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Date: 02-06-15

Higher surface area carbon for supercaps and batteries created by Stanford scientists

Stanford scientists increased the surface area of carbon material for range of applications which also includes super capacitors.

Zhenan Bao, the senior author of the study and a professor of chemical engineering at Stanford and her colleagues synthesized high-quality carbon using less costing and uncontaminated chemicals and polymers. They started the process by using conducting hydrogel, a spongy water-based polymer something like soft contact lenses.

"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."

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

They have used mild carbonization and activation process to convert the polymer organic frameworks into nanometer-thick sheets of carbon.

"The carbon sheets form a 3-D 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."

"We call it designer carbon because we can control its chemical composition, pore size and surface area simply by changing the type of polymers and organic linkers we use, or by adjusting the amount of heat we apply during the fabrication process," To said.

This team could increase the pore volume by 10 times by raising the processing temperature from 400 degrees Celsius) to 900 Deg C.

They could able to produce a carbon material with a very high surface area of 4,073 square meters per gram which is quite higher compared to conventional activated carbon which is about 3,000 square meters per gram.

"High surface area is essential for many applications, including electrocatalysis, storing energy and capturing carbon dioxide emissions from factories and power plants," Bao said.

This team fabricated carbon-coated electrodes and installed them in lithium-sulfur batteries and supercapacitors. They have found out electrical conductivity improved threefold compared to supercapacitor electrodes made of conventional activated carbon.

"We also found that our designer carbon improved the rate of power delivery and the stability of the electrodes," Bao added.

In another test, where they have conducted on lithium-sulfur batteries they could solve the problem of lithium and sulfur reacting to produce molecules of lithium polysulfide, which leak from the electrode into the electrolyte and cause the battery to fail. The electrodes made with so-called designer carbon can trap those polysulfides and improve the battery's performance.

"We can easily design electrodes with very small pores that allow lithium ions to diffuse through the carbon but prevent the polysulfides from leaching out," Bao said. "Our designer carbon is simple to make, relatively cheap and meets all of the critical requirements for high-performance electrodes."

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