Polymers ,Electrolyte and Organic

 Polymers

Polymeric materials, such as p- and n-dopable poly(3-arylthiopene), p-doped poly(pyrrole), poly(3-methylthiophene), or poly(1,5-diaminoanthraquinone) have been suggested by several authors [17–19] aselectrodes for electrochemical capacitors.

The typical cyclic voltammogram of a polymer however is in general not of rectangular shape, as is expected for a typical capacitor, but exhibits a current peak at the espective redox potential of the polymer. In order to be able to use one and the same electrode material on both capacitor electrodes polymers with a cathodic and an anodic redox process were utilized recently [19]. Using a polymeric material for electrochemical capacitor electrodes gives rise to a debate as to whether such devices should still be called capacitors or whether they are better described as batteries. In terms of the voltage transient during charge and discharge and with respect to the CV they are batteries. Compared to metallic oxides, however, the term capacitor is justified. The difference being only that the metallic oxides exhibit a series of redox potentials giving rise to an almost rectangular CV while the polymer typically has only

one redox peak. For such capacitors rather high energy density and power density have been reported [19]. The long-term

stability during cycling, however, may be a problem. Swelling and shrinking of electroactive polymers is well

known and may lead to degradation during cycling.

1. Electrolyte

Another criteria to classify different electrochemical capacitors is the electrolyte used. Most of the presently

available capacitors use an organic electrolyte.

2. Organic

The advantage of an organic electrolyte is the higher achievable voltage. According to Eq. (2) the square of the unit-cell voltage determines the maximum stored energy. Organic electrolytes allow for a unit cell voltage above 2 V. Typically the cell float voltage is 2.3 V with the possibility to increase the voltage for a short time to 2.7 V. The cell voltage is most probably limited by the water content of the electrolyte. In order to achieve higher voltage, some companies plan to go up to a float voltage of 3.2 V, extreme purification procedures of special electrolyte have to be applied and the corrosion of the carbon electrodes has to be reduced by special protective coatings [20]. However, similar problems concerning the potential window of organic electrolyte are known from Li-ion battery production and can be overcome. On the other hand organic electrolytes have a significantly higher specific resistance. Compared to a concentrated aqueous electrolyte the resistance increases by a factor of at least 20, typically by a factor of 50. The higher electrolyte resistance also affects the equivalent

distributed resistance of the porous layer and consequently reduces the maximum usable power, which is calculated according to