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Novel carbon mat'l boosts supercapacitor performance

Posted: 03 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Stanford University? supercapacitor? carbon? batteries?

Stanford University researchers have developed what they describe as a carbon material that increases the performance of lithium-sulfur batteries and supercapacitors.

"We have developed a 'designer carbon' that is both versatile and controllable," said Zhenan Bao, the senior author of the study and a professor of chemical engineering at Stanford. "Our study shows that this material has exceptional energy-storage capacity, enabling unprecedented performance in lithium-sulfur batteries and supercapacitors."

According to Bao, the new designer carbon represents a dramatic improvement over conventional activated carbon, an inexpensive material widely used in products ranging from water filters and air deodorizers to energy-storage devices.

"A lot of cheap activated carbon is made from coconut shells," said Bao. "To activate the carbon, manufacturers burn the coconut at high temperatures and then chemically treat it."

Designer carbon

The activation process creates nano-sized holes, or pores, that increase the surface area of the carbon, allowing it to catalyse more chemical reactions and store more electrical charges.

But activated carbon has serious drawbacks, Bao said. For example, there is little interconnectivity between the pores, which limits their ability to transport electricity.

"With activated carbon, there's no way to control pore connectivity," Bao said. "Also, lots of impurities from the coconut shells and other raw starting materials get carried into the carbon. As a refrigerator deodorant, conventional activated carbon is fine, but it doesn't provide high enough performance for electronic devices and energy-storage applications."

Instead of using coconut shells, Bao and her colleagues developed a new way to synthesise high-quality carbon using inexpensive, and uncontaminated, chemicals and polymers.

The process begins with conducting hydrogel, a water-based polymer with a spongy texture similar to soft contact lenses.

"Hydrogel polymers form an interconnected, 3D framework that's ideal for conducting electricity," said Bao. "This framework also contains organic molecules and functional atoms, such as nitrogen, which allow us to tune the electronic properties of the carbon."

For the study, the Stanford team used a mild carbonisation and activation process to convert the polymer organic frameworks into nanometre-thick sheets of carbon.

"The carbon sheets form a 3D network that has good pore connectivity and high electronic conductivity," said graduate student John To, a co-lead author of the study.

"We also added potassium hydroxide to chemically activate the carbon sheets and increase their surface area."

The result: designer carbon that can be fine-tuned for a variety of applications.

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