WHAT IS AN ULTRACAPACITOR?

Till recently capacitors were divided into two categories- electrostatic or electrolytic capacitors. Capacitance values of these capacitors varied from a few pico-pico-farads (ppF) to several thousand microfarads (μF). Farad was considered too large a unit to be practical.  In recent years a third type of capacitor has been introduced to the world, breaking this barrier, with values ranging from a fraction of a farad to several thousand farads.

Ultracapacitor (also called Supercapacitor) is an electrochemical capacitor (EC), based on nanotechnology materials. It is a long-lasting energy storage device that can store and release energy much faster than a battery. Construction of these capacitors makes extremely large values of capacitance possible, which go in Farad range, or even thousands of Farads. The name ‘ultracapacitor’ or ‘supercapacitor’ was given because of this feature. They are also known by various names like Electrical Double Layer Capacitor (EDLC), Double Layer Capacitor (DLC), Digital Energy Storage Device (DESD) etc. going by their construction and usage.

Electrode of ultracapacitor is made from a highly porous material like activated carbon, having extremely high effective surface area (e.g. 2500 sq. m./gm). A wet or liquid electrolyte fills the porous electrodes, with molecular size small enough to enter spaces in the structure of electrodes. Electrolyte is from a large number of aqueous or organic compositions which can impregnate electrode volume and form contact with the large surface area thus available.

A barrier is naturally formed between the porous electrode and liquid electrolyte throughout the electrode volume. Barrier thickness is of the order of just about 0.2 nanometers, which acts as a dielectric with smallest possible thickness between electrode and electrolyte (the second electrode). This barrier does not allow current to flow unless a minimum voltage is attained across it. This threshold voltage depends on nature of electrode/electrolyte materials, and could vary from, say, 0.8 V to 2.2 V.

A second nanoporous electrode forms the other external electrode, and forms a capacitor with the common electrolyte in similar fashion. The two capacitors thus formed have a membrane in between, wetted by the common electrolyte, and effective capacitance value is a series combination of these capacitors. Current collectors (conducting materials) in contact with electrodes pass on current to an external circuit. Voltage of overall capacitor is combined voltage of the two capacitors. Working voltage of ultracapacitor is thus little below the combined breakdown voltage of the two capacitors in series. Voltage of these capacitors may vary from 1.8 V to 3.6 V depending on construction. Voltage rating is decided by decomposition voltage of electrolyte, which limits their working voltage.

Electrostatic capacitors store charges in the form of accumulation of electrons on plates. Ultracapacitors do not store electrons, but store charges in the form of positive and negative ions, and use flow of ions to facilitate the flow of energy. However, no chemical reactions are involved in the process.

How much energy is stored by ultracapacitor compared to battery?

Energy stored in capacitors is given by E =0.5 C V². For example, if C = 1000 F, V = 2.7 V, Energy stored is 1000 x 2.7 x 2.7 /2 = 7.29 x 1000 / = 3645 Watt .sec (or Joules). This is also mentioned as (3645 W. s / 3600 sec =) 1.01 Watt. Hour (Wh). Energy stored per unit weight of capacitor or a battery (energy density) is usually mentioned in Wh/Kg or Wh/lit. For EDLC using activated carbon electrodes and organic electrolytes, this figure varies between 5 to 8 Wh /Kg, while graphene capacitors may offer up to 15 Wh / Kg. For comparison, lead acid battery has around 45 Wh / kg, Lithium ion capacitors energy density of 80 to 150 Wh / Kg., while pseudocapacitors (a type of ultracapacitor) store up to 45 Wh / Kg.

The Ragone’ chart above gives a comparison of energy densities and power densities of conventional capacitors, ultracapacitors (supercapacitors), batteries and fuel cells. It is seen that ultracapacitors fill the gap between capacitors and battery in both energy density and power density. They are therefore used in several applications today to utilize the benefits of ultracapacitor-battery combinations, while ultracapacitors even replace batteries in certain applications.

applications of ultracapacitors

Ultracapacitors are found in several common applications including digital camera, mobile phones, toys, small gadgets and electronic circuits. One of their major usage is as battery backup. They take up all surges and sudden loads off the battery by supplying these instantly, and battery is saved from these conditions. This makes the battery last much longer, and also enables it to work under extremes of temperatures. Another major application is in power grids and in large scale energy storage. Electric and hybrid vehicles use ultracapacitors for several functions.

Can ultracapacitors replace batteries?

Energy density of ultracapacitors being much smaller than that of batteries, they cannot replace batteries in all places. However, there are small energy applications where instant power availability or quick response is desired. In such cased ultracapacitors are replacing batteries. Further, in remote locations and extreme weather (as on high mountains or undersea, or in extreme cold weather), batteries have reliability and maintenance problems. Ultracapacitors come in handy in such locations, since they can work over a much wider temperature range and have extremely long life without needing any maintenance. Number of electronic circuits, toys, small gadgets etc. also use them in place of batteries. They are also extensively used in vehicles for a number of functions.

With energy densities going high in modern ultracapacitors, they are used as main energy storage in buses, trams and some other applications. Range of these vehicles is not as high as that of batteries, but they can be charged in seconds to minutes, which is very convenient. Advantage is quick charging, high reliability and extremely long life (several times that of battery).