January/February 2012
Automation Update

Flexible power source from soot

Electronic devices such as displays, sensors, and medical implants are on their way to becoming flexible. But flexible power sources to operate them are only starting to catch up. Researchers now report a simple method to fabricate bendable supercapacitors that uses carbon nanoparticles from soot, according to Chemical & Engineering News.

Supercapacitors, like batteries, store energy. While batteries store and release charge through chemical reactions, supercapacitors store it on the surface of their electrodes. Supercapacitors can charge in minutes instead of hours and can recharge millions of times. Unfortunately, they hold less energy per weight than batteries. To improve supercapacitors' energy density, researchers have replaced activated charcoal electrodes with materials with higher surface area, such as carbon nanotubes and graphene, a one atom-thick sheet of carbon. But making and using these materials typically requires complex, costly methods.

Soot  

Carbon nanoparticles, on the other hand, are cheap and easy to make, says Zhong Lin Wang, a materials science and engineering professor at Georgia Institute of Technology. He thought the nanoparticles could form cheap yet effective supercapacitor electrodes. Wang and his colleagues in China started building such electrodes by holding a flexible carbon substrate in front of an ethanol flame for 30 seconds. The flame deposited a thin layer of carbon nanoparticles, each about 7 nm wide, on the fabric. To make the electrode, the researchers then sputtered manganese oxide nanorods on top of the nanoparticle film. Manganese oxide's high charge storage capacity improves the performance of carbon-based supercapacitor electrodes.

Finally, to make the supercapacitor, the researchers took two of the prepared electrodes, infused them with a polymer gel electrolyte, and sandwiched a standard cellulose separator material between the electrodes.

The resulting foldable device stores 4.8 watt-hours of energy per kg and has a power density of 14 kilowatts per kg. Wang's supercapacitors should be sufficient to run small devices such as sensors and radio-frequency identification tags, Wang said.