Pseudocapacitance Polymers

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

In supercapacitors, apart from the electrostatic attraction of ions in the electrode/electrolyte interface, which is strongly affected by the electrochemically available surface area, pseudocapacitance effects connected with faradaic reactions take place. Pseudocapacitance may be realized through carbon modification by conducting polymers [4-7], transition metal oxides [8-10] and special doping via the presence of heteroatoms, e.g. oxygen and/or nitrogen [11, 12]. 

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

Asymmetric Systems Using Pseudocapacitive Oxides or Conducting Polymers... 

Redox polymer film (pseudocapacitance) Polypyrrole, polythiophene, polyacene... 

In the case of a large - pseudocapacitance, e.g., an - electroactive polymer film on the surface, the currenttime decay reflect the - diffusion rate of the -> charge carriers through the surface layer, thus shorter times the decay of the current should conform to the Cottrell equation. At long times, when (Dt)1/2 > L, where L is the film thickness, the concentration within surface film impacts on the film-solution boundary, the chronoam-perometric current will be less than that predicted by the Cottrell equation, and a finite diffusion relationship... 

The enhancement of specific capacitance for carbon materials is generally realized by the usage of pseudocapacitance effects, which depend on the surface functionality of carbon and on the presence of electro-active species (termed supercapacitors) [162-164]. Several modifications (i.e., oxidation of carbon for increasing the surface functionality, formation of carbon/conducting polymers composites or insertion of electroactive particles of transition metals) can be carried out to increase the pseudocapacitance that arises fi om faradaic reactions. However, the enhancement of the capacitance connected with faradaic reactions of surface groups often contributes to an increase in the self-discharge of the capacitor due to the instability of the functionalities [165]. 

The two large families of pseudocapacitive materials under study are conducting polymers [14-17] and metal oxides [8,18-20]. In the case of carbon materials, the surface functionality can also participate to pseudocapacitive redox reactions, as it will be shown later. 

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