Green Car Congress  
Home Topics Archives About Contact  RSS Headlines

« GM Working on New Li-Ion Anode and Cathode Materials with 3X Present Capacity | Main | 2008 Arctic Sea Ice Extent Second-Lowest Measured »

Print this post

Researchers Develop Graphene-Based Ultracapacitors; Potential for Doubling Storage Capacity of Current Materials

16 September 2008

Cgm1
Low and high (inset) magnification SEM images of CMG particle electrode surface. Click to enlarge. Credit: ACS

Researchers at The University of Texas at Austin have developed a new class of carbon material—chemically modified graphene (CMG)—that is made from a 1-atom thick sheet of carbon. CMG materials can be functionalized as needed, and the researchers demonstrated their performance in an ultracapacitor cell. A paper on their work was published online 13 September in the journal Nano Letters.

The surface area of a single graphene sheet is 2,630 m2/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. The group measured specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, in devices using the CMG material.

Through such a device, electrical charge can be rapidly stored on the graphene sheets, and released from them as well for the delivery of electrical current and, thus, electrical power. There are reasons to think that the ability to store electrical charge can be about double that of current commercially used materials. We are working to see if that prediction will be borne out in the laboratory.

—Professor Rod Ruoff

EDLC ultracapacitor units consist of two porous carbon electrodes isolated from electrical contact by a porous separators. The separator and the electrodes are impregnated with an electrolyte, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. Current collectors of metal foil or carbon impregnated polymers are used to conduct electrical current from each electrode.

The CMG system of individual sheets does not depend on the distribution of pores in a solid support to give it its large surface area, rather every chemically modified graphene sheet can “move” physically to adjust to the different types of electrolytes (their sizes, their spatial distribution). Thus, access to the very high surface area of CMG materials by the electrolyte can be maintained while preserving the overall high electrical conductivity for such a network.

—Stoller et al. (2008)

Unlike conventional commercial electrodes which use additives such as carbon black to enable rapid electrical charge transfer from the cell, the high electrical conductivity of the graphene materials eliminates the need for conductive fillers and allows increased electrode thickness. Increasing the electrode thickness and eliminating additives leads to an improved electrode material to collector/separator ratio, which in turn further increases the energy density of the packaged ultracapacitor.

The morphology of the CMG material used in the initial study results in only a portion of the graphene sheets (those at the surface of the particles) being exposed to electrolyte. Nevertheless, the measured specific capacitances showed that the graphene material works well with current commercial electrolytes, has good electrical conductivity, and has very promising charge storage capability.

Chemically modified graphenes with good electrical conductivity and very large (and in principle completely accessible) surface areas, are extremely promising candidates for EDLC ultracapacitors. Our results applying these materials to ultracapacitors show they are compatible with commonly used electrolyte systems. In addition, these CMG materials are based on abundantly available and cost-effective graphite. Ultracapacitors based on these materials could have the cost and performance that would dramatically accelerate their adoption in a wide range of energy storage applications.

—Stoller et al. (2008)

Funding and support was provided by the Texas Nanotechnology Research Superiority Initiative, The University of Texas at Austin and a Korea Research Foundation Grant for fellowship support for Dr. Park.

Resources

  • Meryl D. Stoller, Sungjin Park, Yanwu Zhu, Jinho An, and Rodney S. Ruoff (2008) Graphene-Based Ultracapacitors. ASAP Nano Lett., doi: 10.1021/nl802558y

September 16, 2008 in Batteries | Permalink | Comments (22) | TrackBack (0)

TrackBack

TrackBack URL for this entry:
http://www.typepad.com/services/trackback/6a00d8341c4fbe53ef010534b07811970c

Listed below are links to weblogs that reference Researchers Develop Graphene-Based Ultracapacitors; Potential for Doubling Storage Capacity of Current Materials:

Comments

If they can make this work on a commercial scale this could lead to a drastic reduction in the size of a battery pack for both conventional and plug-in hybrid applications. In short, a vehicle like the Chevrolet Volt could benefit from switching to a ultracapacitor battery pack instead of the battery packs now planned for the Volt.

Most people don't know that the largest ultra-capacitor cell cannot even store the energy thats in a AAA flashlight cell. Some watch batteries store more energy. In theory and in practice a capacitor can deliver large horsepower but only for a few seconds at most. ..HG..

True about ultracapacitors' energy capacity, EEStor and flying pigs notwithstanding.

StephenB

I think this is realistic but not EEStor.

Henri Gibson

The best Ultracapacitor have a energy dnsity storage of 12Whrs/Kg, so this invention will bring it in the 20Whrs/Kg, a lead-acid battery is about 30Whrs/Kg so not bad, then you would need a 25Kg module to replace the battery of the Prius, with a much better efficiency in braking recovery than a NiHydride battery and an unlimited lifetime. Ultracap associated with a battery can make increase the life of a battery by a factor 4.

So better than what you think

Last month JM Energy announced a lithium-ion capacitor due soon at an energy density spec of 14 Wh/kg. If this technology could be combined with University of Texas' CMG concept and it indeed doubles the energy density, the projected energy density would rival that of contemporary lead acids. Particularly after they're not so new, or in cold weather. At the same time it could eliminate almost all features that make lead acids unpalatable: life, cold weather performance, sensitivity to partial charge (sulfation), outgassing, massive toxic ingredients, acid hazard, slow charge etc. It would probably last practically indefinitely and be high-power, very fast-charging and use no geostrategic materials or materials (natural resource) with uncertain economics for competitiveness or uncertain availability for rapid scaling. (The lithium-ion battery-like materials involved are minor quantities.)

Since lead acids have proven good enough for many EV's except for the burdens of their non-density downsides, such a capacitor would make the relaunch of the electric car that much more unstoppable. Suitable for many BEV's, and certainly PHEV's and HEV's.

Also don't forget that a capacitor can use its entire charge window (100% soc) without impact on its life. So 20kw/hr is comparable to a battery with an energy density of 40kw/hr but is limited to using a charge window of 50% (much like the battery in the
Prius and the upcoming Volt)

But Altair's titanate battery appears to have all the benefits of a capacitor (huge power, safety and cycle life) along with 80 Wh/kg and 100% DOD cycling.

Well who is going to fund this to market and how long before we see any benifit. The problem is as the US economy slips into the toilet we need a new catalyst to bring it back from the dead. If this can be made real in 12 months there is not a moment to loose.
Otherwise whats out there thats real right now?
GM made the Volt look like a wind up toy and priced it
out of the middle class reach. So there will not be a huge volume of sales.

Ouch on the yield of the process.

This material isn't graphene, the author's claims notwithstanding. It looks like 10-20 layer thick graphite to me.

@ clett:

The Altairnano unit has most of the qualities required except the $2K+/Kwh price tag.

However, since it can be fully (100%) discharged four times more often,i.e. 10 000 times intead of 2500 times at 50% for the other $500/Kwh lithium units, the Altairnano is, by far, the lowest cost long term unit, if you keep your car for 30+ years.

How many will be prepared to pay (up front) about $100K for a battery pack for the next 30 years? Rolls-Royce owners did. The population at large did not.

i am no engineer, but my understanding of ultracaps is that the voltage drops as they discharge. so while you can theoretically get all of the energy out, it is at an increasingly lower voltage. and most power electronics need voltage within a relatively narrow band. so this limits the useful strage capacity of an ultra cap further. ie. you can only get so much energy out before it is no longer useful. (i suppose you could have some smart electronics that automatically combine the output of varoius caps as the voltage drops to keep the overall output within a range, but i have no idea).
can one of you smart people please confirm and explain?

my understanding is that voltage drops as an ultracap is discharged. so you can't use the full stored eenrgy as the votage quickly drops below the narrow band that the powere electonics are expecting. this limits the usefulsness somewhat. can someone please confirma and explain?

my understanding is that voltage drops as an ultracap is discharged. so you can't use the full stored eenrgy as the votage quickly drops below the narrow band that the powere electonics are expecting. this limits the usefulsness somewhat. can someone please confirma and explain

ABC

Yes, that's true. Q= C*V. Since I = dQ/dt, that means I = C*dV/dt. So dV/dt = I/C. So people either use only a portion of the discharge range, or they add some power electronics to maintain the output voltage during the discharge.

I own some Altairnano. Their great batteries don't ever quite appear in products consumers buy. If the sum wasn't so trivial I would sell. It was a shot.

I have to assume the problem was/is an inability to reduce cost. And that is almost certainly due to a high manufacturing defect rate. Not much solid news from the company.

There are premium applications such as military and science where cost is less critical. Altair is into those.

I also own some Maxwell. As ABC asked and Jim answered, capacitors are inherently variable voltage devices. That makes getting the power into and from them tricky and electronics is usually needed for control.

Said controls have not historically been cheap. But some commentators elsewhere have insisted the control costs are now trivial. I haven't investigated that.

It's definitely easier to get all the juice out of a battery or ultracapacitor when the voltage range you have to accommodate isn't too wide. That's one of the reasons I brought up the LIC, which is a special hybrid of a Li-ion battery and an ultracapacitor but functions as the latter. Almost, except that the bias field that the battery layer makes, which is apparently superimposed on the dielectric space of the capacitor, shifts the working voltage swing (which is unfortunately a very low 0 to 2.5 volts on a standard ultracapacitor) to go from something like 2.2 volts minimum to 3.8 V max (JME's parts). Since the energy you can squeeze into a capacitor is proportional to the square of the voltage, that's how you get so much more energy into them. And the voltage swing is less than 2:1, which is not dramatically worse than what you have to accommodate with most batteries. The worth of the LIC concept (not to be confused with a battery in series or in parallel with a capacitor) is all the more if you have new ways to boost the ultracap part's density.


@harveyD

True, Altair are selling their cells at $2,000 per kWh just now, but that is in very low volumes (almost hand built!)

They have repeatedly said they will manage to produce their cells for less than $500 per kWh when ramped up to volume production. Remember their raw material costs are lower than for cobalt oxide based Li cells, which already wholesale for under $400 per kWh in 18650 format.

My understandng as a interested amateur is that capacitors function as low resistance high efficiency components capable of handling the high voltage / high current fast transients that are important to efficiency savings they also offer extended cycle life.
Batteries, are traditionally deficient in these regard.
We hear of great improvement in battery technology both in storage and also and as importantly in the low resistance extended life aspects.

Capacitors are improved as the storage capacity and higher voltage withstand make for increased utility.

These different aspects of power storage, delivery and conditioning have always been handled by the component most suited to each part of the task. For efficiency it is necessary that energy storage and delivery be able to operate at as high frequency, low resistance environment with as much store as possible.

Ideally we would like to see these qualities in the one component but this is not currently available.

Hybrid battery capacitors are a way of bringing the two aspects together to achieve advantage greater than the sum of the parts.

...the largest ultra-capacitor cell cannot even store the energy thats in a AAA flashlight cell.

Another "Henry fact". It's true ultracaps have very low energy density, but this Maxwell ultracap holds 3 Wh vs. less than 2 for a typical AAA Alkaline:

http://www.maxwell.com/ultracapacitors/products/large-cell/bcap3000.asp

They have repeatedly said they will manage to produce their cells for less than $500 per kWh when ramped up to volume production.

Altair makes lots of claims but very few batteries. Companies who started years after Altair (e.g. A123) ramped production years before them. If you can't deliver in this environment, with virtually unlimited funding, you are either galactically incompetent or you have show-stopper technology problems. Perhaps both.

Anyway, until Altair delivers something to someone you pretty have to classify them with EEStor, PolyPlus and the other wannabes.

doggy - both the Brit Lightning and Phoenix SUV/SUT are Altair powered vehicles. They have not shipped many but they are available if you have the $$$.

http://www.phoenixmotorcars.com/vehicles/phoenix-suv.php
http://www.lightningcarcompany.co.uk/

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Working...
Your comment could not be posted. Error type:
Your comment has been posted. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.

Working...

Post a comment

Green Car Congress © 2017 BioAge Group, LLC. All Rights Reserved. | Home | BioAge Group