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Progress in Organic Coatings 71 (2011) 32–35

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Progress in Organic Coatings

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Electrochemical synthesis and characterization of polyaniline thin film andpolyaniline powder

Ali Ramadan Elkaisa, Milica M. Gvozdenovic a, Branimir Z. Jugovic b, Jasmina S. Stevanovic c,Nebojsa D. Nikolic c, Branimir N. Grgura,∗

a Faculty of Technology and Metallurgy, Karnegijeva 4, 11120 Belgrade, Serbiab Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35/IV, 11000 Belgrade, Serbiac ICTM – Institute of Electrochemistry, University of Belgrade, Njegoseva 12, P.O. Box 473, 11000 Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 2 August 2010Accepted 9 December 2010

Key words:PolyanilineThin filmsPowderElectrochemical synthesis

a b s t r a c t

The polyaniline thin film electrode and powder have been synthesized on graphite electrodes from0.5 M hydrochloric acid solution under galvanostatic conditions. The water insoluble and acetone sol-uble polyaniline mass fractions of the powder, as well as the polymerization efficiency, based on theemeraldine salt have been determined. The morphology of the obtained emeraldine salt powder hasbeen investigated by the optical microscopy.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Polyaniline, due to the excellent characteristics and low priceof the monomer was one of the most interesting electroconduct-ing polymers for different applications. The area of applications isvery high, like active corrosion protection of metals, electrochem-ical power sources (battery and super capacitor), photovoltaic andelectrochromic devices, biosensors, etc. [1].

It is well known that PANI has a variety of oxidation states whichare determined by the doping degree. The doping degree, y, repre-sents the number of inserted anions for the given polymer units.It is generally agreed that PANI has three different fundamentalforms: leucoemeraldine (fully reduced, y = 0), emeraldine (half oxi-dized, y = 0.5), and pernigraniline (fully oxidized, y = 1). Generalizedscheme of the electrochemical reactions of the fundamental PANIoxidation forms are given in Fig. 1 [2].

The transition between those three fundamental forms changethe physicochemical properties, of which the most important are:electrical conductivity and color [3]. Due to the protonation ordeprotonation depending on pH, emeraldine and pernigranilinecan exist as a salt or base. Emeraldine salt, which is the most impor-tant, has a dark green color, shows good conductivity and could beconsidered for many kinds of application. Leucoemeraldine basehas yellow color, emeraldine base has blue color, pernigranilinesalt has black/blue color, while pernigraniline base has purple one.

∗ Corresponding author. Tel.: +381 11 3303 681; fax: +381 11 3303 681.E-mail address: BNGrgur@tmf.bg.ac.rs (B.N. Grgur).

PANI can be obtained by chemical or electrochemical oxidativepolymerization [2]. Due to the simple reaction control by appliedcurrent density, electrochemical polymerization could be favor-able. For relatively short time of electrochemical polymerization,PANI is obtained as thin film, while with prolonged electrolysisPANI can be obtained as a relatively adherent powder. PANI powderis especially interesting for the application as active anticorrosionpigments in classical organic paint.

In this study we investigated the electrochemical synthesis ofthe PANI in the form of thin film and powder from dilute hydrochlo-ric acid solution with addition of aniline monomer on the graphiteelectrode. The main goal of this work was to correlate characteris-tics of the PANI thin film with powder, and to determine the fractionof the electroactive polymers form (emeraldine salt) in the powder.

2. Experimental

Thin PANI film electrode was obtained galvanostatically(j = 2 mA cm−2) on cylindrically shaped graphite electrode insertedin Teflon holder (S = 0.64 cm2) from 0.5 M HCl with the addition of0.3 M aniline monomer (p.a. Merck, previously distilled in the argonatmosphere). After polymerization, electrode was discharged with1 mA cm−2, washed with bidistilled water, and transferred into thethree-compartment electrochemical cell containing pure 0.5 M HCl.Following the transfer, the electrode was conditioned at a potentialof–0.6 V for 600 s to be completely discharged, and characterizedusing cyclic voltammetry.

PANI powder was synthesized galvanostatically with current,I of 0.2 A from the same solution on both side of the graphite

0300-9440/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2010.12.004

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A.R. Elkais et al. / Progress in Organic Coatings 71 (2011) 32–35 33

Fig. 1. Generalized scheme of the electrochemical reactions for the fundamentalPANI oxidation forms [2].

electrode (5 × 10 cm, S = 100 cm2), inserted between two counterelectrodes from stainless steel plates X5CrNi18-10 (S = 50 cm2) inthe polyethylene reactor with a volume of 0.8 dm3.

Before experiments, both graphite electrodes were mechan-ically polished with fine emery papers (2/0, 3/0, and 4/0,respectively) and then with polishing alumina of 0.5 �m (BannerScientific Ltd.) on the polishing cloths (Buehler Ltd.). After mechan-ical polishing, the traces of polishing alumina were removed fromthe electrode surface in an ultrasonic bath in water/ethanol mixtureduring 10 min.

After synthesis, electrode was removed from electrolyte, andpolyaniline powder was collected by scratching with plastic knife.PANI powder was rinsed many times with distilled water to thenegative reaction on chloride ions, and dried at 90 ◦C over nightunder vacuum condition. When the water insoluble PANI mass wasdetermined, acetone soluble oligomers were removed in a Soxhletextractor during 3 h, then washed with distilled water and driedin vacuum at 90◦C. The same procedure was used for the waterinsoluble fraction from the electrolyte.

For all the experiments, saturated calomel electrode (SCE) wasused as a reference electrode. The electrochemical measurementswere carried out using a PAR 273A potentiostat/galvanostat con-trolled by a computer via GPBI PC2A interface.

Micrographs of the PANI powder were obtained with an opticalmicroscope (Olympus CX41) connected to the computer.

3. Results and discussion

3.1. Synthesis and characterization of the PANI thin film electrode

Fig. 2 shows the synthesis and characterization of the PANI thinfilm electrode in 0.5 M HCl. Inset of Fig. 2 shows galvanostaticcurve for the aniline polymerization in 0.5 M HCl and 0.3 M anilinemonomer (ANI) at a constant current density of 2 mA cm−2 with apolymerization charge, qpol, of 0.6 mA h cm−2 (1080 s). It is worthto mention that at higher polymerization current densities, dueto the high polymerization potential more degradation products

1.00.80.60.40.20.0-0.2-0.4-0.6-0.80.0

0.5

1.0

1.5

2.0

DC

B3

B2

B1

A

leucoemeraldine

degradationproducts

overoxidation

perningranilineemeraldine

Graphite/PANIq

pol = 0.6 mA h cm-2

v = 1 mV s-1

0.5 M HClj / m

A c

m-2

E / V (SCE)

050010000.2

0.3

0.4

0.5

0.6

0.7 j = 2 mA cm-2

E /

V (

SCE

)

t / s

Fig. 2. Slow potentiodynamic anodic curve (v = 1 mV s−1) of thin PANI film elec-trode in 0.5 M HCl (open symbols represent experimental data, while lines representpeak deconvolution using Lorentzian peak fit function). Inset: Galvanostatic curveof the aniline polymerization from 0.5 M HCl + 0.3 M ANI at the current density of2 mA cm−2.

and lower current efficiency were obtained. As it can be seen fromthe inset of Fig. 2, aniline polymerization proceeds in the potentialrange between 0.7 and ∼ 0.56 V according to the equation:

n(ANI) + nyCl− → [PANIy+(Cl−)y]n + nye (1)

where y is doping degree.Fig. 2 also shows the anodic part of cyclic voltammogram

(v = 1 mV s−1) of the thin dedoped PANI film electrode in pure 0.5 MHCl for the anodic potential limit of 1.0 V, and the main peak decon-volution. Open symbols represent experimental data, while linesrepresent peak deconvolution using Lorentzian peak fit function.

As it can be seen, doping of the PANI electrode with chlorideanions starts at ∼−0.1 V and proceed up to the potential of 1 Vthrough different oxidation states. The appearing of well definedpeak (marked with A) at 0.1 V is followed by the change of colorfrom deep green to almost black, and it could be attributed tothe changes of the doping degree of emeraldine salt between y > 0and 0.5. At low negative potentials (E < –0.1 V), leucoemeraldineform (y ≈ 0) could exist as well [2]. Further oxidation of emeral-dine salt to pernigraniline salt (y between 0.5 and 1) can beattributed to the peak C at potential higher than 0.4 V. Betweenthese two main oxidation states, few peaks marked with B1 to B3are observed and they correspond to the formation of the PANIdegradation or hydrolysis products [2,4–7]. The main degradationproduct appears to be soluble benzoquinone with the redox ofthe benzoquinone/hydroquinone couple. Other inactive insolubledegradation product has been suggested to remain on the elec-trode surface, including PANI strands containing quinoneimine endgroups, and ortho-coupled polymers. Simultaneously with forma-tion of perningraniline, over-oxidation reaction, marked with peakD, occurred producing the water insoluble, low conducting brownproducts [8].

3.2. Synthesis and characterization of the PANI powder

Fig. 3 shows the galvanostatic curve of the aniline polymer-ization from the solution containing 0.5 M HCl and 0.3 M anilinemonomer on the graphite electrode (S = 100 cm2) at a current of0.2 A (2 mA cm−2) during 22 h. Polymerization starts at a poten-tial of ∼0.7 V and proceeds in the potential range between 0.65 and

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34 A.R. Elkais et al. / Progress in Organic Coatings 71 (2011) 32–35

0.0 0.2 0.4 0.6 0.8 1.00.5

0.6

0.7

Graphite S=100 cm2

0.5 M HCl + 0.3 M Anilinej=2 mA cm-2

I = 0.2 A

E /

V (

SCE

)

t / h

25201510500.50

0.55

0.60

0.65

0.70

0.75

E /

V (

SCE

)

t / h

Fig. 3. Galvanostatic curve for the aniline polymerization in the solution containing0.5 M HCl and 0.3 M aniline on graphite electrode. Inset: magnification of the initialpolymerization curve during 1 h.

Table 1Mass of the water insoluble PANI fractions on the electrode and in the electrolyteduring synthesis with a current of 0.2 A during 22 h.

Fraction m/g water insoluble % Deposit color Filtrate color

Electrode 7.35 91.8 Bluish-green PurpleElectrolyte 0.660 8.2 Brown Purple

0.52 V (SCE). The inset of Fig. 3 shows the magnification of the initialpolymerization curve.

Applying the procedure explained in Section 2, the water insolu-ble fractions of the PANI was determined, and the results are shownin Table 1.

As it can be seen from Table 1, the electrode deposit has thebluish-green color, suggesting that emeraldine salt was mainly pro-duced. Brown color of the water insoluble fraction in the electrolytecould be connected with over-oxidized PANI form [8].

The theoretical mass of the PANI for the given synthesis condi-tions can be calculated assuming the 100% current efficiency duringpolymerization of the aniline, taking into account average molarmass of one monomeric unit in the polymer, and using equation[9]:

mth = It(Mm + yMa)(2 + y)F

(2)

where m, g, is the mass of the PANI polymerized with current,I, A, over time, t, h, Mm and Ma, g mol−1, are molecular mass ofaniline monomer (93.13 g mol−1) and an inserted chloride anion(35.5 g mol−1), F = 26.8 A h mol−1 is Faraday constant, and y is dop-ing degree (0–1).

Based on Eq. (2), Fig. 4 shows calculated theoretical mass of thePANI powder for different doping degrees.

Due to the polymerization potentials of ∼0.52 V, considering thecyclic voltammogram, see Fig. 2, and color of the deposits, it shouldbe suggested that after polymerization PANI powder on the elec-

0.0 0.2 0.4 0.6 0.8 1.07.0

7.1

7.2

7.3

7.4

7.5

7.6

7.7

7.8

7.9

8.0

0.5 M HCl0.3 M ANII = 0.20 At = 22 h

mth /

g

PANI doping degree, y

Fig. 4. Calculated theoretical mass of the PANI powder for different doping degreesunder given synthesis condition.

trode was mainly in the emeraldine salt form with y ∼ 0.5–0.6. FromFig. 4, it could be estimated that for the applied polymerizationcurrent of 0.2 A over 22 h, and y = 0.5, theoretical mass of the PANIshould be ∼7.55 g. Since the experimentally obtained mass of thewater insoluble PANI powder on the electrode was 7.35 g, and 0.66 gin the electrolyte, polymerization current efficiency seams to beover 100%. Unfortunately, during polymerization water insolubleinactive oligomers or other degradation products could be incor-porated in the powder. From these reasons, powders were treatedwith pure acetone in Soxhlet extractor, and the obtained results areshown in Table 2.

As it can be seen from Table 2, the fractions of the acetone solubleproducts from the PANI powder on the electrode were 26%. Due tothe dark green deposit color after acetone leaching, it is reasonableto conclude that practically only emeraldine salt remains. Hence,considering Fig. 4 and obtained results, polymerization efficiencybased on the pure PANI emeraldine salt (y = 0.5) are in the rangeof ∼70%. The rest of 30% could be attributed to the formation ofdifferent, practically inactive PANI forms and degradation products.

The brown color of the water and acetone insoluble powder fromthe electrolyte suggests formation of over-oxidized polyaniline [8].The conductivity of the PANI emeraldine salt is ∼10−3 S cm−1, whilefor the over-oxidized product is ∼10−5 S cm−1. So, former productcannot be considered as useful for any kind of application [10]. Thedifferent colors of the filtrates (purple, rubin red, or dark red) andtheir amounts were not important for this study, although it shouldbe noted that such colors can produce perningraniline base or at themoment unknown water or acetone soluble degradation productswhich decrease polymerization efficiency. For the better undesign-ing of the nature of the degradation products, UV spectra of thefiltrates should be investigated in the future.

3.3. Morphology of the PANI deposit

Morphology of the PANI powder obtained on the electrode wasinvestigated after acetone leaching as shown in Fig. 5 showing pow-der particles recorded at different magnifications. From Fig. 5, it

Table 2The fractions of the acetone soluble product in the water insoluble PANI powders.

PANI fraction Mass before acetoneleaching (g)

Mass after acetoneleaching (g)

Acetone solubleproducts

Deposit Color Filtrate color

Electrode 7.35 5.44 1.91 g (26%) Dark green Rubin redElectrolyte 0.660 0.635 0.021 g (3.2%) Brown Dark red

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A.R. Elkais et al. / Progress in Organic Coatings 71 (2011) 32–35 35

Fig. 5. Micrographs of the typical PANI powder with different magnification after acetone leaching.

Fig. 6. Micrographs of the ultrasonically treated PANI powder in distilled waterduring 30 min.

can be seen that PANI powder are mainly consisted of micrometersconglomerates and small dendritic particles which approached thenano-sized dimensions.

Since, the main fraction of the PANI consisted of relatively largemicrometric conglomerates, the powder was ultrasonically treatedin distilled water for 30 min. After drying, the micrograph wastaken, and results are shown in Fig. 6. As it can be seen in Fig. 6most of the conglomerates were beaked, and the size of the parti-cles are mostly in nanometrics and some fraction in micrometricsdimensions remains.

4. Conclusion

Polyaniline thin coating and powder was successfully syn-thesized and characterized under galvanostatic conditions

(j = 2 mA cm−2) from a solution containing hydrochloric acid andaniline monomer on graphite electrode. It has been concluded thatin real synthesis during the electrochemical polymerization effi-ciency based on the pure PANI emeraldine salt-electroactive form(y = 0.5) are in the range of ∼70%. The rest of 30% are attributedto the formation of different, practically inactive PANI forms anddegradation products.

Acknowledgments

This work is financially supported by the Ministry of Scienceand Environmental Protection, Republic of Serbia, contract No. H172046.

One of the Authors (Ali Ramadan Elkais) is grateful to theGreat Socialist People’s Libyan Arab Jamahiriya for his Ph.D.grant.

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