Exploring supercapacitors to improve their structure

Feb 19, 2013
Exploring supercapacitors to improve their structure
Simplified diagram of a supercapacitor and how it works from the macroscopic scale to the molecular level. Credit: Cehmti-Michael Deschamps

No matter how intimidating their name, supercapacitors are part of our daily lives. Take buses for example: supercapacitors are charged during braking, and supply electricity to open the doors when the vehicle stops. Yet the molecular organization and functioning of these electricity storage devices had not previously been observed. For the first time, researchers from CNRS and the Université d'Orléans have explored the molecular rearrangements at play in commercially available supercapacitors while in operation. The technique devised by the scientists provides a new tool for optimizing and improving tomorrow's supercapacitors. The results are published on-line on Nature Materials' website on 17 February 2013.

are electricity that are quite different from batteries. Unlike batteries, supercapacitors are charged much faster (usually in seconds), and do not suffer rapid wear due to charging/discharging. On the other hand, at equivalent size and although they offer greater power, they cannot store as much electrical energy as batteries (carbon-based supercapacitors supply an energy density of around 5 Wh/kg compared to around 100 Wh/kg for lithium-ion batteries). Supercapacitors are used in the recovery of braking energy in numerous vehicles (cars, buses, trains, etc.) and to open the emergency exits of the Airbus A380.

A supercapacitor stores electricity through the interaction between nanoporous carbon electrodes and ions, which carry positive and negative charges, and move about in a liquid known as an electrolyte. When charging, the (negatively charged ions) are replaced by cations (positively charged ions) in the negative electrode and vice versa. The greater this exchange and the higher the available carbon surface area, the greater the capacity of the supercapacitor.

Using (NMR) spectroscopy, researchers delved deeper into this phenomenon and were able, for the first time, to quantify the proportion in which charge exchanges take place in two supercapacitors using commercially available carbons. By comparing two nanoporous carbon materials, the researchers were able to show that the supercapacitor containing the carbon with the most disordered structure had greater capacitance and improved high-voltage tolerance. This could be due to better electronic charge distribution upon contact with the electrolyte molecules.

Explore further: Graphene sensor tracks down cancer biomarkers

More information: Deschamps, M. et al. Exploring electrolyte organization in supercapacitor electrodes with solid-state NMR, Nature Materials. Published on-line on the 17 February DOI: 10.1038/NMAT3567.

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Eikka
5 / 5 (1) Feb 19, 2013
The reason why supercapacitors aren't suitable for large scale energy storage is that they're fundamentally storing a static charge, and storing too much static charge (energy) in too small a volume makes a highly unstable explosive device. All you have to do is break the insulator and all the charge in the cell is released internally within microseconds. A capacitor like that would be fundamentally akin to chemical explosives, because you could detonate it with a sufficiently strong starting blast that crushes the cells and releases their energy.

You can slow down the release of energy by increasing the electrical resistance of the electrode/electrolyte material, but that would make the capacitor highly inefficient and slow to charge/discharge.

Or you can make individual cells small and isolate them from one another to prevent failure from spreading, but then your capacitor bank will be big and cumbersome due to the filler material.
Eikka
5 / 5 (1) Feb 19, 2013
The point is that while you can improve supercapacitors, there becomes a point when they will become too dangerous to use.

baudrunner
3 / 5 (2) Feb 19, 2013
..the researchers were able to show that the supercapacitor containing the carbon with the most disordered structure had greater capacitance and improved high-voltage tolerance.
That is analogous to making iron into strong steel by adding impurities. The reason the disordered structure appears to give greater capacitance is probably related to the increased opportunity for em induction due to the random occurrence of transverse em fields. This results in increased energy output when the energy is used and may lead to misinterpretation of the effect. I can't make a whole lot of sense out of the article's explanation.
PhysGeek
5 / 5 (3) Feb 19, 2013
@Eikka
Any energy storage technology is dangerous for exactly the reasons you specify. Super capacitors are no different in any respect from this.

We have a need to store more energy in a smaller area. This leads to a higher energy density. ANY storage technology with similar energy densities is basically a bomb.

Personally I prefer the idea of an enormous electrical discharge inefficiently dissipating this energy in an emergency over a high energy density flywheel coming apart. Yes, they are both bombs, but the capacitor has to convert its electrostatic energy into kinetic or heat energy for it to do much damage to people outside it immediate vicinity. On the other hand the flywheel is already kinetic energy. It would convert to shrapnel quite easily.
antialias_physorg
3 / 5 (2) Feb 20, 2013
The point is that while you can improve supercapacitors, there becomes a point when they will become too dangerous to use.

The point is then to find a safety mechanism that will either encase the supercapacitor tothe point where catastrophic damage is very low (e.g. like with hydrogen tanks) - so that the residual danger level/likelyhood of failure is acceptable.

OR

Channel the discharge into a medium with a failsafe device in case of an accident (e.g. by hooking it up to the airbag sensors and sending the discharge into a small block of copper or sand and melt that)

Otherwise I agree with PhysicsGuy: The fuel isn't important. The energy denity in it is the crucial factor. And whrever you have high energy densities you have a potentially dangerous situation.
Eikka
5 / 5 (1) Feb 20, 2013
Any energy storage technology is dangerous for exactly the reasons you specify. Super capacitors are no different in any respect from this.


Not really.

Energy density is secondary. TNT has lower energy density from gasoline, but TNT is explosive and gasoline is not, because the way in which they release energy is different. TNT has an unstable molecule that can release its energy given a sufficient external shock, while gasoline alone won't do anything.

Same thing with capacitors. It's an internally unstable configuration.

To contain the sudden explosive release of energy on the scale of kilowatt-hours is like trying to contain a bucket of dynamite as it goes off.

Channel the discharge into a medium with a failsafe device


If the capacitor's insulator breaks, very little current can flow into an external circuit because it's in parallel with the internal discharge path that has much lower impendance for current to flow.
antialias_physorg
3 / 5 (2) Feb 20, 2013
If the capacitor's insulator breaks, very little current can flow into an external circuit

That's why you'd discharge it before the impact can damage the insulator (i.e. when the sensor that deploys the airbags notices that you have crashed - which is way before any foreign object can penetrate that deep into your car)
Eikka
5 / 5 (1) Feb 20, 2013
To make the point clearer, dynamite has an energy density of 5 MJ/kg whereas gasoline has 46 MJ/kg

This is what happens to capacitors when the internal insulator breaks: http://www.youtub...ge#t=48s
Eikka
5 / 5 (1) Feb 20, 2013
That's why you'd discharge it before the impact can damage the insulator


So you would trust the system to an active failsafe to mitigate the lack of passive safety?

Kinda like our current nuclear reactors, right?

Although I still doubt whether you can safely discharge kilowatt-hours of energy into anything, in millisecond timescales, without causing an explosion. Dumping the contents of a 20 kWh electric car battery into a tub of salt is still equivalent to dropping the equivalent of 15 kg of dynamite on it, and all the intermediate cabling has to withstand that power without instantly vaporizing.
Eikka
5 / 5 (1) Feb 20, 2013
ANY storage technology with similar energy densities is basically a bomb.


A lithium-air battery would not be, because it lacks the necessary components to release any energy on its own. Oxygen needs to be introduced to release energy, which necessarily limits the rate of release to below where any explosion could happen.

You could generalize that any one-component storage system becomes a bomb with high energy density. Whether it's a spinning flywheel or a supercapacitor, or a spring in a box.

And that's something you have to think about when designing things around extremely powerful supercapacitors.
Tausch
1 / 5 (1) Feb 24, 2013
[...that's something you have to think about when designing...-E

...for the demise [of all things] to cover all imagined scenarios.

Fives to all.