Paper has been getting beat by electronics for years. But it may be about to stage a comeback. Researchers are reporting that they've made batteries and other energy-storage devices by printing layers of carbon nanotube–based ink atop standard photocopy paper. The result is a highly conductive sheet that can carry a charge and be easily incorporated into a flexible battery. Because of paper's low cost, that could help lower the price of batteries used in electric vehicles, wind farms, and other renewable sources.
The idea of using paper to make a lightweight, flexible battery isn't new. Researchers led by Robert Linhardt, a chemist at Rensselaer Polytechnic Institute in Troy, New York, first explored the concept 2 years ago. They cast a thin film of cellulose--the same starting material used to make paper--and laid it over conductive carbon nanotubes. The hope was that the cellulose would serve as a sturdy structural material to hold the other components for making a battery, and it did. But the two layers remained independent and could split apart if flexed.
Yi Cui, a materials scientist at Stanford University in Palo Alto, California, had also been exploring using plastics and other types of thin layers as the structural supports for batteries and supercapacitors (which store energy as static charge, unlike batteries that undergo chemical reactions). But the plastic layers also didn't connect well with the conductive nanotubes placed on top. Conventional copy paper has a highly porous structure. So Cui and his colleagues wondered if that could serve as a good support for their nanotubes.
The researchers created an "ink" of carbon nanotubes suspended in water and an organic surfactant. They then heated the paper in an oven to drive off the water. The nanotubes bonded tightly to the paper fibers, creating a highly conductive sheet of paper that functions even when rolled up. The team then used these conductive sheets as components in both lithium-ion batteries and supercapacitors.
The paper batteries can store up to 7.5 Watt-hours per kilogram (Wh/kg), the team reported online this week in the Proceedings of the National Academy of Sciences. That's not quite up to the level of lead acid batteries, which store roughly 30 Wh/kg. But because the cost of nanotubes is coming down, and because paper is cheap and durable, it could open the door to cheaper batteries for large-scale energy storage.
"It's quite innovative and an important contribution," says Linhardt. The fact that the nanotubes and paper fibers hold tight is critical, he adds, because it now enables engineers to make batteries in almost any shape. Paper's strength could also help battery makers reduce the thickness of the electrodes they use to make batteries, which in many cases are made thick to provide structural support for the batteries. And that reduced amount of electrode material could further reduce the battery's cost.