"Designer carbon" bodes well for enhanced energy-storage

A supercapacitor boasting a superior power delivery rate thanks to the "designer carbon" developed at Stanford University(Credit: John To and Zheng Chen)

Activated carbon is a form of carbon that is shot through with nanosized holes that increase the material's surface area and allow it to catalyze more chemical reactions and store more electrical charge. But due to the way it is produced, most of the pores within it aren't interconnected, limiting the material's ability to transport electricity. Now researchers at Stanford University have created a "designer carbon" with greater pore connectivity and therefore greater electronic conductivity, which enables superior energy-storage performance.

Used for everything from gas and water purification to air filters and medicine, activated carbon is generally produced from coconut husks, nutshells, peat, wood and other carbon-rich source materials. These are burnt at high temperatures and then subjected to either physical (aka steam) activation or chemical activation to create nanosized pores. But the random nature of these processes results in a material with little interconnectivity between the pores.

"With activated carbon, there's no way to control pore connectivity," says Zhenan Bao, the senior author of the study and a professor of chemical engineering at Stanford. "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."

To create their new high quality designer carbon, Bao and her colleagues started with a conducting, water-based polymer known as a hydrogel.

"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."

Using a mild carbonization and activation process, the Stanford team converted the hydrogel into nanometer-thick sheets of carbon with a 3D network boasting higher pore connectivity. To activate the carbon sheets and increase their surface area, they then added potassium hydroxide.

"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," says graduate student and co-lead author of the study, John To.

Using this process, the team was able to produce a 10-fold increase in pore volume by raising the processing temperature from 400° C (750° F) to 900° C (1,650° F). They were also able to produce 28 g (1 oz) of carbon material boasting a surface area of 4,073 sq m (43,840 sq ft), or the equivalent of three American football fields. The team says this is a new record, eclipsing conventional activated carbon, whose surface area maxes out at around 3,000 sq m (32,290 sq ft).

To test the performance of the material in real-world conditions, the team coated it on electrodes, which they installed in lithium-sulfur batteries and supercapacitors.

The researchers found the material overcame a major shortcoming of lithium-sulfur batteries. Although they boast superior energy storage capabilities to their lithium-ion counterparts, when lithium and sulfur react, they produce lithium polysulfide at the electrode, which can leak into the electrolyte and cause the battery to fail, resulting in significantly shorter lifetimes.

However, with the ability to tune the size of the pores in the designer carbon, the Stanford team was able to create electrodes with pores big enough to let lithium ions pass through, but small enough to trap the polysulfides, thereby improving the battery's performance and extending their life.

The material also proved beneficial for use in supercapacitors, which boast ultra-fast charging and discharging capabilities. Equipping supercapacitors with the designer carbon, researchers reported a three-fold improvement in electrical conductivity over electrodes made of conventional activated carbon, while also improving the power delivery rate and stability of the electrodes.

The researchers say the surface area attributes, the ability to fine-tune the material, and the simplicity and low cost of the production process gives the designer carbon great potential for a wide range of applications.

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

The team's study appears in the journal ACS Central Science.

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