How a microscopic material thinner than human hair could power a bus

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Graphene sealed in a pouch with electrolytes makes a flexible supercapacitor. Photo by Ashley Jones.

It’s a million times thinner than a human hair. But it’s stronger than steel and can power an entire bus for a mile. That is the ingenuity behind graphene, which is microscopic, specially designed layers of carbon.

The material could be the answer the electric vehicle industry has been looking for.

Clemson University physicists Ramakrishna Podila and Apparao Rao and students Jingyi Zhu and Anthony Childress have discovered that the nanomaterial boosts five-fold the energy capacity of a supercapacitor without sacrificing strength or durability.

But what is a supercapacitor?

Capacitors are similar to batteries, but they deliver more power over a very short period of time. Batteries, on the other hand, deliver less power but store much more energy.

“While the chemical reactions hold much energy, the ion motion in batteries is rather slow, leading to low power,” said Podila. That is why electrically powered cars have struggled to take off and fossil fuels remain the dominant energy source. Because batteries take a long time to charge.

Capacitors, on the other hand, can be charged at a much faster rate. But that rate increases more when it becomes a supercapacitor, which can hold hundreds of times the amount of electrical charge as standard capacitors. Supercapictors store ions on the surface of nanomaterials electrostatically.

Jingyi Zhu, Ramakrishna Podila, Apparao Rao and Anthony Childress with a large-scale model of graphene superconductor material. Photo by Ashley Jones.
Jingyi Zhu, Ramakrishna Podila, Apparao Rao and Anthony Childress with a large-scale model of graphene superconductor material. Photo by Ashley Jones.

 

Graphene is the best option because it’s thin, which allows ions to flow better.

“The high-surface area of graphene provides space for ion storage (high-energy) and the ions are always on the surface ready to race (high power),” Podila said. “The problem, however, has been to effectively use the high surface area.”

He added that ions oftentimes can’t access some of the spaces in nanomaterials due to the lack of connectivity, and that the electrons within some nanomaterials can limit the total energy of a supercapacitor through an effect called “quantum capacitance.”

The researchers decided to create microscopic layers of graphene with nanometer pores. Then they sandwiched them together, which opened new channels for ions to access all the spaces in graphene and increased the quantum capacitance.

They also created the pores in specific configurations, which increased storage capacity to 150 percent. Then the researchers introduced two different electrolytes whose ions were smaller than the pores.

“Testing showed the electrolytes with the larger ions did not increase the capacity, but the smaller ions travel through the pores into untapped parts of graphene. The result was a 500 percent increase in capacity,” Zhu said.

Zhu and Childress also configured graphene into thin, flexible electrodes and inserted them into the flexible pouch. They filled the pouch with the electrolyte containing the smaller ions and sealed it, creating a lightweight, flexible supercapacitor that could withstand more than 10,000 charge-discharge cycles.

That means even faster charging times than what today’s supercapacitors provide. And it also means longer lives, lighter power sources than batteries, reduced dependency on fossil fuels and less air pollution.

Clemson physicists created nanopores in sheets of graphene then sandwiched them together to increase capacity five-fold. Photo by Ashley Jones.
Clemson physicists created nanopores in sheets of graphene then sandwiched them together to increase capacity five-fold. Photo by Ashley Jones.

 

Their research is already garnering international attention.

“A national research and development enterprise in India is interested in the Clemson supercapacitors and visited the Clemson Nanomaterials Institute twice. Negotiations for manufacturing supercapacitors to power a bus are in progress,” Rao said.

Supercapacitors are already powering public buses across the globe.

ABB, the world’s largest maker of power transmission gear, has set up 13 flash-charging stations in Geneva, Switzerland. The stations are located between the airport and a small suburb. At each station, electric buses can charge for 15 seconds and receive a 600-kilowatt blast, which provides enough energy for about a mile.

Each bus runs for about 10 minutes. The buses can recharge in about five minutes. The project is expected to cut carbon dioxide emissions by 1,000 tons per year compared to diesel buses, which emit 0.055kgs of carbon dioxide emissions per passenger mile, according to the Environmental Protection Agency.

Supercapacitors can also be used for regenerative braking in hybrid and electric vehicles to providing the power needed to adjust the direction of turbine blades in changing wind conditions.

For more information, visit newsstand.clemson.edu

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