Ultracapacitors

Technology Insight
February 2006

What has more power than a battery and more energy than a capacitor? Ultracapacitors, also known as supercapacitors or electrochemical double layer capacitors have extremely high power compared to batteries and more energy than standard electrolytic capacitors.

This unique type of capacitor utilizes the phenomenon of the electric double layer effect where charge is stored at the point the solid carbon surface comes into contact with the liquid electrolyte. Two similar electrodes consisting of high surface area carbon become the positive and negative electrodes of the complete cell.

The capacitance “C” of a capacitor is dependant on the:
– area “A” of the electrode plates,
– separation “d” between the parallel plates, and
– dielectric constant “ε” of the medium between the plates.
The relationship between “C” and “A”, “d” and “ε” is given by the following equation:
[1] C = A ε / 4 d
From this formula one sees that capacitance is proportional to area but inversely proportional to the distance between the electrodes.

To obtain ultra-high capacitance, up to 10,000 times greater than ordinary dielectric capacitors, per unit area, the carbon electrode surface area is large and the distance between the electrode plates is very small.

Double layer capacitors use very high surface area carbons which are on the order of 1,000 to 2,000 m 2/g. In other words, one gram of carbon would cover between 2-1/2 and 5 basketball courts!

The separation of charges in electrochemical double layers is on the order of 0.3-0.5 nanometers (where a nanometer (nm) = one billionth of a meter) compared to 10 to 100 nm for electrolytic capacitors. The fact that capacitance is proportional to the universe of the distance yields the ultracapacitor a very large advantage.

Voltage is an important factor when determining energy. The energy of a capacitor, measured in Joules, is directly proportional to the capacitance but proportional to the square of the voltage as shown in the following equation:

[2] E = ½ C (Vi² – Vf²)

As a result, increasing the voltage of a capacitor significantly improves its energy compared to increasing its capacitance.

There are two types of electrolytes used today, aqueous and non-aqueous. Aqueous electrolytes use water as a solvent. Electrolysis of water limits the voltage of aqueous ultracapacitors to approx. 1V, whereas non-aqueous ultracapacitors can be charged up to 2.7V, a large advantage. This advantage is however reduced by the fact that aqueous ultracapacitors have significantly higher capacitance by volume than non-aqueous ultracapacitors.

Another advantage of ultracapacitors over batteries is the very long cycle life. The reaction inside an ultracpacitor involves only storage and delivery of electrostatic charges on the high surface areas of the carbon electrodes; a two-dimensional process. In a battery major chemical and structural changes of electroactive materials occur; a three-dimensional process. As a result, an ultracapacitor can charge and discharge hundreds of thousands to millions of times compared to a battery that is limited to hundreds to thousands of cycles.

Part 2 of this article will describe ultracapacitor applications and which type of ultracapacitor is best suited for each application.

Disclaimer: This information has been compiled from sources considered to be dependable and is, to the best of our knowledge, accurate and reliable as of the date compiled. However, no representation, warranty, or guarantee is made to the accuracy, reliability, or completeness of this information. MPS disclaims any obligation to update such information. This information does not constitute legal advice on transportation regulations, and should not be considered legal advice, nor substitute for obtaining legal advice from competent transportation regulatory authorities and consultants.

  •  
    MPS Newsletter
    NewsletterImage.png Current & Powerful
    Mobile Power Solutions regularly sends out a free newsletter, alerting engineers and other battery power industry professionals on news they can use. Find out the latest on lithium battery testing and cell, battery and capacitor certification by subscribing now.