---
title: "Short note: Super-capacitor"
subject: "Energy Environment and Society"
chapter: 5
key: "semester_7/energy_environment_and_society:super-capacitor"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/super-capacitor
site: BCT Engineering
---

# Short note: Super-capacitor

Energy Environment and Society · Chapter 5: Energy Storage

- Asked in: 2069 Chaitra, 2 marks; year not given
- Video: [Supercapacitors explained - the future of energy storage?](https://www.youtube.com/watch?v=-7T-6XdiRTw) — Undecided with Matt Ferrell
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/super-capacitor
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-5

## Question

Write a short note on 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.

## Answer

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.

```text
 (+) 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does an electric double-layer supercapacitor (EDLC) store energy?
   A. By a reversible chemical reaction at the electrodes
   B. By ions collecting on the electrode surfaces to form a double layer, with no chemical reaction
   C. By storing heat in the electrolyte
   D. By a spinning rotor storing kinetic energy
   Answer: B. In an EDLC, ions gather at the surface of the oppositely charged porous electrodes, forming two layers of charge. No chemical reaction occurs. (Key point: Storage mechanism)

2. Why are porous activated-carbon electrodes used in a supercapacitor?
   A. Their very large surface area (1000-2000 m2/g) gives very high capacitance
   B. They react chemically with the electrolyte to store charge
   C. They raise the cell voltage above 10 V
   D. They make the energy density higher than batteries
   Answer: A. Large surface area lets many ions form the double layer, giving capacitance up to thousands of farads. (Key point: Why porous activated carbon)

3. 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?
   A. Cells are connected in parallel to raise voltage, and it suits long-duration storage
   B. Cells are connected in series for higher voltage, and it suits short, high-power bursts such as regenerative braking
   C. It stores more energy than a battery of the same mass
   D. Its voltage stays constant during discharge
   Answer: B. Series connection raises voltage. Low energy density but high power density suits fast bursts, often combined with batteries. Voltage falls linearly during discharge. (Key point: Limitations and use)