Photo: Riccardo Signorelli/MIT

ELECTRIC SHAG: A cross section of an electrode made with carbon nanotubes.

Small-cell ultracapacitors can be used in cars for purposes other than in the drivetrain. They can be integrated into air-conditioning, electric power steering, power locks, and window systems—components that demand high peak currents, which typically require large-diameter wiring. The need is intermittent, and the average power is low, so having ultracapacitors provide the high current at strategic points would permit thinner wiring to be installed. With the high price of copper these days, such changes can shave an appreciable amount from the cost of a vehicle.

Safety is another motivation. Suppose a car has electrically actuated brakes or door locks and the wiring harness fails because of a defect or an accident. A local ultracapacitor can still provide power for a few precious seconds or minutes.

Such devices are by no means limited to vehicles. Society is in the midst of an energy crisis, and many sources of green energy would benefit from regenerative energy storage. Electric power grids could be 10 percent more efficient if there could be simple, inexpensive ways to store energy locally at the point of use. And if renewable energy is ever to displace fossil fuels, engineers will need to devise better ways to store wind power when the wind is not blowing and solar power when the sun is not shining.

My colleagues and I are not the only ones researching ultracapacitor technology, of course. All the existing ultracapacitor manufacturers—including Maxwell Technologies, NessCap, Panasonic, Nippon Chemi-Con, and Power Systems Co.—are working on improved activated carbons or devices where one electrode functions as a battery and the other as an ultracapacitor. The Japanese government has provided $25 million for nanotube research, money that has supported a promising joint effort between Nippon Chemi‑Con and AIST National Lab to explore nanotube-based techniques. Investigators at Rensselaer Polytechnic Institute, in Troy, N.Y., recently announced, in the Proceedings of the National Academy of Sciences of the United States of America, an exciting combined battery-nanotube ultracapacitor fabric to store electrical energy.

And nanotube forests are not the only way to provide increased porosity. Power Systems, in Japan, for example, has been getting good results with a type of graphene structure that it calls a “nanogate.”

There's a slightly different approach to modified capacitors that has been generating a lot of buzz lately, developed by a start-up called EEStor, in Cedar Park, Texas. EEStor has focused on improving the dielectric, rather than the capacitor's plates. Its design uses barium titanate, which has a high dielectric constant. High-dielectric-constant substances allow for high-value capacitors that are still small in size. The downside is that such materials generally are unable to withstand electrostatic fields of the same intensity as low-dielectric-constant substances such as air. EEStor claims that the capacitors can operate at extremely high voltages, on the order of several thousand volts, leading to very high storage capacities. One concern is that high voltages can cause a dielectric to break down irreversibly in the presence of even slight imperfections in the material. Only time will tell how its design fares.

Improving substantially on the means to store electrical energy would be a welcome development, and high-density capacitive storage is one promising avenue of research. Although batteries and capacitors are old inventions, our particular technique could not have been pursued until recently. Just as semiconductor designers have created smaller and smaller transistors, so have engineers in other areas learned to manipulate objects with ever-more-minuscule dimensions. The ability to sculpt materials at the atomic level is new and evolving. Engineers can use these new techniques to achieve novel properties and, in the case of my lab's research, to move toward a nanoengineered carbon that might usher in the next generation of energy storage.