How does an electric double-layer supercapacitor (EDLC) store energy?

By ions collecting on the electrode surfaces to form a double layer, with no chemical reaction

By a reversible chemical reaction at the electrodes

By storing heat in the electrolyte

By a spinning rotor storing kinetic energy

Why are porous activated-carbon electrodes used in a supercapacitor?

Their very large surface area (1000-2000 m2/g) gives very high capacitance

They react chemically with the electrolyte to store charge

They raise the cell voltage above 10 V

They make the energy density higher than batteries

Cells of a supercapacitor are limited to about 2.5-3 V and its energy density is low (5-10 Wh/kg). Which statement is correct?

Cells are connected in series for higher voltage, and it suits short, high-power bursts such as regenerative braking

Cells are connected in parallel to raise voltage, and it suits long-duration storage

It stores more energy than a battery of the same mass

Its voltage stays constant during discharge

2069 Chaitra · 2 marks
Year not given

Short note: Super-capacitor

Short answer

Supercapacitors store energy electrostatically in an electric double layer using porous carbon electrodes, enabling very high power density and millions of cycles. They have lower energy density than batteries but support fast charge/discharge, used for regenerative braking and short-term power smoothing.

Write a short note on super-capacitor.

A supercapacitor (ultracapacitor or electric double-layer capacitor, EDLC) is an electrical energy storage device that stores energy electrostatically in the electric double layer at the interface between a porous electrode and an electrolyte, with much higher capacitance (up to thousands of farads) than ordinary capacitors.

Construction and working

Two porous activated-carbon electrodes (very large surface area, 1000–2000 m²/g), an electrolyte and a separator.
When voltage is applied, ions in the electrolyte collect on the surface of the oppositely charged electrode, forming two layers of charge (a double layer). No chemical reaction takes place (in EDLC). Energy: E = ½ C V². Cell voltage is limited to 2.5–3 V, so cells are connected in series.

 (+) electrode | electrolyte ions | (-) electrode
  carbon  ====  separator  ====  carbon

Characteristics

Very high power density and very fast charge and discharge (seconds).
Very long life (> 500,000 cycles) and wide temperature range.
Lower energy density (5–10 Wh/kg) than batteries, so not suitable for long-duration storage.
Voltage falls linearly with discharge.

Applications

Regenerative braking in buses, trams and hybrid vehicles; power backup and smoothing of wind and solar output; starting engines; memory backup; used together with batteries as a hybrid storage system.

✓ Answer checked by Claude Opus 5.5 (high effort)

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