At Technology Review's EmTech conference last week, MIT professor Joel Schindall told the audience at a panel on energy storage why ultracapacitors may have a significant role to play in our transportation future. The good properties of these devices—fast charge/discharge cycles and an essentially unlimited number of cycles—make them a compelling choice for powering an electric vehicle. Schindall also explained why their downside, a far lower charge density than batteries, might not be as much of a problem as it might appear at first glance.

Schindall, who had spent some time away from academics, explained that during his first stint at MIT, a capacitor that could hold 350 Farads would have filled the whole stage. Before he returned, someone working on fuel cells had accidentally produced the first ultracapacitor. Now, with refinements, he was able to walk on stage with a 350 Farad ultracapacitor that was about the size of a D battery. The current generation of devices use activated carbon to hold charges, as its highly complex topology creates a lot of surface area across which charge differences can build up.

Although the improvements have been dramatic, Schindall said that ultracapacitors still badly lag batteries in terms of the storage density, holding only about five percent of the charge per volume of lithium batteris. Which is unfortunate, because they have some properties that would make them excellent for a variety of applications, including very rapid charging and the ability to withstand many more charge cycles than a battery. Schindall claimed they could be recharged indefinitely, since "greater than a million times, to me, is indefinite."

Schindall's research group has focused on replacing the disordered structure of activated carbon with a more ordered one that can increase the packing: carbon nanotubes. His research group has developed a vapor deposition process that can grow densely packed, vertically oriented clusters of carbon nanotubes on conducting surfaces. Current industrial processes for the production of carbon nanotubes tend to produce a variety of diameters and lengths, but Schindall told Ars that the process his group has developed keeps everything very regular—he was actually surprised by how even the lengths were.

Right now, he claims that the energy density of the nanotube-based devices are double that of activated carbon, and he expects that he'll be able to get that number up to five times that within a matter of months. At that point it's off to commercialization.

Even if he hits the five-fold improvement target, astute readers will note that that's still only one-quarter of the charge density of a battery. For the right use, however, Schindall said that's OK; he went on to suggest that electric vehicles might be the perfect use case.

Right now, cars are overprovisioned with storage capacity in order to keep the battery operating within a narrow capacity range, which improves its performance and extends its life. Even then, the long term viability is a problem given that many vehicles last more than a decade or see heavy daily use. Replacing it with something that's a bit larger but doesn't require replacement or any sort of careful charge management might, in the long run, make a lot more sense. Schindall also pointed out that the pattern of use from driving—rapid discharge for acceleration, quick bursts of charge from regenerative braking, and the need for things like lunch-hour recharging—all match an ultracapacitor's properties more than a battery's.

Schindall isn't the only one pitching ultracapacitors as a viable solution for vehicles; most notably, a company called EEStor claims to have made a similar charge density breakthrough using a ceramic material. We had the chance to ask Schindall about this. He said that, while the company's choice of materials should provide a viable route to charge storage, a number of people in the field are skeptical of their charge density claims. While he recognizes the need to keep information proprietary, he felt the company could reveal quite a bit more without risking its intellectual property. The fact that it hadn't, he said, made him a bit skeptical.

Since carbon nanotubes appear to be largely an evolution of existing technology, they may provide the boost that ultracapacitors need to take a bit of the market from batteries—providing that Schindall's research group can hit its goals.