22 April 2009—The field of printable electronics has taken off in recent years, with researchers touting prototype wearable sensors, smart packaging labels, and even solar cells made with printing technology. However, little advancement has been made in developing printable, flexible energy-storage devices, such as supercapacitorsand batteries.
Now researchers from Stanford University and the University of California, Los Angeles, have built a flexible supercapacitor out of printable carbon nanotubes and polymer gel electrolyte. Their results were detailed this month in Nano Letters .
George Grüner, a physics professor at UCLA who led the research, says that printing supercapacitors could one day be as simple and cheap as making photocopies or printing a newspaper.
The researchers built the supercapacitor by spraying carbon nanotubes onto two thin plastic substrates. In between the two substrates they sandwiched a polymer gel electrolyte, which acts to separate the charges at the interface of the carbon nanotubes and the gel. Carbon nanotubes are ideal for flexible, printable electronics because they are lightweight and can bend and stretch without breaking or compromising their electrical properties.
”Many people have been trying to build a printable supercapacitor,” says Chongwu Zhou, associate professor of electrical engineering at the University of Southern California, speaking about his own research group as well. Zhou recently demonstrated a flexible and transparent supercapacitor, but it was not made using printing technology, which he is working on now.
The UCLA supercapacitor has a power density of 70 kilowatts per kilogram and an energy density of 9 watt-hours per kilogram, which is comparable to other supercapacitors made using carbon nanotubes. But it also has a high internal resistance, which causes some of the stored energy to be lost. In order for the supercapacitor to be commercialized, the internal resistance would have to be lowered, says Zhou. He says this could be done by simply packing in more carbon nanotubes.
Advances have also been made in flexible batteries, but supercapacitors, also called ultracapacitors or double layer capacitors, are thought to be superior for applications that require quick bursts of energy. Manos Tentzeris, an electrical and computer engineering professor at Georgia Tech, says that for some applications, supercapacitors will eventually replace batteries. For wearable electronics, such as smart clothes or diagnostic devices, ”supercapacitors are much more body-friendly than using a battery,” he says. Supercapacitors are also ideal for sensors that do not need to operate continuously, he adds. And, unlike batteries, supercapacitors do not need to be replaced.
The next step is to demonstrate the supercapacitor on a fully functional flexible chip built with printing technology, says Grüner. Zhou says that this is the only way to truly evaluate the performance of the supercapacitor and figure out what other technology issues need to be worked out. He says the first commercial printed supercapacitor could be available in five years and will likely be in portable devices such as digital cameras or cellphones. Grüner, on the other hand, says that sensor devices, such as temperature or heart monitors or a chemical sensor will come first. Eventually, printable supercapacitors could have applications in drug-delivery devices, solar cells, and even in electric cars.
About the Author
Monica Heger is a writer based in New York City. She wrote about the use of unmanned aerial vehicles in combating Somali pirates in the February 2009 issue of IEEE Spectrum .