By virtue of their dimensions, it struck me that those nanotubes held the promise of even higher porosity than the activated carbon used in commercial ultracapacitors. Together the nanotubes have an enormous surface area, and their dimensions are more uniform than those of the activated-carbon pores, making them more like a paintbrush than a sponge.

There are two major limitations to the conductivity of activated carbon—the high porosity means there isn't much carbon material to carry current, and the material must be “glued” to the aluminum current collector using a binder, which exhibits a somewhat high resistance. If my colleagues and I replaced the activated carbon with billions of nanotubes, we predicted we could make an ultracapacitor that could store at least 25 percent—and perhaps as much as 50 percent—of the energy in a chemical battery of equivalent weight. (To get that much improvement, we'd have to make a number of other changes, as well, such as increasing the number of ions in the electrolyte to reflect that new-found storage space.)

Another advantage of nanotubes over activated carbon is that their structure makes them less chemically reactive, so they can operate at a higher voltage. And certain types of nanotubes, depending on their geometry, can be excellent conductors—which means they can supply more power than ultracapacitors outfitted with activated carbon.["see illustration, "Piling on The Farads"]

Even better, this nanotube-enhanced ultracapacitor would retain all the advantages ordinary ultracapacitors have over batteries: they would deliver energy in quick bursts, they would perform well in cold weather, and they would have much longer life spans. If this ultracapacitor could be developed, it would be revolutionary.

It was clear from the outset that a lot of know-how would be needed to make an ultracapacitor according to our design—knowledge of chemical-vapor deposition, electron microscopy, material science, quantum chemistry. And it's a challenge to get people with all those skills together. One of the strengths of a research university is its incredible diversity of expertise and equipment, plus there's the willingness of faculty to collaborate. Nobody in my lab had experience fabricating carbon nanotubes, but much of the early research in that area at MIT was done in the building next door, at a laboratory under the direction of Mildred Dresselhaus. Using those facilities and aided by Dresselhaus and her lab colleagues, we succeeded in synthesizing a nanotube forest on a small piece of silica in only a few months.

Nanotubes can vary in size, and the ones we're growing are about 5 nm across, or about 1/10 000th the diameter of a human hair. Each tube is about 100 µm long, and they can be spaced as little as 5 nm apart.[see image, "Electric Shag"]

But the sliver of silica was only the start. Silica is an insulator, and we needed a conducting material. After more than a year of false starts, we finally designed and built a custom reactor for chemical-vapor deposition and have used it to grow nanotubes on a conducting substrate. We are now packaging this collection of nanotubes in a prototype ultracapacitor.

We believe that within a few months we'll be able to demonstrate results that outperform today's designs by a wide margin. There will still be a big challenge ahead of us at that point: to see whether our devices can be manufactured at prices that make them attractive for mainstream applications. We are optimistic, though, because chemical-vapor deposition is already used on a huge scale in semiconductor manufacturing, and the raw materials that we need are cheap.

It's not a straight path from high-density ultracapacitors to practical electric cars, but what my colleagues and I have done may constitute one big step along a tortuous route to making such vehicles more convenient and attractive to consumers. Even if it takes many years before ultracapacitors on their own can power either full battery-electric or hybrid cars, we're already at the point where such devices could easily assist lithium-ion batteries.[see illustration, "How to Ultracap A Car"] When the car's electric motor needs high current for a short time, the ultracapacitor supplies it. After the demand eases, the ultracapacitor recharges from the battery. When the motor, working now as a generator, delivers high current for a brief interval—which is typically what happens with regenerative braking—the same thing happens in reverse. A computer would monitor voltages, the state of charge, load, and demand, and then adjust the current flow accordingly using some additional dc-dc power electronics. The added weight and expense involved might not matter if it improves vehicle performance and makes the battery last longer.