Researchers at the University of Arizona—including Palusinski—have been developing a new capacitor technology designed to give hybrid cars the best of both worlds: the ability to rapidly convert mechanical energy into electricity and store that energy as well as to quickly charge and discharge energy to help cars accelerate or brake.

This new device is called a Digitized Energy Storage Device (DESD), which has a capacitance-to-volume ratio that is more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. The researchers make DESD capacitors by using porous membranes as template platforms. The membranes have a pore diameter ranging from 15 nanometers to one micron and a hole density of 10 million to 100 trillion pores per square centimeter (0.16 square inch). DESDs will be able to provide 130 joules per gram (0.35 ounce), Palusinski says, adding that a chemical battery provides about 100 joules per gram. Lithium ion batteries, however, can provide as much as 600 joules per gram.

Palusinski envisions a day when DESDs will replace batteries in automobiles, although currently no carmakers are working with the technology he and his colleagues developed. It's more likely that near- term deployments of DESD technology would be used to power something much less demanding, such as sensors used to detect motion for alarms and lighting systems.

Despite Toyota's success with ultracapacitors, most automakers in recent years have been focused on new developments in the lighter, more stable lithium ion batteries, says Brett Smith, an alternative-fuel analyst at the Center for Automotive Research in Ann Arbor, Mich.

"Moving from the type of lithium battery used in laptops to a nanophosphate lithium ion battery may have been the invention or the paradigm shift that the industry was requiring," Smith says. "Now the challenge is can they get the cost down." That's the pivotal question that is likely to ultimately determine the fate of ultracapacitors used in hybrid cars.