High-performance capacitor could lead to better rechargeable batteries

Jan 26, 2011 By Lisa Zyga feature
The unique 3D array of nanopores in zeolite-templated carbon enables it to be used as an electrode for high-performance supercapacitors that have a high capacitance and quick charge time. Image credit: Hiroyuki Itoi, et al. ©2011 American Chemical Society.

(PhysOrg.com) -- In order to develop next-generation electric vehicles, solar energy systems, and other clean energy technologies, researchers need an efficient way to store the energy. One of the key energy storage devices for these applications and others is a supercapacitor, also called an electric double-layer capacitor. In a recent study, scientists have investigated the possibility of using a material called zeolite-templated carbon for the electrode in this type of capacitor, and found that the material’s unique pore structure greatly improves the capacitor's overall performance.

The researchers, Hiroyuki Itoi, Hirotomo Nishihara, Taichi Kogure, and Takashi Kyotani, from Tohoku University in Sendai, Japan, have published their results on the high-performance electric double-layer capacitor in a recent issue of the .

To store energy, the electric double-layer capacitor is charged by ions that migrate from a bulk solution to an electrode, where they are adsorbed. Before reaching the electrode’s surface, the ions have to travel through narrow nanopores as quickly and efficiently as possible. Basically, the quicker the ions can travel down these paths, the quicker the capacitor can be charged, resulting in a high rate performance. Also, the greater the adsorbed ion density in the electrode, the greater the charge that the capacitor can store, resulting in a high volumetric capacitance.

Recently, scientists have been testing materials with pores of various sizes and structures to try to achieve both quick ion transport and high adsorption ion density. But the two requirements are somewhat contradictory, since ions can travel more quickly through larger nanopores, but large nanopores make the electrode density low and thus decrease the adsorbed ion density.

“In this work, we have successfully demonstrated that it is possible to meet the two seemingly contradictory requirements, high power density and high volumetric capacitance, with zeolite-templated carbon,” Nishihara told PhysOrg.com.

The zeolite-templated carbon consists of nanopores that are 1.2 nm in diameter (smaller than most electrode materials) and that have a very ordered structure (whereas other pores can be disordered and random). The nanopores’ small size makes the adsorbed ion density high, while the ordered structure – described as a diamond-like framework – allows the ions to quickly pass through the nanopores. In a previous study, the researchers showed that zeolite-templated carbon with nanopores smaller than 1.2 nm cannot enable fast ion transport, suggesting that this size may provide the optimal balance between high rate performance and high volumetric capacitance.

In tests, the zeolite-templated carbon’s properties exceeded those of other materials, demonstrating its potential to be used as an electrode for high-performance electric double-layer capacitors.

“We are now trying to further increase the energy density of the zeolite-templated carbon up to the same level of secondary batteries,” Nishihara said. “If such an electric double layer is developed and used for mobile devices, such as cellular phones, their charging time can be shortened to only a few minutes. Another important future application of electric double layer capacitors is a support of secondary batteries in to prolong the battery's lifetime. Also for this purpose, achieving a higher density is one of the key issues.”

Explore further: Researchers make nanostructured carbon using the waste product sawdust

More information: Hiroyuki Itoi, et al. “Three-Dimensionally Arrayed and Mutually Connected 1.2-nm Nanopores for High-Performance Electric Double Layer Capacitor.” Journal of the American Chemical Society. DOI:10.1021/ja108315p

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User comments : 9

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lengould100
not rated yet Jan 26, 2011
Hmmmm... Combine this electrode with EEStor's fantastically high di-electric insulator, we'd have a monster. Quick, call Disney, we've got another fairy tale.
TabulaMentis
not rated yet Jan 26, 2011
Supercapacitorsteins. Well, they can't all be gems.
TStidham
not rated yet Jan 26, 2011
MIT was working on carbon nanotube capacitors back in 2006. I haven't seen anything more about this until this article, so it's nice to see the idea wasn't abandoned. It won't let me post a direct link, but google 'nano tube capacitor' for more info.
yoatmon
not rated yet Jan 26, 2011
@ TStidham:
Here is the latest news I'm aware of concerning progress on graphene ultra capacitors.
(http://)pubs.acs.org/doi/abs/10.1021/nl102661q
Note: Use above web address without parantheses.
Eikka
5 / 5 (2) Jan 26, 2011
Hmmmm... Combine this electrode with EEStor's fantastically high di-electric insulator, we'd have a monster. Quick, call Disney, we've got another fairy tale.


This invention is for an electrolytic capacitor, whereas the EESTOR scam is about ceramic capacitors.

Electrolytic capacitors deal with high charge densities at low voltages, whereas ceramic capacitors deal with low charge densities at extremely high voltages. All supercapacitors are of the electrolytic sort, and their dielectrics break down over few volts, because they resemble chemical batteries to a great degree.
Eikka
5 / 5 (3) Jan 26, 2011
Though the problem of fitting together batteries and capacitors is their different voltage-charge curves.

Because chemical batteries operate on chemical reactions that happen over a somewhat constant voltage difference, they tend to keep their voltage steady right up to the point when they're almost completely empty of charge, and while the voltage may change, it usually does so in a more or less linear way. A NiMh cell usually stays between 1.2-1.0 volts for most of the time as you discharge it.

Capacitors on the other hand don't. Their stored energy is in relation to the square of the voltage, so at 100% full you might have 10 volts. 50% full is 7 volts, 25% full is 5 volts, 10% full is 3 volts... etc. all the way down to zero.

To use the capacitor, you need a special voltage booster to use it all the way to empty, or otherwise your device will simply not work when the voltage drops below the limit it can use. Unfortunately that booster drops the efficiency significantly.
DrJim
3.5 / 5 (2) Jan 27, 2011
....To use the capacitor, you need a special voltage booster to use it all the way to empty, or otherwise your device will simply not work when the voltage drops below the limit it can use. Unfortunately that booster drops the efficiency significantly.


May I beg to differ? Let's say you had capacitors capable of 100V. If you put 5 in series, you get 500V. If voltage drops to 450 volts, simply link in another freshly charged cell. This way the output voltage varies from 450 to 550 with no booster. With a little more planning, and lots of little cells, when you drop to 450 you might find a partially charged cell that brings you to 500. So with proper management, you might keep the voltage between 490 and 510. Just a thought.
Eikka
5 / 5 (1) Jan 27, 2011
If voltage drops to 450 volts, simply link in another freshly charged cell. This way the output voltage varies from 450 to 550 with no booster.


What you are describing there is essentially a charge pump, which is a type of voltage booster.

The loss in efficiency in a charge pump comes from the fact that you need switching elements to connect the cells, and PN junctions in semiconductor switches exhibit what is called a forward voltage drop.

That means that whenever current is flowing through the switch, energy is lost at a rate which is proportional to the voltage drop and the current (P=VI). Furthermore, the voltage drop is greater the more current you put through the switch. This presents a problem because the switches will heat up in operation, and you need so many of them that it becomes a cost issue anyways.

For moderate powers, like 100 Watts, your 100 Volt capacitor would lose 1-3% of its energy in the charge pump. For 20 kilowatts, it will be more like 10-30%.
Eikka
5 / 5 (1) Jan 27, 2011
Many low power applications do use voltage boosters with batteries, though, because the efficiency in low current applications can be high.

Your LED flashlight will typically use a "driver chip" which has an integrated charge pump and a current regulator to keep the light output steady with varying battery voltages.

Lithium cells have a hard lower limit of 2.5 volts that you should not break or the cell will be destroyed, but for alkaline batteries, you can use e.g. a "Joule thief" circuit that draws the battery completely dry to extend the operating time well beyond what you would normally get.

Google it up. It's rather useful.