Super-capacitors are just one of the many hoped for silver bullets to make electric vehicles a viable mainstream option for consumers. The reason everyone wants capacitors is their ability to absorb energy rapidly without degrading the way batteries do. An ideal capacitor energy storage would have huge capacity that could be replenished in just minutes the way we fill a fuel tank today. 

Unfortunately large capacitors have remained both stubbornly expensive and short on total energy storage capacity. Mazda may well have the best near term application for capacitors in its oddly-named i-ELOOP system. Like the brake energy recuperation systems that have become common in Europe in recent years, i-ELOOP let's the alternator mostly coast when then vehicle is cruising or accelerating to minimize the parasitic load on the engine. 

Once the driver releases the accelerator, the alternator switches modes to charge the battery while absorbing kinetic energy from the vehicle as it slows. Previous systems have relied on larger capacity advanced glass matt batteries like those typically used on cars with automatic start stop. Even these AGM batteries have limits on how fast they can absorb energy. Since the braking periods of the vehicle tend to be limited in duration, the total energy recovered is reduced. 

Using a capacitor as the energy sink potentially allows more of the available energy to be absorbed in less time optimizing the system. I'll be curious to see how much of a real world benefit drivers of the Mazda6 get with this system.