This document discusses electric double layer capacitors (EDLCs), also known as supercapacitors or ultracapacitors. It describes their construction, working principle, advantages, and applications. EDLCs store energy electrostatically at the interface between an electrode and electrolyte, giving them a higher power density than batteries. They have low internal resistance allowing rapid charging and discharging. The document outlines how EDLCs use activated charcoal or carbon nanotubes as electrodes to provide a large surface area for energy storage. Their applications include UPS systems, hybrid vehicles, and consumer electronics requiring bursts of power.