Laser Writes Storage into Graphite Sheets
HOUSTON, Aug. 4, 2011 — Using a CO2 laser, a Rice University team etched patterns of reduced graphite oxide (RGO) into thin sheets of graphite oxide (GO), effectively turning them into free-standing supercapacitors with the ability to store and release energy over thousands of cycles.
Rice University graduate student Neelam Singh holds a supercapacitor made from a single sheet of graphite oxide. The heat from writing a pattern in the material with a laser turns it into electrically conducting reduced graphite oxide. (Images: Jeff Fitlow/Rice University)
The surprising find was that GO, when hydrated, can hold ions and serve as a solid electrolyte and an electrically insulating separator. “This is quite easy, as GO soaks up water like a sponge and can hold up to 16 percent of its weight,” said Wei Gao, lead author of the study and a graduate student in the lab of professor Pulickel Ajayan.
"The fundamental breakthrough here is that GO, when it contains water, acts as an ionic conductor," said Ajayan. "So we're able to convert a sheet of GO into a supercapacitor without adding anything. All you need are a pattern and the electrodes, and you have a device. Of course, the devices also perform in the presence of external electrolytes, which is even better.
Rice University researchers in the lab of professor Pulickel Ajayan made supercapacitors by burning patterns into graphite oxide with a laser. Pictured, left to right, are: graduate student Wei Gao, faculty fellow in mechanical engineering and materials science Robert Vajtai, graduate student Neelam Singh and postdoctoral researcher Arava Leela Mohana Reddy.
"I think you're going to see a lot of tiny devices that need smaller power sources. Intermediate-sized devices might also be powered by this material; it's very scalable."
As a control experiment, the team sucked all the water out of an RGO-GO-RGO device in a vacuum to kill its ionic conductivity. Exposing it to air for three hours completely restored its supercapacitor function, another potentially handy characteristic.
Burning patterns into graphite oxide with a laser turns the thin sheets into fully functional supercapacitors.
To build a fully functional supercapacitor, conducting electrode materials must be separated by an insulator that contains the electrolyte. When laser-written patterns of conducting RGO are separated by GO, the material becomes an energy storage device, Gao said. The patterns can be layered top and bottom or on the same plane.
In their experiments, heat from a laser sucked oxygen out of the surface to create the dark, porous RGO, which provided a level of resistance and restrained the GO-contained ions until their controlled release. Patterns were written in the GO with nearly one-micron accuracy.
Essentially, the devices exhibited good electrochemical performance — without the chemicals.
Testing of the devices at Rice and by colleagues at the University of Delaware showed their performance compares favorably with existing thin-film microsupercapacitors. They exhibit proton transport characteristics similar to those of Nafion, a commercial electrolyte membrane discovered in the 1960s, Ajayan said.
While the lab won't make flat supercapacitors in bulk anytime soon, Ajayan said the research opens the way to interesting possibilities, including devices for use in fuel cells and lithium batteries.
He said the discovery is surprising "because a lot of people have been looking at graphite oxide for five or 10 years now, and nobody has seen what we see here. We've discovered a fundamental mechanism of graphite oxide — an ionic conducting membrane — that is useful for applications."
For more information, visit: www.rice.edu
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