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December 10, 2009
3 Min Read
A piece of standard paper with black carbon nanotube ink on it can function as a battery. (Photo by PNAS)
If someone were to tell you that he can power his cellphone using a sheet of standard photocopy paper, you'd think that too much texting has caused him to lose his grip on reality. But not so fast.Led by materials science professor Yi Cui, a group of researchers at Stanford University (Palo Alto, CA) have demonstrated that ordinary office paper can be turned into a battery electrode by coating it with carbon-nanotube inks. Strong, flexible, and highly conductive, the resulting electrodes could conceivably be used to make cheap energy storage devices to power portable electronics, including medical devices. The team's work is described in the Proceedings of the National Academy of Sciences (PNAS).The process of making paper-based storage devices is simple. After a piece of paper is coated with nanotube ink and dried, it becomes highly conductive.In addition to being strong, light weight, and conductive, carbon nanotubes are a promising material for printing batteries because they can store large amounts of energy. Energy-storage capacity is the quality that helps portable electronics run longer between charges.
A scanning electron micrograph image shows that in paper electrodes, nanotubes absorb into the paper and bind strongly with its cellulose fibers. (Image by Stanford University)
Paper soaks up carbon nanotubes like a sponge, enabling it to be used as a material for electrodes for batteries and supercapacitors. The advantage of paper, Cui explains, is that it's cheap and interacts strongly with nanotubes without the need for putting additives in the ink. "We take advantage of the porous structure of paper," he remarks. "Carbon nanotubes absorb into the paper and stick on really tightly."By testing the thin films as electrodes in supercapacitors, the Stanford team found that they can store more total energy and operate at higher currents than previous printed nanotube devices. The group also tested the paper electrodes as current collectors in lithium-ion batteries. Although metal current collectors used in batteries are much heavier than paper electrodes, the latter performed as well.While other researchers have attempted to use paper as a substrate for making electrodes, Cui notes that their attempts have been much more complicated, requiring growing the nanotubes on the paper or developing novel paper formulations. In contrast, the dipping method is simple and straightforward.The paper-nanotube electrodes are robust, Cui remarks. While untreated paper dissolves in water, nanotube-treated paper remains intact, and the nanotubes do not come off when the paper is scratched or rolled up. "The supercapacitor has been tested over 40,000 charge cycles for six months and it's still working," Cui says.The Stanford researchers are now working to improve the performance of their device by testing different printing methods and materials. So far, Cui has utilized an ink composed of a mixture of semiconducting and metallic nanotubes. Purely metallic inks are likely to perform better, but they are more expensive than carbon-nanotube inks. The group is also experimenting with different ways of bringing nanotubes and paper together, including painting the inks with a pen or brush to form complex patterns.More information about this technology can be found in "Batteries Made from Regular Paper" published in MIT's Technology Review. For recent Medtech Pulse Blog posts on up-and-coming battery technologies, see "Betavoltaic Battery Expected to Live 25 Years," "Liquid Semiconductor Nuclear Batteries Could Be a Blast in Small Devices," and "Algae Enables Battery Breakthrough."
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