Transforming biochar into activated carbon

The pyrolysis process turns plant materials into bio-oil and biochar, according to assistant professor Zhengrong Gu of the South Dakota State University agricultural and biosystems engineering department. When the bio-oil is further processed, it becomes biofuel.

Gu is converting biochar, a charcoal-like material, into graphene which can be used in place of expensive, activated carbon to coat the electrodes of energy storage devices -- supercapacitors.

Small engines use start-up and run capacitors, Gu explained, but supercapacitors have more rapid charge and discharge rates as well as a higher energy storage capacity. Unlike conventional batteries, supercapacitors can withstand low temperatures.

To manufacture these storage devices, the United States now imports most of its activated carbon from Asia -- including Japan, Thailand and China. "We can use these abundant agricultural materials as biofuel to reduce our dependence on petroleum and, at the same time, generate good active carbon to export," Gu said.

Increasing product value

Gu estimated that approximately 2.2 pounds of graphene is worth at least $1,000. A pound of DDGS costs 7.5 to 9 cents and converts to approximately 7 ounces of graphene.

"That's the increased value of the product," Gu said. "We can convert agricultural residue to a high-value product that is easy to ship."

Once the DDGS or corn stover is transformed to biochar, Gu mixes chemical called a catalyst with the biochar and heats the mixture to 1,292 degrees Fahrenheit for one hour to make porous graphene.

"It's a one-step process," he said. He estimated production costs, including feedstock, at about $1.36 for a pound of graphene.

Using the native grass big blue stem as the feedstock, Gu said, "we save more on feedstock."

In addition, Gu hopes to adapt a new plasma processing technique developed at SDSU that reduces the processing time to five minutes and the temperature to 302 degrees Fahrenheit to convert biochar to graphene.

That could result in a significant cost savings, he added.

Optimizing material properties

Though Gu's processing method generates graphene with the properties needed to capture and discharge electricity, he admitted, "We don't know how the reaction happens."

Through a $775,155 grant from the National Science Foundation along with $332,210 in university matching funds, Gu and a team of SDSU researchers studying carbon materials and biofuel technologies have purchased a transmission electron microscope will help advance this and other projects campuswide. The instrument should arrive this spring.

"We can find out how the process happens and learn how to change the parameters to improve the end product," Gu said.

With the microscope, he and his team can determine the internal material structure and how the morphology changes its energy storage properties much like doctors use a CT scan to examine the human body, Gu explained. They can also find out whether biochar from one type of feedstock produces better graphene than another.