**3.1 Mechanism**

80 Electropolymerization

polyaniline, formation of polarons as charge carriers explained high conductivity of polyaniline (Mu et al., 1998; Patil et al., 2002). As mentioned, unique property of polyaniline is conductivity dependence on the doping (proton) level (Chiang et al., 1986; Wallace et al., 2009). The maximal conductivity of polyaniline is achieved at doping degree of 50%, which corresponds to polyaniline in the form of emeraldine salt (Tanaka et al., 1989). For higher doping degrees some of the amine sites are protonated, while lower doping degrees means that some of the imine sites were left unprotonated (Wallace et al., 2009), explaining why, in the light of the polaron conductivity model, reduction of emeraldine salt to leucoemeraldine and oxidation to perningraniline states decrease the conductivity. The order of magnitude for conductivity varies from 10-2 S cm-1, for undoped emeraldine, up to

103 S cm-1 for doped emeraldine salt (Inzelt, 2008; Wallace et al., 2009).

Fig. 2. Schematic presentation of polyaniline conductivity

also found to had influence on the polyaniline conductivity.

Beside the fact that doping degree has the pronounced effect on the conductivity, various other factors such as: moisture amount (Kahol et al., 1997; Patil et al., 2002), morphology (Monkman & Adams, 1991; Zhou et al., 2007), temperature (Probst & Holze, 1995) etc. were

**3. Mechanism and kinetics of the electrochemical polymerization of aniline**  Generally, electroconducting polymers are obtained by either chemical or electrochemical oxidative polymerization, although reductive polymerization was also reported (Heinze et al., 2010; Inzlet et al., 2000; Yamamoto & Okida, 1999;Yamamoto, 2003, as cited in Inzelt, 2008). Chemical polymerization is used when large quantity of polymer is requested. Electrochemical polymerization is favorable, since in the most cases the polymer is directly deposited on the electrode facilitating analysis. On the other hand, electrochemical polymerization is especially useful if polymer film electrode is needed. By proper design of

Both the mechanism and the kinetics of the electrochemical polymerization of aniline were extensively investigated (Andrade et al. 1998; Arsov et al. 1998; Carlos et al. 1997; Hussain & Kumar, 2003; Inzelt et al., 2000, Bade et al. 1992; Lapkowski, 1990; Malinauskas et Holze, 1998; Mandić et al., 1997; Mu & Kan, 1995; Mu et al., 1997). Electrochemical, similarly to chemical, polymerization of aniline is carried out only in acidic electrolyte, since higher pH leads to formation of short conjugation oligomeric material, with different nature (Wallace et al., 2009). As stated before, it is generally accepted that the first step of the polymerization process of aniline involves formation of aniline cation radicals, by anodic oxidation on the electrode surface, which is considered to be the rate-determining step (Zotti et al., 1987, 1988). The existence of aniline radical cation was experimentally confirmed, by introducing molecules, (resorcinol, hydroquinone, benzoquinine etc.), capable of retarding or even stopping the reaction, which evidenced a radical mechanism (Mu et al., 1997). The oxidation of the aniline monomer is an irreversible process, occurring at higher positive potentials than redox potential of the polyaniline (Inzelt, 2008).

The following step is dependent on numerous factors such as: electrolyte composition, deposition current density, or potential scan rate, nature and state of the anode material, temperature etc. (Inzelt, 2008). There is a request for relatively high concentration of radical cations near the electrode surface. Radical cations can be involved, depending on reactivity, in different reactions. If it is quite stable, it may diffuse into the solution and react to form soluble products of low molecular weights. On the other hand, if is very unstable, it can react rapidly with anion or the solvent, in the vicinity of the electrode and form soluble products with low molecular weights (Park & Joong, 2005). In favorable case, coupling of the anilinium radicals would occur, followed by elimination of two protons and rearomatization leading to formation of dimer (lately oligomer). The aniline dimer, or oligomer, is further oxidized on the anode together with aniline. The chain propagation is achieved by coupling radical cations of the oligomer with anilinium radical cation. Finally, the counter anion originating from the acid, normally present in the electrolyte, dopes the polymer, meeting the request of electroneutrality. The mentioned mechanism of aniline electrochemical polymerization is schematically presented in Fig. 3 (Hussain & Kumar 2003, Wallace et al. 2009).

Electrochemical Polymerization of Aniline 83

instantaneous nucleation, depending on electrolyte concentration and composition (Bade et al., 1992; Córdova et al., 1994; Mandić et al., 1997). The mass transfer controlled early stage of the polyaniline growth leads to formation of a compact layer (Inzlet 2010). In the case of perchloric acid, depending on monomer concentration, the nucleation process proceeds from progressive, at lower, to instantaneous nucleation at higher concentration (Mandić et al., 1997). At advanced stage, characterized by exponential current increase, 1D growth was assumed, resulted in continual branching and formation of the open structure (Cruz &

Electrochemical synthesis of electroconducting polymers is strongly dependent on numerous parameters involved, such as: nature of the doping anion (affecting morphology, order of the polymer rate growth, nature and the composition of the solvent (to nucleofilic solvent would react with cation radicals formed by monomer oxidation on the anode) electrode material (depending on its surface energy controls the ease of the desired polymer deposition), temperature of the electrochemical polymerization etc. (Heinze et al., 2010;

Electrochemical polymerization of aniline, as mentioned before, is practically always carried out in strong acidic aqueous electrolytes. Doping anions incorporated in polyaniline originate from the acid, and represent its conjugated base. The dopant anions are inserted during electrochemical polymerization fulfilling the request of electroneutrality, and therefore their concentrations are on the stoichiometric levels, for its reasonable that their presence have strong influence on, polyanilne morphology, conductivity, and electrochemical activity and the polymerization process itself (Arsov et al., 1998; Cordova et al,. 1994; Dhaoui et al., 2008; Koziel, 1993, 1995; Lapkowski, 1990, 1993; Lippe & Holze, 1992;

It was experimentally confirmed that polyanilne obtained in the presence of so called "large dopant anions", originated from hydrochloride acid, sulfuric acid, nitric acid, *p*toluensulfonic acid, and sulfosalicylic acid promoted formation of more swollen and open

formation of a more compact structure (Nunziante & Pistoria, 1989; Pruneanu et al., 1998; Zotti et al., 1988). The order of the polyaniline growth was also proved to increase with the size of the dopant anion (Inzelt et al., 2000). It was shown that addition of polyelectrolytes in polymerization electrolyte resulted in insertion of these molecules as dopants (Hyodo & Nozaki 1988, as cited in Wallace et al. 2009). It was also possible to obtain optically active polyaniline by electrochemical polymerization in the presence of (+) or (-) camphorsulfonic

As mentioned previously, electrochemical polymerization of aniline is usually performed in aqueous electrolytes. There is limited number of studies referred to electrochemical polymerization in non-aqueous solvents (Genies & Lapkowski, 1987; Lapkowski, 1990; Pandey & Singh, 2002; Şahin et al., 2003). In the early studies acetronitrlie was mostly used as solvent, for example Watambe et al. (Miras et al. 1991; Watambe et al. 1989, as cited in


**3.3 Factors affecting electrochemical polymerization of aniline** 

Okamoto & Kotaka, 1998; Pron et al., 1992; Pron & Rannou, 2002).

acid, leading to insertion of chiral dopants (Majidi et al., 2009).

**3.3.2 Electrolyte composition**

structured film, while the presence of "small ions" such is ClO4- or BF4

Inzelt, 2008; Pron & Rannou, 2002; Pruneanu et al., 1998; Wallce et al., 2009).

Ticianelli, 1997)

**3.3.1 Doping anions** 

Fig. 3. Schematic presentation of mechanism of electrochemical polymerization of aniline.

It was evidenced that electrochemical polymerization of aniline is an autocatalytic process (Inzelt, 2008; Mu & Kan, 1996; Mu et al.,1997; Stilwell & Park, 1996; Wallace et al., 2009). It was observed that current increased over time, at constant potentials higher than 0.80 V and that anodic peak potentials shifted to more negative values upon increasing cycle number (Mu et al., 1997). Generally, it means that the more polymer formed on the anode, the higher the rate of the electrochemical polymerization.

#### **3.2 Nucleation and growth of polyaniline**

Explanation of the kinetics of the nucleation and growth process during electrochemical synthesis of electroconducting polymers is usually relaying on the metal deposition theory (Heinze et al., 2010; Kankare, 1998). According to the theory, two kinds of nucleation process exist, instantaneous and progressive, with three types of growth refering to: one- (1D), two- (2D), and three- (3D) dimensional processes. Instantaneous nucleation implies constant number of nuclei, growing without the further formation of nuclei. In the case of progressive nucleation, nuclei are constantly generated. 1D growth implies growth in only one direction, e.g. perpendicular to the electrode surface. In the 2D growth, the nuclei has preference to grow parallel to the electrode surface, while the 3D growth is characterized by the similar rates for these processes perpendicular and parallel to the electrode are quite similar (Heinze et al., 2010).

It was shown, mostly base on potentiostatic experiments, that several stages of polyaniline growth during electrochemical polymerization of aniline were involved, proceeding through different mechanism (Mandić et al.,1997). Cyclic voltammetry studies indicated that polyaniline growth was strongly dependent on type and concentration of anion in the electrolyte (Zotti et al.,1988). Studies on the early stages of the polyaniline growth indicate progressive nucleation of the polyaniline film, with 2D or 3D growth mechanism or 3D

Fig. 3. Schematic presentation of mechanism of electrochemical polymerization of aniline. It was evidenced that electrochemical polymerization of aniline is an autocatalytic process (Inzelt, 2008; Mu & Kan, 1996; Mu et al.,1997; Stilwell & Park, 1996; Wallace et al., 2009). It was observed that current increased over time, at constant potentials higher than 0.80 V and that anodic peak potentials shifted to more negative values upon increasing cycle number (Mu et al., 1997). Generally, it means that the more polymer formed on the anode, the higher

Explanation of the kinetics of the nucleation and growth process during electrochemical synthesis of electroconducting polymers is usually relaying on the metal deposition theory (Heinze et al., 2010; Kankare, 1998). According to the theory, two kinds of nucleation process exist, instantaneous and progressive, with three types of growth refering to: one- (1D), two- (2D), and three- (3D) dimensional processes. Instantaneous nucleation implies constant number of nuclei, growing without the further formation of nuclei. In the case of progressive nucleation, nuclei are constantly generated. 1D growth implies growth in only one direction, e.g. perpendicular to the electrode surface. In the 2D growth, the nuclei has preference to grow parallel to the electrode surface, while the 3D growth is characterized by the similar rates for these processes perpendicular and parallel to the electrode are quite

It was shown, mostly base on potentiostatic experiments, that several stages of polyaniline growth during electrochemical polymerization of aniline were involved, proceeding through different mechanism (Mandić et al.,1997). Cyclic voltammetry studies indicated that polyaniline growth was strongly dependent on type and concentration of anion in the electrolyte (Zotti et al.,1988). Studies on the early stages of the polyaniline growth indicate progressive nucleation of the polyaniline film, with 2D or 3D growth mechanism or 3D

the rate of the electrochemical polymerization.

**3.2 Nucleation and growth of polyaniline** 

similar (Heinze et al., 2010).

instantaneous nucleation, depending on electrolyte concentration and composition (Bade et al., 1992; Córdova et al., 1994; Mandić et al., 1997). The mass transfer controlled early stage of the polyaniline growth leads to formation of a compact layer (Inzlet 2010). In the case of perchloric acid, depending on monomer concentration, the nucleation process proceeds from progressive, at lower, to instantaneous nucleation at higher concentration (Mandić et al., 1997). At advanced stage, characterized by exponential current increase, 1D growth was assumed, resulted in continual branching and formation of the open structure (Cruz & Ticianelli, 1997)
