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DVD writer spins out graphene electrodes for new class of supercapacitor

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March 26, 2012

Schematic showing the structure of laser scribed graphene supercapacitors created by UCLA ...

Schematic showing the structure of laser scribed graphene supercapacitors created by UCLA researchers (Image: UCLA)

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The wonders of graphene seem to know no bounds. Not only is it one of the strongest materials known, is both highly conductive and piezoelectric, it can generate electricity from flowing water and now it is being used to make better supercapacitors. Using a DVD writer, a team of UCLA researchers has invented a new process for making high quality graphene electrodes and used these electrodes to make a new species of supercapacitor. Though the work is in the early stages of development, it could lay a foundation for supercapacitor-based energy storage systems suitable for flexible portable electronic devices.

Lithium-ion batteries are electrically fragile, can explode on charging, and must be slowly recharged over a period of hours to avoid an early death. Supercapacitors, more formally known as electric double layer capacitors, are rugged and can be charged in a minute or so. They also can provide plenty of power and last through millions of recharge cycles. Why, then, don't we hear more about their use?

The short answer is energy density. Supercapacitors store about 20 watt-hours per kilogram, or one-seventh of the energy per kilogram of a lithium-ion battery.

To understand what more widespread adoption will require, let's take a look at how supercapacitors work. A supercapacitor is able to store a charge as a coating of ions adsorbed on the surfaces of its electrodes.

A schematic drawing of a supercapacitor, or electric double layer capacitor (Image: NASA)
A schematic drawing of a supercapacitor, or electric double layer capacitor (Image: NASA)

Ions are separated from the electrolyte by a charging current, and are propelled toward their respective electrodes. The membrane serves to separate the ions so that a net charge separation can be maintained. Notice that the ions on the electrode surface are neutralized (save for a residual dipole field) by the opposite charge attracted to just below the surface of the electrode. This dual surface layer is called an electric double layer. All other parameters being equal, the number of ions stored is proportional to the surface area of the electrodes. Energy storage of a capacitor is proportional to the amount of charge stored, so a tenfold increase in capacitance will require new electrodes that are highly conductive (so large power levels can be generated) and provide more surface area than conventional supercapacitors.

Now researchers at UCLA have used a standard DVD writer to make such electrodes. The electrodes are composed of an expanded network of graphene that shows excellent mechanical and electrical properties as well as exceptionally high surface area.

The process is based on coating a DVD disc with a film of graphite oxide that is then laser treated inside a DVD writer to produce high-quality graphene electrodes. Graphite oxide is a compound of carbon, oxygen, and hydrogen made by treating graphite with sulfuric and phosphoric acids combined with potassium permanganate, an extremely strong oxidizer. When graphite oxide is placed in a basic solution, it exfoliates into monomolecular layers with a graphene-like structure. These layers were then collected on an ordinary DVD disc. The disk was then written on, a number of passes being made.

The action of the 5 milliwatt IR laser on the graphite oxide was to reduce the material, thereby producing isolated but intertangled graphene monolayers. The surface area of the resulting electrodes was 1,520 square meters per gram - about a third of an acre, and 3-5 times the surface area of activated carbon electrodes. You can't do much better, as graphene's intrinsic surface area is 2,630 square meters (about two thirds of an acre) per gram. Also of importance is the conductivity of the electrode material - at 10-100 times that of activated carbon and previous graphene electrodes, we are assured that high power densities are at hand.

The UCLA research team investigated several different types of supercapacitor chemistry using the laser scribed graphene electrodes. They found the supercapacitors were surprisingly robust against flexure, surviving thousands of folds with no significant change in capacitance. Their highest energy storage supercapacitor was based on using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as the electrolyte. The supercapacitor exhibited a capacitance of 276 Farads per gram, and an operating voltage of 4 volts. This corresponds to an energy density of over 600 watt-hours per kilogram (2.2 lb), or about four times that of lithium-ion batteries. In practice, the energy density will be smaller, owing to support structures, but such supercapacitors should be able to give lithium-ion batteries a run for their money.

The UCLA team's paper is published in the journal Science.

Source: UCLA

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
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11 Comments

Then imagine this incorporated in an EV like the Tesla Model S. That would give it a range of 1000 miles (1600 km) in one charge. And each charge would only take a moment if I understand this correctly.

Patrik Nordberg
27th March, 2012 @ 05:00 am PDT

Patrik, charging times are limited by the capabilities of the supporting hardware and infrastructure, but should not be as limited by the supercap itself like batteries are.

This tech is truly a game-changer, even if only half of this claimed energy density is achieved in a real-world application. It allows electric cars to compete directly with fossil-fuel vehicles - even surpass them in many respects, potentially even cost! Great stuff! Lets get it to market, folks!

MzunguMkubwa
27th March, 2012 @ 05:25 am PDT

My obvious thought is that you would use a charging station with a built in supercapacitator that can discharge fast.

Patrik Nordberg
27th March, 2012 @ 07:48 am PDT

I'm hoping someone can provide some technical information for my curiousity.

As far a I remember capacitors offer no resistance to charging.

So would a super capacitor energy supply be able to provide regenerative braking?

Thanks

Facebook User
27th March, 2012 @ 05:37 pm PDT

Compare electrolytic cans most people are familiar with are rated in microfarad (0.000001 farad) with the two large and dangerous bulk capacitors often found in a computer power supply commonly somewhere around 0.00082 farad at 200 volts. Two hundred and seventy six farads per gram is a huge energy storage medium even considering the limited voltage.

Michael Gene
27th March, 2012 @ 10:36 pm PDT

Assuming the system is 100% scalable (they already say it is not, due to support structures), that is a total power density of about 5 times the most likely battery (either LiMnO2 or LiFePO4 with energy density around 110Wh/Kg) used in the Tesla Roadster and new Model S sedan. Their ranges depending on load and road conditions about 200 and 300 miles, respectively. Using 100% scalability, this becomes ranges of 1000 and 1500 miles respectively. If after scaling up to size the density is just that of the batteries currently used by Tesla (80% power loss after scaling), this becomes a very viable product.

The vehicle costs COULD (not would) significantly decrease as the charging/discharging system would be much simpler. Also the recharge time would drop to probably minutes (theoretically it could be less than a second). This allows for creation of commercial recharge stations without long waits for cross country trips.

If this technology is able to be developed, it would be the final nail in the coffin of personal land based petroleum based vehicles.

NatalieEGH
28th March, 2012 @ 12:19 am PDT

Most of you are looking at this technology as a means of propelling vehicles. The major implication that I see for this is to allow the advent of REAL off grid solar HOMES.

The prices/watt of solar production are dropping weekly. Storage has, in the last decade, been the real limiting factor in terms of BOTH efficiency and space required per watt or amp of storage capacity.

Here is where this technology has implications equal to or greater than the auto industry.

markadedo
28th March, 2012 @ 04:59 am PDT

It's a perfect complement to conventional batteries in an electric car. Capacitors are great for rapid charging & discharging, but don't hold a charge well, while chemical batteries hold a charge well, but rapidly wear out under rapid cycling. The super capacitor would store the energy from braking & feed it back to the motor when accelerating, and provide sudden pulses of power as needed. Regular batteries would provide power for cruising and charging the super-capacitor. (Hence, extending the life of the batteries, raising overall efficiency, reducing total costs, etc, maybe even reducing the number of batteries).

Dennis Sweitzer
28th March, 2012 @ 09:39 am PDT

This is an interesting development. Right now I am experimenting with a couple of ultracapacitor modules having recoverable capacity (for my application) of about 2 watt-hours each, and they weigh just over a pound each. Their cycle efficiency is around 98%, but they do leak charge at about 1% per day. They work out less expensive than any current battery in terms of cost per unit energy handled over their lifetime, since they last 10 to 15 years and can be cycled a million times or more. If these can be commercialized as modules (with the cell leveling circuitry) at even 25 Wh per pound, (all other characteristics roughly equal,) they will make a huge difference.

Robert DeDomenico
28th March, 2012 @ 03:02 pm PDT

It is great to see all you guys considering this as a technical problem that is being solved. Possibly a far greater problem in bringing this type of technology to market is how the financial bounty from fossil fueled power can be incorporated.

Taxes, Dealer margins for vehicles and the ongoing revenue that our current systems deliver to both government and industry.

I hope that there is as much energy being devoted to a new financial model as there is to the technology.

Nick Hill
28th March, 2012 @ 09:18 pm PDT

I think the energy density of 600 Whr/kg looks wrong.

When I calculate it I get only ~ 300 Whr/kg using 276 Farads per gram at 4V.

lotuselanplus2
20th March, 2013 @ 08:59 am PDT
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