Where Do Batteries End and Supercapacitors Begin?

Electrochemical measurements can distinguish between different types of energy storage materials and their underlying mechanisms. Batteries keep our devices working throughout the day–that is, they have a high energy density–but they can take hours to recharge when they run down. For rapid power delivery and recharging (i.e., high power density), electrochemical capacitors known as supercapacitors ( 1 ) are used. One such application is regenerative braking, used to recover power in cars and electric mass transit vehicles that would otherwise lose braking energy as heat. However, supercapacitors have low energy density.