High-performance, low-cost ultracapacitors built with graphene and carbon nanotubes

April 22, 2014
A scanning electron microscope image shows the ultracapacitor's composite film containing graphene flakes and single-walled carbon nanotubes. Credit: Journal of Applied Physics

By combining the powers of two single-atom-thick carbon structures, researchers at the George Washington University's Micro-propulsion and Nanotechnology Laboratory have created a new ultracapacitor that is both high performance and low cost.

The device, described in the Journal of Applied Physics, capitalizes on the synergy brought by mixing graphene flakes with single-walled carbon nanotubes, two with complementary properties.

Ultracapacitors are souped-up energy storage devices that hold high amounts of energy and can also quickly release that energy in a surge of power. By combining the high energy-density properties of batteries with the high power-density properties of conventional capacitors, ultracapacitors can boost the performance of electric vehicles, handheld electronics, audio systems and more.

Single-walled carbon nanotubes and graphene both have unique and excellent electronic, thermal, and mechanical properties that make them attractive materials for designing new ultracapacitors, said Jian Li, first author on the paper. Many groups had explored the use of the two materials separately, but few had looked at combining them, he said.

"In our lab we developed an approach by which we can obtain both single-walled carbon nanotubes and graphene, so we came up with the idea to take advantage of the two promising carbon nanomaterials together," added Michael Keidar, a professor in the Department of Mechanical and Aerospace Engineering in the School of Engineering and Applied Science at GW, and director of the Micro-propulsion and Nanotechnology Laboratory.

The researchers synthesized the graphene flakes and nanotubes by vaporizing a hollow graphite rod filled with metallic catalyst powder with an electric arc. They then mixed the two nanostructures together to form an ink that they rolled onto paper, a common separator for current commercial capacitors.

The combination device's specific capacitance, a measurement of the performance of a capacitor per unit of weight, was three times higher than the specific capacitance of a device made from carbon nanotubes alone.

The advantage of the hybrid structure, Li explained, is that the graphene flakes provide and good in-plane conductivity, while the carbon nanotubes connect all of the structures to form a uniform network.

While other types of ultracapacitors have also achieved the high specific capacitance of the graphene/nanotube hybrid, the researchers say, the main advantage of the combination approach is its low costs, since the team has developed a simple way to manufacture large amounts of the desirable mix of carbon nanostructures.

The hybrid ultracapacitor is also small and light, an advantage as electronic devices get ever smaller.

Explore further: Hybrid nanotube-graphene material promises to simplify manufacturing

More information: "Paper-based ultracapacitors with carbon nanotubes-graphene composites" by Jian Li, Xiaoqian Cheng, Jianwei Sun, Cameron Brand, Alexey Shashurin, Mark Reeves and Michael Keidar, Journal of Applied Physics April 22, 2014 DOI: 10.1063/1.4871290 . http://scitation.aip.org/content/aip/journal/jap/115/16/10.1063/1.4871290

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5 / 5 (2) Apr 22, 2014
This is interesting research. Will these eventually replace current capacitor technologies or will they form more of a niche for specific functions?
5 / 5 (5) Apr 22, 2014
Will these eventually replace current capacitor technologies or will they form more of a niche for specific functions?

They won't necessarily replace most capacitors. Lots of them are very tiny, made out of ceramics and used to do things like filtering power going into an IC. Larger electrolytic capacitors (the ones made with paper) could benefit from this technology if it makes them cheaper to produce, and it probably will. The big benefit is probably going to be with capacitors used for energy storage, rather than power conditioning. Look to applications like smart phones that can be charged in 5 minutes and last for 5 days, or electric cars that can charge in a few hours and go 500 miles. The energy density is key.
5 / 5 (3) Apr 22, 2014
Here is hoping. I wish they had given us some performance numbers so that I could do some modeling.
3 / 5 (2) Apr 22, 2014
Yeah, the real key will be scalability.

If this trchnology can be deployed at grid strength, then we would finally be able to make major reductions in fossil fuel use, and eventually to phase them out entirely.

And that will be a very good day for every living thing on this planet.

5 / 5 (3) Apr 23, 2014
Typically ultracapacitors (and there are others, not just this one "brand") have an inherently low energy density, but a very high power density. What that means in practice is that they can hold very little power, but they can release it - or replenish it - very quickly.

Hybrid ultracapacitors are devices that attempt to increase the energy density of the capacitor by sacrificing some, though not all, of their power density. Even so, the energy density of these hybrids is by necessity much lower than that of a Li-ion battery.

So while these might have a *place* in electric vehicles (especially racecars), that place won't be replacing their battery. It will be helping short term performance by allowing surges in available power output for certain situations (hills, etc).
not rated yet Apr 24, 2014
Ultracapacitors are also limited in operating voltage by the electrolyte to typically less than three volts, unlike aluminum or ceramic capacitors whose operating voltage is determined by the breakdown voltage of the insulating layer of aluminum oxide or ceramic.
This means that to make higher voltage capacitors, Ultracaps would need to be stacked in series. Since they can't share electrolyte between each "cell", each of Ulracap "cells" put in series would need to be completely independent and sealed from the others.
Ultracaps won't be replacing other commonly used capacitors in most applications.
Except for trying to replace batteries for energy storage, they don't have much of a market.
not rated yet Apr 26, 2014
When do they go on the market? All these discoveries are nice but worthless if they never come to market.
Bob Osaka
not rated yet Apr 28, 2014
The ultimate goal should be a design capable of withstanding and storage of 30-300 kiloampere, 5 coulombs and 500 megajoule. Lightning in a bottle. Keep up the good work!
5 / 5 (2) Apr 28, 2014
When do they go on the market? All these discoveries are nice but worthless if they never come to market.

There is quite a ways to go from discovery to market. Don't exepect that to take much less than a decade.
You need to optimize, find a cheap manufacturing process that scales well, build factories, integrate with existing electronics, ... all of that takes a lot of time.

I agree with gopher: this will be good for surges of power - not to replace battery storage. It will make stuff like reclaiming power from braking very light weight.

The image still looks like this is a very 'unordered' setup. Reminds me of the 'brown soup' used during the early days of harddrives. I bet with some effort put into growing these in a structured way the performance can be greatly increased.

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