Graphene’s high conductivity, which improves with increased surface area, makes it an attractive material for EC electrodes. The laser-scribing method creates a layer less than 8 µm thick with an exceptionally high surface area.
The ECs made with laser-scribed graphene electrodes exhibited high energy density and high power density values. The researchers also showed that the ECs could be cycled even when subjected to bending.
“Our study demonstrates that our new graphene-based supercapacitors store as much charge as conventional batteries,” said Kaner, “but can be charged and discharged a hundred to
a thousand times faster.”
Yvonne Carts-Powell is a freelance science writers who specialize in optics and photonics.