It's incorrect to say that capacitor storage will drastically increase the energy recovered from braking in an electric or hybrid vehicle compared to battery storage. The problem is that both the inverter (power transistor) electronics and motor have efficiency losses on the order of 5 to 7% each. Batteries do have additional losses when charging or discharging of about 15% in each direction, which a capacitor largely avoids, but there are still significant losses from the motor and power transistors. Remember those losses get applied twice, both then regenerating (decelerating the vehicle) and when using the energy to accelerate the vehicle again. There are also significant losses in the gears, tires, etc.

There ain't no such thing as a free lunch. If regenerative braking and acceleration in an EV or hybrid were 100% efficient, they would be perpetual motion machines.

That said, capacitors would allow and EV to make use of significantly more regenerative braking energy than a battery electric vehicle. But the overall improvement is going from about 50% efficiency to 70% or about a 20% range improvement. That is very significant, but it's not perpetual motion and can't be due to the other losses.

That also said, practical capacitors would be a major shift in how we use energy for transporation and in general.

American Superconductor Corporation (NASDAQ:AMSC - News), a leading energy technologies company, announced today it has achieved commercial levels of electric current for the first time in long lengths of second generation (2G) high temperature superconductor (HTS) wire. This is the first time commercial levels of electrical current have been successfully achieved in long lengths (over 300 feet) by a low cost industrial process, making possible the emergence of this technology from the laboratory into the marketplace. HTS wires conduct large quantities of electricity with 100% efficiency when cooled with environmentally friendly liquid nitrogen, the coolant of choice for superconductor electric power transmission and distribution cables.