Ultracapacitors are also known as supercapacitors and both terms are used interchangeably.
Normal capacitors work by physically separating electrical charges across a barrier that allows electrical potential to build without actually completing a circuit. Chemical batteries do this chemically drawing negative ions to one node and positive ions to the other, but a capacitor has an actual physical barrier between its regions of positive and negative charge. The electrons want to jump the barrier to the area of positive attraction, but can't, so they build up on the negative charge surface. A good way to think of this is like water building up behind a dam. The water wants to rush into the open space beyond the dam, but is held back by the barrier.
And like the water behind a dam, once released the electrons flow in a raging torrent, so to speak, dumping all their built-up electrical potential all at once.
The trick to increasing a capacitor's stored up electrical energy is by increasing the surface area that separates the areas of negative and positive charge. To extend the analogy a bit, the longer and higher the dam, the more water it can hold behind it, and the more energy it can release once taken down. Advanced capacitors use a number of different efficient geometries to maximize their stored potential within the space they work with, allowing greater energy to build up in them for various applications.
Ultracapacitors does this method one better, by creating the barriers between the areas of charge on a molecular and atomic scale. In other words, the capacitor geometry is nanoengineered on the scale of billionths of a meter in order to ultimately maximize the space within for energy storage.
Ultracapacitors uses molecule-thin layers of polarized electrolytic solution separated by a dielectric barrier to store the charged regions electrostatically. Even though it uses a chemical solution, no actual chemical reactions are involved, allowing the solution to be charged and discharged hundreds of thousands of times without significant wear. In the most advanced form of ultracapacitor yet engineered, vertically-aligned carbon nanotubes hold individual atoms of the electrolytic solution, allowing for an even greater surface area to be achieved between the individual particles themselves. Many engineers believed that nanotube-enhanced ultracapacitors, once fully perfected, will be able to compete head-to-head with chemical batteries in most applications.
Current ultracapacitors can store much more current than other types of capacitors, but are not quite yet up to the same level of storage a chemical battery enjoys. As new techniques and geometries are employed in their manufacture, this may change, but ultracapacitors now and in the near future will remain merely as a supplementary technology to other energy storage and generation devices. They are also only able to store their charge for a few hours at a time, though this has been slowly but steadily improving.
The big advantage of any capacitor over a battery, however, is that they can release some or all of their stored up energy all at once, where as a battery can only offer a comparatively small but steady stream. And as an ultracapacitor can store much more power than its technological predecessors, it can be used in a much wider range of applications than normal capacitors.
Ultracapacitors can also be charged much more quickly than chemical batteries. A typical ultracapacitor can take ten seconds or less to fully charge, compared to several hours for the best commercial rechargeable battery.
Because they are much more energy-dense, compact ultracapacitors can be used alongside side advanced batteries in electronic devices without adding significant weight or bulk. The presence of a fully-charged ultracapacitor will not only allow the device to extend its potential use by several hours, but allow it to perform more high-end functions longer and more efficiently. For example, laptop computers could used an ultracapacitor to run DVDs or high-end computers for several hours without significant battery drain. Cellphones could temporarily boost their signal strength for better broadcast power in remote areas, speakers can use them to produce better quality and louder sounds without distortion, and so on.
One of the major areas ultracapacitors are expected to make a significant impact is in motor vehicles. A car will still use an alternate source of energy as its main means of propulsion, whether that be from a gasoline engine, a fuel cell array, or a battery. Ultracapacitors will be present in the vehicle to improve its efficiency and motor power. Breaking a vehicle can create a lot of electromechanical energy, which can be fed into the onboard ultracapacitors. When the vehicle is ready to go again, it draws upon this stored energy to give that extra oomph to get up to speed again with minimal overall energy loss, thus greatly increasing the car's potential fuel and battery efficiency.
And because ultracapacitors can charge quickly and hold that charge for a few hours, busses in China have been experimenting in using them as their main motive power. The bus pulls into a stop, hooks up to a power outlet for thirty seconds while it takes on and unloads passengers, then disengages, fully charged until the next stop. Thus, the busses never need refueling, and the ultracapacitors can be recharged by some estimates over a million times, making them ideal for this kind of system.
Ultracapacitors may also see a lot of military applications, especially with electrically-powered weapons such as laser and electrothermal chemical artillery and firearms. The ultracapacitors can provide the large amounts of current fast for these power-hungry but effective weapon systems.