By Will Soutter
for Ultracapacitor Technology
An ultracapacitor, also known as a supercapacitor, or
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
times over without degradation.
Figure 1. Schematic of an ultracapacitor cell.
Image source: NREL.gov
Nanotechnology research from the last few years has allowed us to
to explore the potential of ultracapacitors, by providing materials
which have the necessary properties for wide-ranging commercial
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
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.
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.
It is currently possible to make graphene ultracapacitors with
equivalent characteristics to more established materials, but at far
greater cost. In the near future, however, graphene technology is
likely to take over this market, as the potential performance benefits
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
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)