It's almost engineering heresy to suggest that a capacitor could power a car. Indeed, the common capacitor stores a puny amount of energy. At equivalent voltage, a chemical battery can store at least a million times as much energy as a conventional capacitor of the same size. Put two ordinary capacitors the size of a D-cell battery in your flashlight, each charged to 1.5 volts, and the bulb will go out in less than a second, if it lights at all. An ultracapacitor of the same size, however, has a capacitance of about 350 farads and could light the bulb for about 2 minutes.

Before delving into our methods, I should explain the basics of capacitors and ultracapacitors. Capacitors have been around since 1745, beating batteries to the scene by half a century. Ultracapacitors are much more recent, but they're not exactly new, either. Engineers at Standard Oil patented ultracapacitor technology in 1966, an unanticipated product of their fuel-cell research. Standard Oil licensed the technology to NEC Corp., of Tokyo, which commercialized the results as “supercapacitors” in 1978, to provide backup power for maintaining computer memory.

A capacitor consists of two electrodes, or plates, separated by a thin insulator. When a voltage is applied to the electrodes, an electric field builds up between the plates. A capacitor's energy is stored in such an electric field, without requiring any sort of chemical reaction. Thus a capacitor has an almost unlimited lifetime. It's also fast. Depending on its physical structure, typical charge and discharge times are on the order of a microsecond; sometimes they are as quick as a picosecond.

Three main factors determine how much electrical energy a capacitor can store: the surface area of the electrodes, their distance from each other, and the dielectric constant of the material separating them. However, you can push conventional capacitor designs only so far. What the Standard Oil engineers did was to develop a capacitor that functions differently. They coated two aluminum electrodes with a 100-micrometer-thick layer of carbon. The carbon was first chemically etched to produce many holes that extended through the material, as in a sponge, so that the interior surface area was about 100 000 times as large as the outside. (This process is said to “activate” the carbon.)

They filled the interior with an electrolyte and used a porous insulator, one similar to paper, to keep the electrodes from shorting out. When a voltage is applied, the ions are attracted to the electrode with the opposite charge, where they cling electrostatically to the pores in the carbon. At the low voltages used in ultracapacitors, carbon is inert and does not react chemically with the ions attached to it. Nor do the ions become oxidized or reduced, as they do at the higher voltages used in an electrolytic cell.

This approach allowed the engineers at Standard Oil to build a multifarad device. At the time, even large capacitors had nowhere near a farad of capacitance. Today, ultracapacitors can store 5 percent as much energy as a modern lithium-ion battery. Ultracapacitors with a capacitance of up to 5000 farads measure about 5 centimeters by 5 cm by 15 cm, which is an amazingly high capacitance relative to its volume. The D-cell battery is also significantly heavier than the equivalently sized capacitor, which weighs about 60 grams.

Hundreds of thousands of ultracapacitors are manufactured each year, for applications that require rapid recharging, high power output, and repetitive cycling. In 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source, and it's growing, especially in the automotive sector. Though ultracapacitors have generally remained a niche player, the situation may soon change.

My laboratory at MIT—the Laboratory for Electromagnetic and Electronic Systems—works with several automobile manufacturers to investigate ways to improve vehicle performance. About four years ago, I assisted on a project to evaluate commercial ultracapacitors for use in cars. While on a flight from Boston to Detroit, I read an article describing a way to grow vertically aligned carbon nanotubes on a flat surface. This is a truly amazing process. A sheet of silica is covered with a nanometer-thick layer of an iron catalyst. The sheet is placed in a vacuum, heated to 650 ºC, and exposed to a thin hydrocarbon gas, perhaps ethanol or acetylene. The heat causes the iron to form tiny droplets, which steal carbon molecules from the gas. The carbon molecules then begin to self-assemble into tubes, which grow upward from each of the droplets.