Readers who are familiar with hybrid and electric vehicle drivetrains will already know what a capacitor is, but here is a brief explanation. A battery is a chemical means of storing electricity, while a capacitor is a mechanical means of storing electricity. Both can be recharged, and both can be discharged. One reason to choose a capacitor over a battery is that the mechanical capacitor can generally charge and discharge its electrical energy much more quickly than the chemical battery can. Alternatively, one reason to choose a chemical battery is that they generally store a longer lasting charge of electricity at lower voltages than the mechanical capacitor can. A quick high charge of current (capacitor), or a slower, longer lasting charge of current (battery). Hope that makes sense.
Carmakers have already realized the benefits of a hybrid battery\capacitor energy storage system. Aftermarket and automotive supplier companies have been working on capacitor technology as well; EEStor springs right to mind as one to watch. Researchers at the University of Arizona have used nanotechnology to create a new type of capacitor called Digitized Energy Storage Devices. or DESD's. According to them, "DESDs have a very high capacitance-to-volume ratio that's more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. This makes for a device with large capacitance in a small package."
This is a rather exciting technology to watch, and if you are interested in more of the science behind their work, click here and read the whole article.
EDIT: Added the link to the source article above.
[Source: University of Arizona]