Conventional ultracapacitors, which have the advantage of delivering fast bursts of power and can be recharged hundreds of thousands of times without losing much capacity, are too expensive and store too little energy to replace batteries.
Nanotune, however, which has raised $3 million from the venture capital firm Draper Fisher Jurvetson, says its ultracapacitors are close to competing with batteries in terms of energy storage, and could soon surpass them. Using a conventional electrolyte, the company has demonstrated energy storage of 20 watt-hours per kilogram, as opposed to roughly five watt-hours for a conventional ultracapacitor. Using a more expensive ionic-liquid electrolyte, it has made ultracapacitors that store 35 watt-hours per kilogram. By the end of the year, the company hopes to approximately double this storage capacity, says Nanotune CEO Kuan-Tsae Huang. At 40 watt-hours per kilogram, the ultracapacitors would be an improvement over the batteries used in some hybrid vehicles.
In recent months, several startups have announced that they’re using nanotechnology to make better ultracapacitors. Each hopes to help solve one of the biggest problems with electric cars today: their batteries’ high cost and limited storage capacity. Nissan, for example, to make its electric Leaf affordable, had to limit the size of the battery pack, resulting in a range of just 73 miles.
Part of the reason battery systems are so expensive and bulky is that the batteries degrade as they’re used, especially when exposed to extreme temperatures—so automakers often augment them with cooling and heating systems, and add extra battery cells to offset losses in performance over time. Ultracapacitors could sidestep this problem, because they can be recharged without degrading and can work well in a wide range of temperatures.
Eventually, Huang says, it may be possible to make ultracapacitors that store 500 kilowatt-hours per kilogram—about three to four times more than the lithium-ion batteries used in cars today. The practical benefit could be even greater. Cars are often engineered to use only half the storage capacity of their batteries, to keep them from degrading. But almost all of an ultracapacitor’s storage capacity can be used.
Nanotune’s technology is very expensive now—between $2,400 and $6,000 per kilowatt-hour. (The Department of Energy has proposed a goal of $250 per kilowatt-hour to make electric vehicles competitive with conventional ones.) Nanotune says, however, that its costs could come down to less than $150 per kilowatt-hour if the prices of some key materials, such as electrolytes, continue to fall, and as manufacturing is scaled up.
The company’s energy-storage projections are based on several advances it is working on. Nanotune is currently making electrodes with pores that are about 4 to 5 nanometers across, but it says it can make them smaller (high porosity leads to high surface area, which makes it possible to store a large amount of charge) and tune them to match the needs of different electrolytes—the ion-conducting materials the electrodes are immersed in.
The company is also looking into using ionic liquids rather than conventional organic electrolytes. These increase the voltage of the system, greatly increasing energy storage, but typically they aren’t compatible with conventional ultracapacitor electrodes. Finally, the company hopes to make use of recent academic findings that suggests that adding small amounts of ruthenium to the ultracapacitors can increase energy storage.
Nanotune isn’t the first company to claim it can make ultracapacitors with very high energy storage. Others have found this promise hard to deliver. Increasing surface area can improve storage capacity only so much, since at some point the storage is limited by the ions in the electrolyte. Ionic liquids help with this, but they have significant shortcomings, says Joel Schindall, a professor of electrical engineering and computer science at MIT. (A company called FastCap Systems, which is developing ultracapacitors using carbon nanotubes, was spun out of his lab.) They’re very expensive, for one thing, and some operate well only in a limited temperature range, making them impractical for cars.
Schindall says, however, that Nanotune can fall short of its very high energy goals and still improve the competitiveness of electric vehicles and hybrids. Given the durability of ultracapacitors, even achieving energy storage of 100 watt-hours per kilogram—close to that of lithium-ion batteries—"would be fantastic."
By Kevin Bullis, www.technologyreview.com/