**1. Introduction**

130 Electropolymerization

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The electronically conducting polymers (ECPs), such as polypyrrole (PPy), polythiophene (PT) and polyaniline (PANI) are known to possess unusually high electrical conductivity in the doped state. Due to this, these materials have been of great interests for chemists as well as physicists since their electrical properties were reported (Diaz et al., 1979). The ECP films behave like a redox polymer and have potential applications in electrocatalysis, solar energy conversion, corrosion, electronics, etc. The redox polymer reaction is accompanied by a change in the electrical properties of the film from an insulator to an electrical conductor involving both electron and ion transport within the film (Kaplin & Qutubuddin, 1995).

Conducting polymers can be synthesized either chemically or electrochemically. Electrochemical synthesis is the most common method as it is simpler, quick and perfectly controllable. PPy is one of the most interesting conducting polymers since it is easily deposited from aqueous and non-aqueous media, very adherent to many types of substrates, and is well-conducting and stable. Electrochemical polymerization produces thin films with a thickness of few micrometers on an electrode surface (Diaz et al., 1979), while a chemical oxidation yields a fine-grained material. However,the yield and quality of the resulting polymer films are influenced by several factors, such as nature and concentration of monomer and the counter ion, solvent, cell conditions (e.g. electrode and applied potential), temperature and pH (Sadki et al. 2000; Ansari, 2006 & Pina et al. 2011).

ECPs can be modified in several ways (Juttner et al., 2004) to obtain tailored materials with special functions: (i) derivatization of the monomer by introducing aliphatic chains with functional groups; (ii) variation of the counterion, incorporated for charge compensation during the polymerization process; (iii) inclusion of neutral molecules with special chemical functions and (iv) formation of compounds with noble metal nanoparticles as catalyst for electrochemical oxidation and reduction processes.

The electropolymerization reaction is a complex process and its mechanism is still not fully understood. A number of mechanisms have been proposed (Genies et al., 1983; Kim et al., 1988; Asavapiriyanont et al., 1984; Qui & Reynolds, 1992) and are comprehensively reviewed (Sadki et al. 2000; Ansari, 2006 & Pina et al. 2011). Among these, Diaz's mechanism is the most accepted one (Genies et al., 1983) and supported by Waltman and Bargon (1984 & 1985) also. In this mechanism, the pyrrole (Py) activation occurs through electron transfer

Polypyrrole Composites: Electrochemical Synthesis, Characterizations and Applications 133

Electrochemical film formation is often followed by stoichiometric determination of the number of electrons donated by each molecule. This value is generally found to be in between 2 and 2.7, where 2 electrons serve in the film formation and the excess charge is

The final step in the polymerization is not clear and different hypotheses have been proposed. Funt and Diaz (1991) believe that the reaction with water could be responsible for the polymerization quench. While, Street (1986) believes that the growth of the chain stops because the radical cation becomes relatively unreactive towards the chain propagation or

This article presents an overview of electrochemical synthesis of PPy nanocomposites and their structural and electrochemical properties and applications in electrochemical devices. Applications of nanocomposites described include solar cells, fuel cells, batteries, corrosion protection coatings and super capacitors. Nanocomposites consists of PPy and one or more

PPy films are obtained through electropolymerization of Py on suitable substrates/working electrodes by using different electrochemical techniques such as cyclic voltammetry (CV), potentiodynamic, galvanostatic, potentiostatic, reversal potential pulsing technique, etc. To carry out the electropolymerization of Py, a three-electrode one-compartment cell (50 or 100 ml capacity) with provision of passing an inert gas (N2/Ar) into the electrolyte before the start of the oxidation and above the surface of the electrolyte during the polymerization process is employed. Saturated calomel electrode (SCE) or Ag/AgCl (saturated KCl) is used as the reference electrode. PPy composites prepared were either in the form of sandwich-

Generally, the electroplymerization is carried out on noble metals or inert materials such as Pt, Au, glassy carbon (GC), or indium tin oxide (ITO). It is because of the fact that the standard oxidation potential of pyrrole is fairly high (Eox = 0.70 V vs. SCE) and so, when scanning the potential the dissolution of most of metal supports takes place prior to the oxidation of monomer is reached and, thus, the electropolymerization reaction is inhibited. However, when we are using supports which are oxidizable, for example, Fe, Zn, Al, etc, as the working electrode, it is necessary to find new electrochemical conditions for slowing the dissolution of the working electrode without preventing electropolymerization. Recently, a new electrosynthesis process of PPy films on oxidizable metals Al and Fe has been reported (Bazzaoui et al. 2005). In this process, the formation of homogeneous and strongly adherent

components, which can be carbon nanotubes, metals, oxide nanomaterials, etc.

(a) (b)

because the reactive chain ends become sterically blocked.

consumed by the polymer oxidation.

Fig. 1.

**2. Synthesis** 

type or simple films.

**2.1 Polypyrrole (PPy) films** 

from the monomer forming a radical cation-rich solution near the electrode in several steps. Details of steps involved are given in Scheme 1.

Scheme 1.

The propagation continues via the same sequences: oxidation, coupling, deprotonation until the final polymer product is formed (Fig.1a). The electropolymerization does not give the neutral nonconducting polymer but its oxidized conducting form (doped). The final polymer chain, in fact, carries a positive charge every 3-4 Py units, which is counter balanced by an anion. The structure of the doped polymer can be given as shown in Fig.1b.

#### Fig. 1.

132 Electropolymerization

from the monomer forming a radical cation-rich solution near the electrode in several steps.

The propagation continues via the same sequences: oxidation, coupling, deprotonation until the final polymer product is formed (Fig.1a). The electropolymerization does not give the neutral nonconducting polymer but its oxidized conducting form (doped). The final polymer chain, in fact, carries a positive charge every 3-4 Py units, which is counter balanced by an anion. The structure of the doped polymer can be given as shown in Fig.1b.

Details of steps involved are given in Scheme 1.

Scheme 1.

Electrochemical film formation is often followed by stoichiometric determination of the number of electrons donated by each molecule. This value is generally found to be in between 2 and 2.7, where 2 electrons serve in the film formation and the excess charge is consumed by the polymer oxidation.

The final step in the polymerization is not clear and different hypotheses have been proposed. Funt and Diaz (1991) believe that the reaction with water could be responsible for the polymerization quench. While, Street (1986) believes that the growth of the chain stops because the radical cation becomes relatively unreactive towards the chain propagation or because the reactive chain ends become sterically blocked.

This article presents an overview of electrochemical synthesis of PPy nanocomposites and their structural and electrochemical properties and applications in electrochemical devices. Applications of nanocomposites described include solar cells, fuel cells, batteries, corrosion protection coatings and super capacitors. Nanocomposites consists of PPy and one or more components, which can be carbon nanotubes, metals, oxide nanomaterials, etc.
