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Shocking Sheets: Power paper packs a punch

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12:20pm, August 15, 2007
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A new, ultrathin material made from cellulose, the main ingredient in paper, could power future electronic gadgets, medical implants, and even hybrid vehicles. Developed by researchers at Rensselaer Polytechnic Institute in Troy, N.Y., the material can be rolled into a tube, folded, and cut into different shapes with no effect on its function.

"This new, paperlike energy device could fundamentally change the way we power things," says Ning Pan, a materials scientist at the University of California, Davis.

Rensselaer biomaterials specialist Robert Linhardt and his students conceived the power paper while experimenting with cellulose membranes for kidney-dialysis machines. To strengthen the membranes, the researchers thought of mixing the cellulose with carbon nanotubes.

Working with Pulickel Ajayan, a carbon-nanotube researcher at Rensselaer, the Linhardt team dissolved cellulose in an ionic liquid—a liquid salt—and poured the solution over an array of vertically aligned carbon nanotubes on a silicon wafer. The researchers then washed away most of the ionic liquid and dried the material.

"When they peeled it off [the silicon], it was black on one side and white on the other," says Linhardt. Except for the black, "it looked like a regular piece of paper."

After enlisting Rensselaer electronics expert Omkaram Nalamasu, the team soon realized that this material could have an application altogether different from dialysis: energy storage.

Simply folding the paper in half with the black carbon-nanotube layer on the outside yielded a supercapacitor, a device that can provide quick bursts of energy. Coating the white side of the paper with a layer of lithium oxide produced a battery.

In the battery configuration, the paper's carbon-nanotube layer and the lithium oxide serve as positive and negative electrodes. The tiny amount of ionic liquid remaining in the paper functions as the electrolyte. Applying a voltage to the material charges the battery by causing the electrodes to pull apart the ions in the liquid. The cellulose acts as a separator, holding the charges apart.

The difference in the supercapacitor scheme is that the two outside layers of carbon nanotubes serve as electrodes. Preliminary tests showed that the paperlike supercapacitors have the same energy-storage capacity as commercial devices do. The paper is "pretty fabulous," says Linhardt. "And it hasn't even been optimized yet."

By combining the paper battery and supercapacitor, the researchers made a hybrid device that uses the battery to charge the supercapacitor. Such a setup could be especially useful in hybrid vehicles, which require an efficient means of storing and releasing energy on demand, says Linhardt.

"This is a very significant step toward developing a hybrid battery-supercapacitor structure," says Gehan Amaratunga, an electrical engineer at the University of Cambridge in England.

The researchers found that their supercapacitors worked well when a bodily fluid—such as blood or sweat—was used in place of the ionic liquid as the electrolyte. This opens up the possibility of using the material to power medical implants such as pacemakers and drug-delivery chips. Linhardt and his colleagues describe their material in an upcoming Proceedings of the National Academy of Sciences.

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