Energy storage turns a new leaf

Aug. 31, 2007

A lightweight, thin and flexible energy-storage device that resembles a sheet of black paper has been developed by researchers at Rensselaer Polytechnic Institute (RPI), Troy, N.Y. That’s not surprising because more than 95% of it consists of cellulose. The remainder comprises carbon nanotubes, which serve as electrodes, and an ionic liquid electrolyte.

“The components are molecularly attached to each other: the carbon nanotube print is embedded in the paper, and the electrolyte is soaked into the paper. The end result is a device that looks, feels, and weighs the same as paper,” explains Robert Linhardt, a professor in RPI’s Chemical and Biological Engineering Department.

The device, which can function both as a lithium-ion battery and as a supercapacitor, reportedly can provide long, steady power output like a conventional battery, as well as a supercapacitor’s quick burst of high energy. Lab-scale units can power an LED, but with improved efficiency such devices will be able to handle small-scale applications, notes Victor Pushparaj, a professor in the Materials Science and Engineering Department.

The sheets can be rolled, twisted, folded or cut into shapes without loss of mechanical integrity or efficiency. Because there’s nothing in the devices to freeze or evaporate, they can be used from -100°F to 300°F, claim the RPI researchers, adding that the high cellulose content and lack of toxic chemicals mean they’re environmentally safe.

“Presently, at the lab scale, the batteries are made up to 1 in. in diameter but [this is] easily scalable to 3 in. in diameter samples with the present setup,” explains Pushparaj. “We are scaling up the process so that [the devices] can be made into paper rolls on a large scale.”

While ionic liquids are costly, the amount used is so small that the devices still are inexpensive, notes Pushparaj.

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