**4. Applications**

140 Electropolymerization

The EIS spectra (i.e. Nyquist plots) of several composites such as S/T-PPy (Tubular PPy) and S/G-PPy (Granular PPy) (Liang et al., 2010), PEDOT/h-PPy (Poly 3,4 ethylenedioxythiophene/horn like-PPy) (5:1) (Wang et al., 2007), PPy films on DuPont 7012 carbon composite (Li et al., 2005), S-PPy(Wang et al., 2006) and Si/PPy (9:1) (Guo et al. 2005) reported in literature displayed a small semi-circle at high frequencies and a straight line at lower frequencies. The observed linearity at low frequencies has been ascribed to the anion diffusion in the composite matrix and the semicircle in the high frequency region, to the contact resistance and charge transfer resistance. Only in the case of PEDOT/h-PPy, a line indicating a capacitive behavior related to the film charging mechanism at low frequencies

The EIS study of sandwich-type composite film electrodes of PPy and a mixed valence oxide have generally produced more or less similar Nyquist plots. Each Nyquist curve indicated a capacitive behavior at low frequencies, a small semicircle (or an arc) at high frequencies and more or less a linear curve at intermediate frequencies. The observed semicircle at high frequencies is considered to be produced due to contribution of the charge transfer reaction (Singh et al., 2004). In some cases, no semicircle corresponding to the charge transfer reaction is observed (Malviya et al., 2005; Singh et al., 2007a); in these cases the charge transfer resistance is thought to be much smaller than the sum of the film and solution resistances. In fact, at higher frequencies, the diffusion of anions (ClO4¯, Cl¯ or SO42-) into the polymer matrix dominates the impedance results and that a conventional semi-infinite Warburg response is observed. At relatively lower frequencies, the ∆Zim/∆Zreal value increases with decreasing frequency and that the impedance response results a capacitivetype behavior. Thus, a change over seems to take place from the semi-infinite Warburg regime to the finite Warburg regime with the variation of frequency from high (100 KHz) to

The spectra obtained were analyzed by fitting the equivalent circuits, Rs(RQ)Q', Rs(R1Q1)(R2C2)(R3C3)C4W4, RsQWC, LR(RQ)(RQ)Q and LR(RQ)(RQ), where Rs is solution resistance, Q is constant phase elemnt, W is Warburg resistance and C is capacitance.The apparent diffusion coefficient (Da) for transport of anions particularly SO42- in the polymer matrix were also estimated at varying potentials. Estimates of Da values for SO42- anions at a constant potential, E = -0.1 V vs SCE in deoxygenated 0.5M K2SO4 + 5mM KOH at 25°C were 1.1, 12.1, 2.5, 0.7, 0.60 × 10-7 cm2 s-1 in binary composites of PPy and LaNiO3, LaMnO3,

During the CV, the PPy film electrode-electrolyte system involves the redox couple, PPy+/ PPy (Kaplin & Qutubuddin, 1995), where PPy is the neutral species (associated with three to four monomer units) and PPy+ is the radical cationic species or polaron (one positive charge localized over three to four monomer units). In the oxidized state, the PPy is positively charged and so, some anions get migrated from solution into the polymer matrix so as to maintain electroneutrality. Similarly, on reduction, as electrons are injected and the positive charge on the polymer chain disappears, some anions leave the PPy matrix and enter into the solution. The formation of a second redox couple, PPy++/ PPy+, has also been proposed (Genies and Pernaut, 1985) to explain two reduction peaks observed in some CVs. PPy++ is the dicationic species or bipolaron. The counterion concentration will be greater for a PPy film containing both polaron and bipolaron species than for a film containing only polaron

La0.8Sr0.2MnO3, La0.7Sr0.3MnO3 and La0.6Sr0.4MnO3, respectively.

was observed.

to a low value (20 mHz).

**3.6 Cyclic Voltammetry (CV)** 

species (Kaplin and Qutubuddin, 1995).

#### **4.1 Corrosion protection coatings**

Conductive polymers are presently being considered as potential materials for corrosion protection of metals. They can substitute conventional protection materials such as chromates and phosphates used in the electroplating and paint industries with stronger more resistant and environmentally friendly coatings. The traditional anticorrosion materials provide excellent corrosion protection coatings but their toxicity has been severely questioned.

A conductive organic coating forms a physical barrier against corrosive agents and a passive layer on the metal surface. The corrosion protection efficiency of this organic coating can be

Polypyrrole Composites: Electrochemical Synthesis, Characterizations and Applications 143

oxygen reduction). The mechanism of oxygen reduction reaction (ORR) is very complex. The most accepted mechanisms in acid media are, the direct four electron pathway (O2 + 4H+ + 4e- ↔ 2H2O, Eº = 1.229 VSHE) and a series two-electron pathway (O2 + 2H+ + 2e- ↔ H2O2 + 2H+ + 2e- ↔ 2H2O). The ORR and Methanol/Ethanol oxidation reaction (MOR/EOR) are traditionally catalyzed by Pt or Pt-based alloys dispersed on high surface

PPy has been investigated as carbon-substitute supports for fuel cell catalysts. Polymersupported metal particles present the higher specific surface area and the higher tolerance to poisoning due to the adsorption of CO species, in comparison to the serious problem of poisoning of bulk and carbon supported metals. Moreover conducting polymers are not only electron conducting, but also proton conducting materials, so they can replace Nafion in the catalyst layer of the fuel cell electrode and provide enhanced performance also.

Several PPy-based anodes for fuel cells have recently been prepared and investigated for

In the MOR, Pt nano-particles decorated PPy-MWCNTS composite electrodes showed higher catalytic stability than Pt/MWCNTs binary catalyst, due to the synergic interaction between PPy and the carrier (Qu et al, 2010). Zhao et al. (2008a) demonstrated that the Pt nano-particles deposited on PPy-C with naphthalene sulfonic acid as dopant exhibit better catalytic activity than those on plane carbon in fuel cells. They also reported (Zhao et al., 2009) that bimetallic Pt-Co nano- particles co-deposited on PPy-MWCNT composite via over-oxidation treatment had higher catalytic activity towards methanol oxidation. Further, the MOR was observed to improve on the Pt-Fe/PPy-C catalyst compared to commercial

Mohana Reddy et al. (2008) examined the suitability of the cobalt-polymer-MWCNT composite electrode for the ORR in DMFCs and DEFCs by using Pt-Ru/MWCNT and Pt-Sn/MWCNT, respectively, as anode electrocatalysts. The study indicated an improved power densities for hydrogen, methanol and ethanol based fuel cells compared to the previously non Pt based electro-catlysts. Hammache et al. (2001) observed a higher catalytic activity for MOR on the dispersed gold micro particle on PPy coatings on Fe than a bare

The application of PPy film containing nanometer-sized Pt and Pt/Pd bimetallic particles on ITO glass plates has also been investigated as anode for MOR. The modified electrode was found to exhibit significant electrocatalytic activity. The enhanced electrocatalytic activities may be due to the uniform dispersion of nanoparticles in the PPy film and a synergistic effect of the highlydispersed metal particles so that the PPy film reduces electrode poisoning by adsorbed CO species (Selvaraj et al. 2006). A Pd/PPy/Pd/G generated by sequential electrodeposition method (Ding et al., 2011) showed a satisfactory electrocatalysis toward

As early as in 1983, Bull et al. observed that Fe tetrasulfonated phthalocyanines-doped PPy film on GC electrode catalyzes the reduction of O2 at potentials 250-800 mV less negative than at bare GC or at PPy-coated GC electrode. Co-doped PPy films on metal electrodes also showed the electrocatalytic activity for O2 reduction (Ikeda et al., 1983). Osaka et. al. (1984)

their applications in DAFCs. Most of them are decribed briefly in following lines.

area carbon support materials.

**4.2.1 PPy-based anodes** 

Pt/C catalyst (Zhao et al., 2008b).

gold electrode in acidic media.

the formic acid oxidation.

**4.2.2 Oxygen Reduction Reaction (ORR)** 

greatly improved by introducing organic or inorganic materials embedded within the polymer structure by electropolymerization. It is reported that the composites with micronand submicron ceramic particles such as TiO2 (Ferreira et al., 2001), WO3 (Yoneyama et al., 1990), Fe3O4 (Garcia et al., 2002), MnO2 (Yoneyama et al., 1991) and Zn3(PO4)2 (Lenz et al., 2007) with PPy improved the mechanical and corrosion resistance of the coatings.

Electrochemically polymer (PANI+PPy) coated stainless steel plates showed improved corrosion resistance with acceptable contact resistance under proton-exchange membrane fuel cell (PEMFC) condition (Joseph et al., 2005). Recently, it has been demonstrated (Ren & Zeng, 2008) that a bilayer conducting polymer coating, composed of an inner layer of PPy with large dodecylsulfate ionic groups and an external PANI layer with small SO4 <sup>2</sup>− groups, reduced the corrosion of the type 304 stainless steel, used for bipolar plates of a PEMFC, much more effectively than the single PPy coatings in 0.3M HCl. Besides these, some PPy-based composites such as PPy-CMC (carboximethylcellulose), PPy-SDS (sodium dodecylsulfate), PPy-TiNT (Titanate nanotube) and PPy-zinc phosphate were observed to protect steel (Herrasti & Ocoan, 2001), 1Cr18Ni9Ti stainless steel (Zhang &Zeng, 2005), stainless steel type 904L (Herrasti et al., 2011), and AISI 1010 steel (Lenz et al., 2007) surfaces effectively from corrosion, respectively. Electrochemically prepared PPy-WO4 2- (Sabouri et al., 2009), PPy/PANI (Panah & Danaee, 2010) and Pt/PPy (Rahman, 2011) composite films provided noticeable corrosion inhibition for carbon steel, whereas PPy-TiO2 (Lenz et al., 2003), PPhe (polyphenol)/PPy (Tuken et al., 2004), PPy-P (Hosseini et al., 2007), PPy-DGEBA (Riaz et al., 2007), PPy-PMo12O4 3- (Kowalski et al., 2008), and PPy-PANAP (poly(5-amino-1-naphthol)) (Bereket & Hur, 2009) provided better protection for corrosion of mild steel.

PPy-based coatings were also found to reduce the corrosion of oxidizable metals such as Fe (Bazzaoui et al., 2006; Bazzaoui et al., 2005; Lee et al., 2005), Al (Bazzaoui et al., 2005; Lehr & Saidman, 2006a; 2006b)and Al alloys(Lehr & Saidman, 2006a; 2006b). The efficiency of corrosion protection of PPy depends on the nature of the doping agents (Balaskas et al., 2011).

Electrochemically deposited PPy films on Cu displayed good protection against Cu corrosion in a 3.5% NaCl solution (Herrasti et al., 2007). The effectiveness of the protection is enhanced when the PPy film is electrosynthesized from a solution of dihydrogen phosphate (Redondo & Breslin, 2007) or sodium saccharinate (Bazzaoui et al., 2007).

PPy films electrodeposited onto Ni-Ti alloy employing sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or AOT) solutions improved the corrosion performance of the alloy at the open circuit potential and at potentials where the bare substrate suffers pitting attack (Flamini & Saidman 2010). PPy coatings on Mg alloy AZ91D, obtained respectively from aqueous solutions of a dicarboxylic organic acid salt (Turhan et al., 2011a) and sodiumsalicylate (Turhan et al., 2011b) by cyclic voltammetry (CV) method, demonstrated good corrosion protections of the alloy.
