End of the Line for Metal Batteries?

New technology could produce lightweight and simple energy storage using nanotubes and paper.

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By Rachel Ehrenberg, Science News

Those who are quick to dismiss paper as old-fashioned should hold off on the trash talk. Scientists have made batteries and supercapacitors with little more than ordinary office paper and some carbon and silver nanomaterials. The research, published online December 7 in Proceedings of the National Academy of Sciences, brings scientists closer to lightweight printable batteries that may one day be molded into computers, cell phones or solar panels.

“Power storage is one of the very important aspects of solving the energy issue,” comments Robert Linhardt of Rensselaer Polytechnic Institute in Troy, N.Y. The paper-based devices show excellent performance.

That performance is largely due to paper’s porous nature: at the nano scale, paper is a tangled matrix of fibers. This vast surface area helps inks stick, says Yi Cui of Stanford University, coauthor of the new work. This holds true for carbon nanotube ink as well. When carbon-nanotube ink touches paper, the nanotubes “get caught very tightly to the cellulose,” says Cui, probably just via good old electrostatic forces.

The paper acts as a scaffold, and the carbon nanotubes act as electrodes that electrolytes in solution react with. This nanotube-paper combination offers a lightweight alternative to traditional energy storage devices that rely on metals.

By sandwiching a piece of untreated paper between two pieces inked with carbon nanotubes and then placing the layers in an electrolyte solution, the researchers made conductive paper that could be bent and rolled. When adhesive tape was applied to the paper and then pulled off, the ink didn’t peel off with the tape, a problem with other energy storage materials made with plastic films, the researchers note. Silver nanowires also made the paper conductive.

Calculations suggest that conductive paper coated with a kilogram of the carbon nanotubes could power a 40-watt bulb for an hour, making the paper more efficient than plastic-based versions of flat energy-storage devices. The scientists also used the conductive paper to collect current inside lithium-ion batteries and were able to power a light-emitting diode, or LED.

While previous work has used cellulose as a backbone for conductive materials, this demonstration is the first with ordinary office paper, says Cui. The next step is to take the technology to a larger scale, which might not be a huge hurdle, he says, since paper making and printing are well-developed technologies.

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