SuperCapacitors

Characteristics

Capacitors store energy in an electrostatic field rather than as a chemical state as in batteries.
No chemical actions involved which means very long cycle life is possible.
No limit on the cell voltage imposed by the "cell chemistry" as with galvanic cells.
The terminal voltage directly proportional to the State of Charge (SOC) which limits range of applicability somewhat.
Low power capacitors
Capacitors are probably the most common form of non-chemical energy storage and are widely used in low power applications.
Typical specification: 20 µF to 2 Farads 5.5 to 6.3 Volts
Supercapacitors, Ultracapacitors or EDLC (Electric Double Layer Capacitors) as they are also called, look very much like batteries. They have double layer construction consisting of two carbon electrodes immersed in an organic electrolyte. See below

During charging, the electrically charged ions in the electrolyte migrate towards the electrodes of opposite polariity due to the electric field between the charged electrodes created by the applied voltage. Thus two separate charged layers are produced. Although similar to a battery, the double layer capacitor depends on electrostatic action. Since no chemical action is involved the effect is easily reversible and the typical cycle life is hundreds of thousands of cycles.

They have a low energy density of less than 15 Wh/Kg but a very high power density of 4,000 W/Kg and capacitance values of thousands of Farads are possible. Although the power density is very high the cell voltage is limited to about 2.3 Volts to avoid electrolysis of the electrolyte with the consequent emission of gas.
 
Voltage equalisation to spread the available charge evenly between the capacitors in a series chain may also be needed for many applications.
Typical specification of capacitor banks for automotive applications: 10 to 200 Farads 100 Volts

Advantages

Cell voltage determined by the circuit application, not limited by the cell chemistry.
Very high cell voltages possible (but there is a trade-off with capacity)
High power available.
High power density.
Simple charging methods. No special charging or voltage detection circuits required.
Very fast charge and discharge. Can be charged and discharged in seconds. Much faster than batteries.
No chemical actions.
Can not be overcharged.
Long cycle life of more than 500,000 cycles at 100% DOD.
Long calendar life 10 to 20 years
Low impedance

Shortcomings

Linear discharge voltage characteristic prevents use of all the available energy in some applications.
Power only available for a very short duration.
Low capacity.
Low energy density. (6Wh/Kg)
Cell balancing required for series chains.
High self discharge rate. Much higher than batteries.

Applications

Applications requiring a short duration power boost.
Low power
Capacitors are extensively used as power back-up for memory functions in a wide range of consumer products such as mobile phones, laptops and radio tuners.
Used in pulsed applications to share the load and for providing peak power assistance to reduce the duty cycle on the battery to prolong battery life in products or devices using mechanical actuators such as digital cameras. See also Load Sharing.
Also used for energy storage for solar panels, and motor starters.
High power
The shortcomings above render supercapacitors unsuitable as primary power source for EV and HEV applications however their advantages make them ideal for temporary energy storage for capturing and storing the energy from regenerative braking and for providing a booster charge in response to sudden power demands.
Since the capacitor is normally connected in parallel with the battery in these applications, it can only be charged up to the battery upper voltage level and it can only be discharged down to the battery lower discharge level, leaving considerable unusable charge in the capacitor, thus limiting its effective or useful energy storage capacity.
Using supercapacitors in EVs and HEVs to facilitate regenerative braking can add 15% to 25% to the range of the vehicle.
At the same time, supercapacitors can provide an effective short duration peak power boost allowing the prime battery to be downsized.
It should be noted however that while supercapacitors can be used to to provide the increased range and short term power, it is at the cost of considerable added weight and bulk of the system, and this should be weighed against the advantages of using higher capacity batteries.

Supercapacitors are also used to provide fast acting short term power back up for UPS applications. By combining a capacitor with a battery-based uninterruptible power supply system, the life of the batteries can be extended. The batteries provide power only during the longer interruptions, reducing the peak loads on the battery and permitting the use of smaller batteries.

Carbon Nanotube Enhanced Supercapacitors

Recent developments at MIT have shown that the performance of supercapacitors can be significantly improved by using nanomaterials. The energy storage capability of a capacitor is directly proportional to its capacitance which in turn is proportional to the area of the plates or electrodes. Likewise the current carrying capability is directly proportional to the area of the electrodes. By using vertically aligned, single-wall carbon nanotubes which are only several atomic diameters in width instead of the porous, amorphous carbon normally employed, the effective area of the electrodes can be dramatically increased. While the achievable energy density of 60Wh/Kg still can not match the level obtainable in Lithium Ion batteries (120Wh/kg), the power densities achieved of 100kW/kg are three orders of magnitude better than batteries.
Commercial products are not yet available but should be soon.