Electrodes Based on Electron-Conducting Polymers ECPs

2 ELECTRODES BASED ON ELECTRON-CONDUCTING POLYMERS (ECPs) 

1 Main Properties of ECPs as Regards Their Application in PsCs 

One of the achievements of electrochemistry in the past 20 years has been the development of electron-conducting polymers (ECPs). Electron conductivity of 

ECPs appears in the course of its doping by counterions because of formation of delocalized n-electrons or holes and their transport under the action of electric field through the system of polyconjugated double bonds characteristic of any ECPs. ECPs include polyacetylene (Pac), polyaniline (PAni), poly(p-phenylene) (PPh), polythiophene (PT), polypyrrole (PPy), polyporphyrin (PP), and their derivatives. Eigure 28.3 shows structural formulas for some ECPs used in ECSCs. 

One can see that all of these have polyconjugated double bonds providing electron conductivity as a result of doping by counterions. ECPs of the PP type have been lately developed by Vorotyntsev et al. (2011). 

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Electrodes of various types are used in hybrid (asymmetric) supercapacitors (HSCs). For example, one of the electrodes is highly dispersed carbon, that is, a double-layer electrode, and the other electrode is a battery one or one of the electrodes is carbon and the other one is a pseudocapacitor, for example, based on electron-conducting polymer (ECP). The main advantage of HSCs as compared EDLCs is an increase in energy density because of the wider potential window. The main fault of HSCs, meanwhile, as compared to electric double-layer capacitors (EDLCs), is a decrease in cyclability following the limitations posed by the nondouble-layer electrode. 

Electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy) and po 1 y(3.4-cthy 1 cncdi oxyth iophcnc) (PEDOT) have been applied in supercapacitors, due to their excellent electrochemical properties and lower cost than other ECPs. We demonstrated that multi-walled carbon nanotubes (CNTs) prepared by catalytic decomposition of acetylene in a solid solution are very effective conductivity additives in composite materials based on ECPs. In this paper, we show that a successful application of ECPs in supercapacitor technologies could be possible only in an asymmetric configuration, i.e. with electrodes of different nature. 

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

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