Supercapacitor

Supercapacitors, also known as electrochemical capacitors or ultracapacitors, are energy storage devices that store energy primarily in the electric field between two porous electrodes separated by an electrolyte. They differ from conventional capacitors in their high surface-area electrodes and, in some cases, the involvement of fast surface reactions that contribute to energy storage.

They operate through two mechanisms: electric double-layer capacitance, arising from the accumulation of ions at the electrode–electrolyte interface, and pseudocapacitance, produced by fast, reversible surface redox reactions of certain electrode materials. This combination gives supercapacitors their characteristic high power output and long cycle life.

Most commercial units are electrochemical double-layer capacitors using activated carbon electrodes. They offer very high power density and long cycle life, with typical energy densities around 5 Wh/kg and voltages per cell near 2.5–2.7 V; cycle life can exceed 100,000 cycles.

There are three main categories: electric double-layer capacitors (EDLCs), pseudocapacitors based on materials such as metal oxides or conducting polymers, and hybrid capacitors that combine a capacitive electrode with a battery-like electrode. Each type emphasizes different trade-offs between energy density, power density, and lifespan.

Applications include regenerative braking in vehicles, uninterruptible power supplies, power smoothing for renewable energy, and emergency backup power. They are commonly used alongside batteries to improve response time and reliability, especially where rapid charge–discharge cycles are important.

Limitations include lower energy density than lithium-ion batteries, higher self-discharge than some batteries, cost of certain electrode materials, and safety considerations related to electrolytes. Ongoing research aims to raise energy density and reduce cost through new materials, electrolytes, and hybrid designs.