**3.3.2 Electrolyte composition**

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

Electrochemical Polymerization of Aniline 85

formation of passive layer consisted of iron oxalate, on which aniline polymerize. It was also showed that *p*-toluen sulfonic acid can be used for electrochemical polymerization of aniline, the deposition occurred after passivation. The passive film, in contrast to oxalic acid, was consisted mainly of iron oxide (Camalet et al., 1998). Other approach involves the pretreatment of the steel surface by polypyrrole, which can be easily formed electrochemically on iron and steel, with low extent of the metal dissolution, after this treatment aniline is easily electrochemically polymerized (Lacroix et al., 2000). The problem with electrochemical polymerization of aniline on aluminum is occurrence of two simultaneous processes electrochemical polymerization and passivation of the electrode by very stable protective oxide (Biallozor & Kupniewska, 2005). The studies of electrochemical polymerization on aluminum and its alloys involved pretreatment of the metal, and further polymerization (Huerta-Vilca et al., 2005; Wang & Tan, 2006). Similarly to steel, oxalic acid and *p*-toluen sulfonic acid electrolytes were used to grow polyaniline on aluminum (Conroy & Breslin, 2005; Karpagam et al., 2008). It was shown that sodium benzoate could be used to electrochemically polymerize aniline, without need for pretreatment, on steel, copper and aluminum (Gvozdenović & Grgur, 2009; Gvozdenović et al. 2011; Popović & Grgur, 2004). Electrochemical polymerization of substituted anilines on various metals and alloys were also carried out (Chaudhari & Patil 2007; Chaudhari et al.,

Generally, experimental set-up for electrochemical synthesis of electroconducting polymers in laboratory conditions is simple. It involves, in majority of cases, standard three-electrode electrochemical cell, although in some cases of galvanostatic polymerization, two electrode cell can be used (Wallace et al., 2009). The polymer obtained by this procedure is deposited directly on the electrode. Novel experimental set-up, enabling electrochemical generation of polyaniline colloids, using flow-through electrochemical cell, was also reported (Aboutanos et al., 1999; Innis et al., 1998). In this novel electrochemical cell, anode was separated from two and cathodes by ion exchange membrane. The anodic and cathodic electrolytes were passed through electrode compartments at specified flow, while polymerization was

The most common experimental techniques used for electrochemical polymerization of aniline are: cyclic voltammetry (potentiodynamic), galvanostatic and potetntiostatic techniques. Polymerization using cyclic voltammetry is characterized by cyclic regular change of the electrode potential and the deposited polymer is, throughout the experiment, changing between its non-conducting and conducting (doped) state, followed by exchange of the electrolyte through polymer (Heinze et al., 2010). At the end of polymerization the obtained polymer is in its non-conducting form, moreover, cyclic voltammetry favors formation of disordered chains and open structure (Heinze et al., 2010) As stated before, relatively high potential is required for electrochemical oxidation of aniline monomer, therefore at first 2 – 10 cycles, the upper potential limit is high, but owing to autocatalytic nature of aniline electropolymerization, the upper potential limit can be decreased to avoid degradation resulted from over oxidation of perningranilin form of polyaniline (Inzelt, 2008). Recently, it was shown that cyclic voltammetry can even be useful for formation of nanostructure polyaniline . It was shown that different sized polyaniline nanofibres were electrochemicaly polymerized, by different scan rates, in the presence of ferrocensulfonic

2009; Pawar et al., 2007).

**3.4 Experimental performance** 

achieved at constant potential.

acid (Mu & Yang, 2008).

Wallace et al., 2009) made first electrochemical polymerization of aniline in acetonitrile solution containing lithium perchlorate, the resulted polymer exhibited similar redox properties as "ordinary" prepared polyaniline.

Lately, apart from acetonitrile, the studies involved use of: dichlormethane, nitrobenzene with various electrolytes such as: sodium tetraphenylborate, tetraethilammonium tetrafluoroborate and tetraetilammoniumperchlorate and resulted films showed different microstructures and electrochemical activity (Pandey & Singh, 2002). Other studies referred to electrochemical polymerization of aniline and fluoro- and chloro- substituted anilines and their coopolymers with aniline in acetonitrile containing tetrabutilammonium perchlorate and perchloric acid. The obtained polymers exhibited similar electrochemical and UVbehavior to "ordinary" polyaniline, but their conductivities were remarkably lower, explained by the steric effects of the substituent (Şahin et al. 2003). Various alkyl substituted anilines were electrochemically polymerized in both acetonytrile and dimethiylsulfoxide, their conductivities were also very low suggested that there were not in the typical state of the emeraldine salt (Yano et al., 2004). Successful electrochemical co-polymerization of aniline and pyrrole was also carried out in acetonitrile.

Finally, electrochemical polymerization of aniline was performed in various ionic liquids (Heinze et al., 2010; Innis et al., 2004; Li et al., 2005; Mu, 2007). For example, using IR and NMR spectroscopy Mu showed that the ionic liquid, namely 1-ethyl-3-methylimidazolium ethyl sulfate was incorporated in polyaniline during electrochemical polymerization. It was also observed that resulted polyaniline had exhibited good electrochemical activity in solutions with pH 12, and also considerably wider window of the detectable color changes at higher pH values, this effect was explained by the fact that used ionic liquid possessed high buffer capacity, which improved the redox activity and the electrochemical activity in broader pH range (Mu, 2007). Apart from the strong influence of the solvent, the presence of other components in the electrolyte solution, used for electrochemical polymerization of aniline, also had influence, primarily, on the morphology of the deposit (Inzelt, 2008). It was shown that presence of alcohols in the electrolyte would lead to polyaniline in the form of nanofibres agglomerated into interconnected network, FTIR spectra of the resulted polymer revealed strong interactions between alcohol and polyaniline molecules (Zhou et al., 2008).

#### **3.3.3 Electrode material**

Electrochemical polymerization of aniline is easily performed at so called inert electrodes, such as: platinum, gold, various graphite, carbons or indium-tin-oxide glasses, according to previously described mechanism. But the fact that relatively high electrode potential is required for oxidation of aniline, restricts the usage of other materials. The electrochemical polymerization of aniline on active metals is usually considered for application in corrosion protection (Biallozor & Kupniewska, 2005; Tallman et al., 2002). The problem connected to electrochemical polymerization onto active metals is either dissolution, or formation of nonconducting passive layer, on the potentials necessary for oxidation of aniline.

In the case of iron and steel the potential at which polymerization starts is in the region of active dissolution, leading to lost of the metal and contamination of the electrolyte, therefore it necessary to find a suitable electrolyte that would enable strong passivation of the metal without suppressing further electrochemical polymerization. The most common electrolyte used to electrochemical deposition of polyaniline on steel and aluminum is oxalic acid (Camalet et al., 1996, 2000a, 2000b; Martyak et al., 2002). The use of oxalic acid permitted

Wallace et al., 2009) made first electrochemical polymerization of aniline in acetonitrile solution containing lithium perchlorate, the resulted polymer exhibited similar redox

Lately, apart from acetonitrile, the studies involved use of: dichlormethane, nitrobenzene with various electrolytes such as: sodium tetraphenylborate, tetraethilammonium tetrafluoroborate and tetraetilammoniumperchlorate and resulted films showed different microstructures and electrochemical activity (Pandey & Singh, 2002). Other studies referred to electrochemical polymerization of aniline and fluoro- and chloro- substituted anilines and their coopolymers with aniline in acetonitrile containing tetrabutilammonium perchlorate and perchloric acid. The obtained polymers exhibited similar electrochemical and UVbehavior to "ordinary" polyaniline, but their conductivities were remarkably lower, explained by the steric effects of the substituent (Şahin et al. 2003). Various alkyl substituted anilines were electrochemically polymerized in both acetonytrile and dimethiylsulfoxide, their conductivities were also very low suggested that there were not in the typical state of the emeraldine salt (Yano et al., 2004). Successful electrochemical co-polymerization of

Finally, electrochemical polymerization of aniline was performed in various ionic liquids (Heinze et al., 2010; Innis et al., 2004; Li et al., 2005; Mu, 2007). For example, using IR and NMR spectroscopy Mu showed that the ionic liquid, namely 1-ethyl-3-methylimidazolium ethyl sulfate was incorporated in polyaniline during electrochemical polymerization. It was also observed that resulted polyaniline had exhibited good electrochemical activity in solutions with pH 12, and also considerably wider window of the detectable color changes at higher pH values, this effect was explained by the fact that used ionic liquid possessed high buffer capacity, which improved the redox activity and the electrochemical activity in broader pH range (Mu, 2007). Apart from the strong influence of the solvent, the presence of other components in the electrolyte solution, used for electrochemical polymerization of aniline, also had influence, primarily, on the morphology of the deposit (Inzelt, 2008). It was shown that presence of alcohols in the electrolyte would lead to polyaniline in the form of nanofibres agglomerated into interconnected network, FTIR spectra of the resulted polymer revealed strong interactions between alcohol and polyaniline molecules (Zhou et al., 2008).

Electrochemical polymerization of aniline is easily performed at so called inert electrodes, such as: platinum, gold, various graphite, carbons or indium-tin-oxide glasses, according to previously described mechanism. But the fact that relatively high electrode potential is required for oxidation of aniline, restricts the usage of other materials. The electrochemical polymerization of aniline on active metals is usually considered for application in corrosion protection (Biallozor & Kupniewska, 2005; Tallman et al., 2002). The problem connected to electrochemical polymerization onto active metals is either dissolution, or formation of non-

In the case of iron and steel the potential at which polymerization starts is in the region of active dissolution, leading to lost of the metal and contamination of the electrolyte, therefore it necessary to find a suitable electrolyte that would enable strong passivation of the metal without suppressing further electrochemical polymerization. The most common electrolyte used to electrochemical deposition of polyaniline on steel and aluminum is oxalic acid (Camalet et al., 1996, 2000a, 2000b; Martyak et al., 2002). The use of oxalic acid permitted

conducting passive layer, on the potentials necessary for oxidation of aniline.

properties as "ordinary" prepared polyaniline.

aniline and pyrrole was also carried out in acetonitrile.

**3.3.3 Electrode material** 

formation of passive layer consisted of iron oxalate, on which aniline polymerize. It was also showed that *p*-toluen sulfonic acid can be used for electrochemical polymerization of aniline, the deposition occurred after passivation. The passive film, in contrast to oxalic acid, was consisted mainly of iron oxide (Camalet et al., 1998). Other approach involves the pretreatment of the steel surface by polypyrrole, which can be easily formed electrochemically on iron and steel, with low extent of the metal dissolution, after this treatment aniline is easily electrochemically polymerized (Lacroix et al., 2000). The problem with electrochemical polymerization of aniline on aluminum is occurrence of two simultaneous processes electrochemical polymerization and passivation of the electrode by very stable protective oxide (Biallozor & Kupniewska, 2005). The studies of electrochemical polymerization on aluminum and its alloys involved pretreatment of the metal, and further polymerization (Huerta-Vilca et al., 2005; Wang & Tan, 2006). Similarly to steel, oxalic acid and *p*-toluen sulfonic acid electrolytes were used to grow polyaniline on aluminum (Conroy & Breslin, 2005; Karpagam et al., 2008). It was shown that sodium benzoate could be used to electrochemically polymerize aniline, without need for pretreatment, on steel, copper and aluminum (Gvozdenović & Grgur, 2009; Gvozdenović et al. 2011; Popović & Grgur, 2004). Electrochemical polymerization of substituted anilines on various metals and alloys were also carried out (Chaudhari & Patil 2007; Chaudhari et al., 2009; Pawar et al., 2007).
