What is an Ultracapacitor?

By Will Soutter

Topics Covered

Introduction

An ultracapacitor, also known as a supercapacitor, or electrochemical capacitor, is a device for storing electrical energy which is growing rapidly in popularity. The design and mechanism of operation is somewhere between an ordinary capacitor and a battery, which opens up some interesting and valuable applications.

Like a battery, a single ultracapacitor cell consists of a positive and negative electrode, separated by an electrolyte. However, ultracapacitors store energy electrostatically, like a regular capacitor, not chemically like a battery - there is a dielectric separator dividing the electrolyte, also like a capacitor.

The small separation between electrodes permitted by this structure lead to much higher energy storage density than a normal capacitor. Whilst an ultracapacitor stores less energy than an equivalently sized battery, it can release it much quicker, as the discharge is not dependent on a chemical reaction taking place.

Because no physical or chemical changes occur when charge is stored, ultracapacitors can also be used many times over without degradation.

Figure 1. Schematic of an ultracapacitor cell. Image source: NREL.gov

Nanotechnology in Ultracapacitors

Nanotechnology research from the last few years has allowed us to begin to explore the potential of ultracapacitors, by providing materials which have the necessary properties for wide-ranging commercial applications.

Their energy density, short charging cycle, and wide range of operating temperatures makes them well suited for applications from efficient large-scale energy storage to very small portable/wearable devices. In the coming years, many experts predict that ultracapacitors will being to replace or augment battery and fuel cell systems in many areas of technology.

The electrodes for commercial ultracapacitors are usually made from nanostructured carbon-based materials, like carbon nanotubes, porous activated carbons, or carbon aerogels. These materials have a high surface area, and good conductivity, making them ideal for use in ultracapacitors.

There is a compromise to be made in the design of the electrode materials, however, as smaller nanopores have excellent surface area but restrict the movement of conducting ions, reducing the conductivity. The pore size must therefore be selected to suit the application of each specific ultracapacitor design.

Future Developments for Ultracapacitors

Ultracapacitors using graphene electrodes show great promise, due to the remarkable electrical properties of the material. The technology is still in its infancy, however, and the degree of control over the electrode's structure which is needed is still difficult to achieve.

Figure 2. Electron micrograph of highly porous activated graphene, for use as an electrode in an ultracapacitor. Image source: "Activated Graphene Makes Superior Supercapacitors for Energy Storage" - BNL.gov

Applications for Ultracapacitors

The performance benefits of ultracapacitors come into play most effectively when a large spike in power consumption, or a large-scale, repeated charging cycle is required.

• Electric or hybrid vehicles - to provide a quick burst of acceleration over short distances, or to start the main motor.
• Quick-charging electronic devices
• Waterproof/weatherproof energy storage (e.g. remote wind farms)
• Military vehicles (starter engines for tanks, submarines, compact power for missiles)

References

• "Report on Energy: Batteries and SuperCapacitors" - EU ObservatoryNano
• Supercapacitor Briefing - Mitre.org
• Ultracapacitors - NREL.gov