Liberating devices from their power cords: New structural 'supercaps' take a lickin', keep on workin'

May 19, 2014 by David Salisbury
Close-up of structural supercapacitor. Credit: Joe Howell / Vanderbilt

( —Imagine a future in which our electrical gadgets are no longer limited by plugs and external power sources. This intriguing prospect is one of the reasons for the current interest in building the capacity to store electrical energy directly into a wide range of products, such as a laptop whose casing serves as its battery, or an electric car powered by energy stored in its chassis, or a home where the dry wall and siding store the electricity that runs the lights and appliances.

It also makes the small, dull grey wafers that graduate student Andrew Westover and Assistant Professor of Mechanical Engineering Cary Pint have made in Vanderbilt's Nanomaterials and Energy Devices Laboratory far more important than their nondescript appearance suggests.

"These devices demonstrate – for the first time as far as we can tell – that it is possible to create materials that can store and discharge significant amounts of electricity while they are subject to realistic static loads and dynamic forces, such as vibrations or impacts," said Pint. "Andrew has managed to make our dream of structural energy storage materials into a reality."

That is important because structural energy storage will change the way in which a wide variety of technologies are developed in the future.

"When you can integrate energy into the components used to build systems, it opens the door to a whole new world of technological possibilities. All of a sudden, the ability to design technologies at the basis of health, entertainment, travel and social communication will not be limited by plugs and external power sources," Pint said.

Liberating devices from their power cords: New structural 'supercaps' take a lickin', keep on workin'
Side view of a structural supercapacitor shows the blue polymer electrolyte that glues the silicon electrodes together. Credit: Joe Howell / Vanderbilt

The new device that Pint and Westover has developed is a supercapacitor that stores electricity by assembling electrically charged ions on the surface of a porous material, instead of storing it in chemical reactions the way batteries do. As a result, supercaps can charge and discharge in minutes, instead of hours, and operate for millions of cycles, instead of thousands of cycles like batteries.

In a paper appearing online May 19 in the journal Nano Letters, Pint and Westover report that their new structural supercapacitor operates flawlessly in storing and releasing electrical charge while subject to stresses or pressures up to 44 psi and vibrational accelerations over 80 g (significantly greater than those acting on turbine blades in a jet engine).

Furthermore, the mechanical robustness of the device doesn't compromise its energy storage capability. "In an unpackaged, structurally integrated state our supercapacitor can store more energy and operate at higher voltages than a packaged, off-the-shelf commercial supercapacitor, even under intense dynamic and static forces," Pint said.

One area where supercapacitors lag behind batteries is in capability: Supercaps must be larger and heavier to store the same amount of energy as lithium-ion batteries. However, the difference is not as important when considering multifunctional energy storage systems.

"Battery performance metrics change when you're putting energy storage into heavy materials that are already needed for structural integrity," said Pint. "Supercapacitors store ten times less energy than current , but they can last a thousand times longer. That means they are better suited for structural applications. It doesn't make sense to develop materials to build a home, car chassis, or aerospace vehicle if you have to replace them every few years because they go dead."

Liberating devices from their power cords: New structural 'supercaps' take a lickin', keep on workin'
The engineers suspended a heavy laptop from the supercapacitor to demonstrate its strength. Credit: Vanderbilt Nanomaterials and Energy Devices Laboratory

Westover's wafers consist of electrodes made from silicon that have been chemically treated so they have nanoscale pores on their inner surfaces and then coated with a protective ultrathin graphene-like layer of carbon. Sandwiched between the two electrodes is a polymer film that acts as a reservoir of charged ions, similar to the role of electrolyte paste in a battery. When the electrodes are pressed together, the polymer oozes into the tiny pores in much the same way that melted cheese soaks into the nooks and crannies of artisan bread in a Panini. When the polymer cools and solidifies, it forms an extremely strong mechanical bond.

"The biggest problem with designing load-bearing supercaps is preventing them from delaminating," said Westover. "Combining nanoporous material with the polymer electrolyte bonds the layers together tighter than superglue."

The use of silicon in structural supercapacitors is best suited for consumer electronics and solar cells, but Pint and Westover are confident that the rules that govern the load-bearing character of their design will carry over to other materials, such as carbon nanotubes and lightweight porous metals like aluminum.

The intensity of interest in "multifunctional" devices of this sort is reflected by the fact that the U.S. Department of Energy's Advanced Research Project Agency for Energy is investing $8.7 million in research projects that focus specifically on incorporating energy storage into structural materials. There have also been recent press reports of several major efforts to develop multifunctional materials or structural batteries for use in electric vehicles and for military applications. However, Pint pointed out that there have not been any reports in the technical literature of tests performed on structural materials that show how they function under realistic mechanical loads.

Explore further: New device stores electricity on silicon chips

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5 / 5 (1) May 20, 2014
Interesting research, but for a large energy capacity, you need to store opposite charges in very thin capacitors and theses opposite charges at very short distance recombine quite fast through the very thin dielectric, so that theses capacitors are for short time storage.
If you want stable storage like with separate chemical fuels, you lose all the simple reversibility of charge storage in capacitors.
5 / 5 (1) May 20, 2014
The million dollar question is, what happens when the capacitor does get damaged?

If you have a wall sized capacitor and someone throws a knife in it, will the whole thing discharge at once and go up in flames? Or worse yet, explode?

There's plenty of videos online for people overcharging or reversing the polarity of a supercapacitor, and they do go up spectacularily and very violently, but nobody seems to have tried what happens when you charge one up and smash it with a big hammer.
not rated yet May 20, 2014

but nobody seems to have tried what happens when you charge one up and smash it with a big hammer.

Or when one is tossed into a fire or otherwise catches fire.

Besides that, it seems apparent to me that the proportion of added material weight in any given device becomes an issue pretty early on, as so much additional capacitor-y would be required to attain equivalence with battery power density.

It would probably work ok for a car or train, but a cellphone or laptop? I don't see how they could possibly be thinking in terms of "saying goodbye to power cords", since charging frequency would have to be increased accordingly.

Seems they've got their thinking turned every-which-way, and I would suspect that this is primarily being developed for military applications, as it tis difficult to see where it would fit in in terms of consumer products which would likely end up being more trouble to use than they were worth.
1 / 5 (1) May 20, 2014
If you have a wall sized capacitor and someone throws a knife in it, will the whole thing discharge at once and go up in flames? Or worse yet, explode?
It would be something like this I think

Seriously it couldnt be any woorse than this
not rated yet May 21, 2014
Seriously it couldnt be any woorse than this

Well, it can.

Gas explosions have a limited rate of reaction because they need oxygen. A rapid capacitor discharge to a gas explosion is like C4 is to wet gunpowder.
1 / 5 (1) May 21, 2014
Seriously it couldnt be any woorse than this

Well, it can.

Gas explosions have a limited rate of reaction because they need oxygen. A rapid capacitor discharge to a gas explosion is like C4 is to wet gunpowder.
Well of course it's more complex than this? Speed of propagation is only part of the equation. Quantity is a determining factor in the extent of damage as well. And by the time a gas explosion occurs, a house can be completely full with a thoroughly-mixed gas/air mixture.

Macht's nichts. What we have now is every bit as dangerous as what we might have in the future. And engineers can design to limit both the quantity of structural material which could detonate at any one time as well as the extent of damage it may do.

This should be easier to do than with gas.
not rated yet May 26, 2014
Well of course it's more complex than this?

No doubt, but think about it this way: a house full of gas mixed in the explosive concentration range has a rather limited energy density. You only have a few grams of gaseous fuel per cubic meter of air before it goes too rich to explode. It'll throw your walls out and collapse your roof - sure.

But compare and contrast to a wall sized supercapacitor that contains a thousandfold more energy , and will release said energy a thousand times faster than the exploding gas inside a building. That'll throw your walls and your roof out, and level half the street.

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