Ultracapacitors, also known as supercapacitors, store energy through a physical process of ion adsorption rather than through chemical reactions like batteries. They have much higher capacitance than regular capacitors, allowing them to charge and discharge rapidly. While ultracapacitors have advantages over batteries such as longer lifespan and ability to handle short bursts of power, they currently have lower energy density and higher self-discharge than lithium-ion batteries. Modern designs use carbon nanotubes as electrodes to improve performance. Ultracapacitors find applications where high power output or regeneration is needed, such as in hybrid vehicles, trains, and military equipment.