Classification of electrochemical capacitors

                    Electrochemical capacitors may be distinguished by several criteria such as the electrode material utilized, the electrolyte, or the cell design.With respect to electrode materials there are three main categories: carbon based, metal oxides and polymeric materials.

A comprehensive review of possible electrode materials suitable for ECs is given by Sarangapani [6]

3.1. Electrode material

          3.1.1. Carbon

                Carbon in various modifications is the electrode material used most frequently for electrodes of electrochemical

capacitors. Reasons for using carbon are manifold such as (i) low cost, (ii) high surface area, (iii)

availability, and last but not least (iv) established electrode production technologies. Carbons are available

with a specific surface area of up to 2500 m2:g as powders, woven cloths, felts, or fibers.

Charge storage on carbon electrodes is predominantly capacitive in the electrochemical double layer.

Carbon based electrochemical capacitors come close to what one would call an electrochemical double layer capacitor. 

          There are however contributions from surface functional groups which are in general present on

activated carbons and which can be charged and discharged giving rise to pseudocapacitance.

A typical cyclic voltammogram (CV) for an activated glassy carbon electrode is reproduced in Fig. 3 for

electrochemically activated glassy carbon in aqueous and non-aqueous electrolyte [7,8]. The rectangular

shape expected for an ideal capacitor is best approached in organic electrolyte, while the redox peak

around 0.4 Vsce in aqueous electrolyte illustrates the contribution from surface functional groups. This corresponding

pseudocapacitance is significantly reduced in organic electrolyte because protons are not available.

      The effect of surface functional groups containing oxygen on the stability of carbon electrodes in EC using

organic electrolyte was investigated by Nakamura et al. [9]. These authors found that the stability of the activated

carbon increases with the oxygen content when the carbon is used for the anode and decreases when

used for the cathode. In general one can observe that both the stability and conductivity of the activated high

surface area carbon decrease with increasing surface area