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NC State researchers identify mechanism enabling PVDF-based capacitor’s high-speed energy storage

Researchers at North Carolina State University have identified the origin of the nonlinear dielectric response and high energy density of polyvinylidene-fluoride-based (PVDF) polymers enabling capacitors to store and release large amounts of energy quickly.

NC State physicist Dr. Vivek Ranjan had previously found that capacitors which contained PVDF in combination with the polymer chlorotrifluoroethylene (CTFE) were able to store up to seven times more energy than those currently in use.

In research published in the journal Physical Review Letters, Ranjan and colleagues used first-principles simulations to see how the atomic structure within the polymer changed when an electric field was applied. Applying an electric field to the polymer causes atoms within it to polarize, which enables the capacitor to store and release energy quickly.

They found that when an electrical field was applied to the PVDF mixture, the atoms flipped from a non-polar to a polar state simultaneously, requiring a very small electrical charge to do so.

Usually when materials change from a polar to non-polar state it’s a chain reaction—starting in one place and then moving outward. In terms of creating an efficient capacitor, this type of movement doesn’t work well—it requires a large amount of energy to get the atoms to switch phases, and you don’t get out much more energy than you put into the system.

In the case of the PVDF mixture, the atoms change their state all at once, which means that you get a large amount of energy out of the system at very little cost in terms of what you need to put into it. Hopefully these findings will bring us even closer to developing capacitors that will give electric vehicles the same acceleration capabilities as gasoline engines.

—Vivek Ranjan

Resources

  • V. Ranjan, M. Buongiorno Nardelli and J. Bernholc (2012) Electric Field Induced Phase Transitions in Polymers: a Novel Mechanism for High Speed Energy Storage. Physical Review Letters doi: 10.1103/PhysRevLett.108.087802

Comments

Davemart

Electric vehicles already have the same acceleration as gasoline ones, so I don't really know what they are talking about, although of course I welcome any way in which capacitors improve.

kelly

"..capacitors which contained PVDF in combination with the polymer chlorotrifluoroethylene (CTFE) were able to store up to seven times more energy than those currently in use"

If true, 7X energy density capacitors could start replacing expensive Li-ion batteries - needing only seconds to recharge.

Nick Lyons

Hopefully these findings will bring us even closer to developing capacitors that will give electric vehicles the same acceleration capabilities as gasoline engines.

This statement is a puzzler. The implication is that capacitors will be able to deliver a lot of power at once. I thought that's what capacitors did already, and that energy density is the issue.

Anton Krivosheyev

Interesting, but I'd rather see the actual numbers.

Ultracapacitors have energy density of 5.52Wh/kg, currently cost ~$16/Wh, >1,000,000 charge cycles, and max continuous discharge current of 150A (2.7V 3000F 3Wh Maxwell Ultracapacitor)

In contrast, a LiFeYPO4 cell energy density is 91Wh/kg, current cost of $0.4/Wh, 2000 to 3000 charge cycles, and max continuous discharge current of 300A (Thunder sky LiFeYPO4 3.2V 100Ah Cell).

So the real question is, ho does this new technology compare to these two examples?

Engineer-Poet

Nick is correct; this article is short on details which would spell out the implications.

There are already ultracapacitors which would be competitive with batteries in applications like GM's BAS II.  What's needed isn't more energy-storage capability, but greater power-handling capacity (esp. to absorb energy during braking).  For instance, the total kinetic energy of a 4000 lb vehicle moving at 60 MPH is only 653 kJ, or 181 Wh. The problem is that you can't charge a 300 Wh battery fast enough to absorb the power of braking from speed even relatively gradually (10 seconds, roughly 200 kW peak); you need a very large battery, perhaps 20 kWh taking a surge charge rate of 10C. But a capacitor of much smaller energy capacity can take the same power, and return it just as fast.

kelly

Using AK's above 'Ultracapacitors have energy density of 5.52Wh/kg' and the article "were able to store up to seven times more energy than those currently in use." implies ~38Wh/kg.

This would surpass Lead-Acid battery energy density, but there is not enough information.

kelly

http://www.sciencedaily.com/releases/2012/02/120223182646.htm - perhaps an 'acceleration' context.

HarveyD

eventually a cross between lithium an super caps may potential?

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