electrochemical preparation of composite coatings of 3,4-etylenodioxythiophene (edot) and...

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Materials Chemistry and Physics xxx (2014) 1e7

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Materials Chemistry and Physics

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Electrochemical preparation of composite coatings of 3,4-etylenodioxythiophene (EDOT) and 4-(pyrrole-1-yl) benzoic acid(PyBA) with heteropolyanions

Lidia Adamczyk a,*, Krystyna Giza a, Agata Dudek b

aDivision of Chemistry, Department of Materials Processing, Technology and Applied Physics, Czestochowa University of Technology, Al. Armii Krajowej 19,PL 42-200 Czestochowa, Polandb Institute of Materials Engineering, Department of Materials Processing, Technology and Applied Physics, Czestochowa University of Technology,Al. Armii Krajowej 19, PL 42-200 Czestochowa, Poland

h i g h l i g h t s

* Corresponding author.E-mail address: [email protected] (L. Adamczy

0254-0584/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2014.01.012

Please cite this article in press as: L. Adamczand 4-(pyrrole-1-yl) benzoic acid (PyBA)j.matchemphys.2014.01.012

g r a p h i c a l a b s t r a c t

� The composite SiMo12-PEDOT/PyBAcoatings on carbon and steel sub-strates were investigated.

� The coatings have exhibited verygood adhesion to the substrate.

� The coatings are capable of largelyblocking the access of chloridesanions.

a r t i c l e i n f o

Article history:Received 2 October 2012Received in revised form7 January 2014Accepted 10 January 2014

Keywords:Composite materialsPolymersCorrosion testElectrochemical properties

a b s t r a c t

The possibility of incorporating 4-(pyrrole-1-yl) benzoic acid, (PyBA), and heteropolyacids (SiMo12)during the electrodeposition of poly (3,4-ethylenedioxythiophene), PEDOT, is demonstrated in thepaper. The formed novel composite material was applied on the electrode surface as a moderately thin(ca. 0.9e1 mm thick) PEDOT/PyBA/SiMo12 coating. The physicochemical identity of our composite coatingwas established with the use of electrochemical, spectroscopic, and microscopic techniques. The fact thatcarboxylate-containing PyBA units link with positively charged and PEDOT structures tend to improvethe overall stability and adherence of composite coatings to glassy carbon and stainless steel. The PEDOT/PyBA composite serves as a stable host matrix for large negatively charged silicium hetero-polymolybdates inorganic species. Consequently, due to the formation of denser polymeric structuresand due to the existence of electrostatic repulsion effects, the large polyanion-containing compositecoatings are capable of blocking the access of smaller pitting-causing anions (chlorides) to the surface ofstainless steel.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Over the last several years, there has been growing interest inconducting polymer coatings on electrodes [1e13] due to the

k).

All rights reserved.

yk, et al., Electrochemical prewith heteropolyanions, M

prospects of their application in various microtechnological sys-tems, including charge storage devices, sensors, gas separatingmembranes, molecular electronics, displays and light emitting di-odes, and corrosion protection. From among the many conductingpolymers, poly(3,4-ethylenedioxytiophene) or PEDOT has beenrecently of interest as a particularly stable, highly conductive andelectroactive organic polymer [14e20]. For example, PEDOT has

paration of composite coatings of 3,4-etylenodioxythiophene (EDOT)aterials Chemistry and Physics (2014), http://dx.doi.org/10.1016/

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L. Adamczyk et al. / Materials Chemistry and Physics xxx (2014) 1e72

been used as an antistatic coating [21], a conductive electrode inlight emitting diodes [22], and as a material for electrochromicdevices [23]. It is commonly accepted that PEDOT is very stable inthe doped (oxidized) state [24], and it may reach a conductivity ashigh as 200 S cm�1 [25]. Although the latter parameter is depen-dent on the coating morphology, methods of preparation, mea-surement and experimental conditions, and it has been reported tobe lower by approximately an order of magnitude under in situconditions [26], the overall conductivity of the polymer is high inthe oxidized state [27]. The exact nature of the electrochemicalprocesses occurring in the PEDOT coating is fairly complex, and thesystem is believed to undergo several overlapping fast redoxtransitions characterized by high diffusion coefficients for chargepropagation. The above properties make PEDOT attractive as apotential material for fabrication of composite matrices withimmobilized metals [13,20,28,29].

PEDOT and its derivatives have been recognized as the moststable conducting polymer systems currently available. PEDOTcoatings can be formed either chemically or electrochemically, andthe overall polymerization involves the oxidation of the 3,4-ethylenedioxythiophene (EDOT) monomer, typically performed inorganic non-aqueous solvents rather than in aqueous solutions[30]. The possibility of using aqueous anionic micellar media con-taining sodium dodecyl sulphate for making PEDOT coatings onelectrode surfaces was shown [31]. The possibility of incorporating4-(pyrrole-1-yl) benzoic acid (PyBA) during the electrodepositionof PEDOT was demonstrated recently [32]. The morphology of theresulting PyBA-modified PEDOT coatings was found to be granularand much denser compared to the PyBA-free PEDOT. Upon theincorporation of PyBA, the overall stability of the PEDOT coatings(resistance to dissolution) was improved.

The purpose of this study was to develop the means ofimproving the dissolution of the EDOT monomer in an aqueoussolution, as well as to stabilize the surface of the glassy carbon andsteel substrate during the electrodeposition of PEDOT-based com-posite coatings and during oxidative voltammetric potentialcycling. We have proposed a novel preparative solution consistingof PyBA with PEDOT and containing Keggin-type polyoxometalate,such as silicododecamolybdic acid (H3SiMo12O40). The firstcarboxylate-derived organic component was demonstrated to form(with Prussian blue) crack-free composite materials to be used inthe construction of optical [33] and amperometric sensors [34].Another important issue is that the positively charged polymer(PEDOT) backbones are not only stabilized with carboxylate-containing PyBA units, but also attract electrostatically robustinorganic polyanions. In addition to the stabilization effect, it isnoteworthy that the anion exchange membrane properties of theconducting polymer are reversed, and the composite system (pro-tective coating) starts to repel anions (including pitting chlorides).Finally, the existence of silicium heteropolymolybdates may stabi-lize the interface formed at the steel surface presumably by formingsparingly soluble ultra-thin deposits [with chromium(III) andiron(II) or iron (III)] on the steel. We have established the physi-cochemical identity of our composite coating using electro-chemical, spectroscopic, and microscopic techniques.

2. Experimental

All chemicals used were of analytical grade purity. The elec-tropolymerization of PEDOT/PyBA was performed in an aqueousmedium consisting of 0.068mol dm�3 3,4-ethylenedioxythiophene(Aldrich), 10 mg of 4-(pyrrole-1-yl) benzoic/PyBA (Aldrich) in1 dm�3 of water. Heteropolyacids, H3SiMo12O40 (SiMo12), wereobtained from Fluka. Solutions were prepared using doubly

Please cite this article in press as: L. Adamczyk, et al., Electrochemical preand 4-(pyrrole-1-yl) benzoic acid (PyBA) with heteropolyanions, Mj.matchemphys.2014.01.012

distilled and subsequently deionized (Millipore Milli-Q) water.Experiments were carried out at room temperature.

Electrochemical measurements were performed using a CH In-struments (Model CHI 660) workstation (Austin, USA). A glassycarbon disk (with a geometric area of 0.2 cm2) supplied by Mineral(Warsaw, Poland), or an X20Cr13 stainless steel (C, 0.17%; Cr, 12.6%;Si, 0.34%; Ni, 0.25%; Mn, 0.30%; V, 0.04%; P, 0.024%, and S < 0.005%)disk (with a geometric area of 0.2 cm2) embedded in epoxy resinacted as a working electrode. The experiments were performed in aconventional three-electrode system, where platinum wire servedas a counter electrode, while an Ag/AgCl electrode was the refer-ence electrode.

Prior to each experiment, the (stainless steel) electrode surfaceswere polished with waterproof emery paper (Nos. 600, 1000 and2000) and rinsed with distilled water. The glassy carbon electrodewas activated by polishing it with aqueous alumina slurries (with agrain size of 0.05 mm) on polishing cloth and then rinsed withdistilled water.

Unless indicated otherwise, SiMo12-containing PEDOT/PyBA(PEDOT/PyBA/SiMo12) coatings were produced on glassy carbonand on stainless steel by cycling the potential from�0.2 to 1.0 V fora duration of 1056 s at 50 mV s�1 in a modification solution con-taining 30 mmol dm�3 SiMo12 and 0.068 mol dm�3 EDOT (togetherwith 10 mg of PyBA in 1 dm�3 water). To describe the corrosionbehaviour of uncoated and coated samples, potentiodynamic po-larization curves were recorded at a scan rate of 10 mV s�1 byvarying the potential between �0.8 and 1.6 V (vs. Ag/AgCl) inacidified 0.5 mol dm�3 K2SO4 (pH ¼ 2) solutions with the additionof 0.5 mol dm�3 KCl.

The coating thicknesses were determined using a profilometer(Talysurf 50, Rank Taylor Hobson). The thickness of the PEDOT/PyBA/SiMo12 coating, as determined based on the data obtainedusing the profilometer, was equal to 0.95 mm. The coating wasdeposited on glassy carbon. Scanning electron microscopy (SEM)images were produced using a JOEL. The coating was deposited onstainless steel. Inverted microscope images were obtained usingOLYMPUS Model GX71. Infrared spectra were measured with aShimadzu 8400 Fourier transform infrared (FTIR) spectrometer.

In the present work we present the coating adhesion in-vestigations which were performed by scratch tape test. Before theadhesion test the modified electrode was rinsed with distilledwater and dried in the ambient conditions. Next the scotch tapewas sticked to the surface. The tape was detached rapidly. Theamount of modified material on the scratch tape indicated theadhevisity of the surface coating. Only after ten times of detachingof scotch tape from the PEDOT/PyBA/SiMo12 coating there wasobserved the beginning of decohesion process. This kind of adhe-sion test allows to assess how strongly the coating is attached to thesubstrate.

3. Results and discussion

3.1. Fabrication of polyoxometallate/PEDOT/PyBA coatings

The controlled electrodeposition of composite coatings ofPEDOT/PyBA with Keggin-type SiMo12 heteropolyanions wasconveniently achieved by voltammetric potential cycling inrespective modification mixtures, as described in Experimentalsection. Fig. 1A illustrates the growth of the SiMo12-PEDOT/PyBAcoating, as evidenced by the increase in the peak currents, duringthe first 22 voltammetric cycles. The further growth of the coatingduring potential cycling was much slower. Due to the possibility ofSiMo12 structural reorganization, the negative potential excursionswere limited to �0.2 V in Fig. 1A.


Fig. 2. (A) Cyclic voltammogram responses of the PEDOT/PyBA/SiMo12 (a) and SiMo12(c) coating on glassy carbon. For comparison, the response of the PEDOT/PyBA coating(b) is provided. (B) Inset: The CV responses of PEDOT/PyBA/SiMo12-coatings depositedon glassy carbon from modifying solutions of varying solution concentration:30 mmol dm�3 SiMo12 and 0.068 mol dm�3 EDOT (together with 10 mg of PyBA in1 dm�3 water) (a), 15 mmol dm�3 SiMo12 and 0.068 mol dm�3 EDOT (together with10 mg of PyBA in 1 dm�3 water) (b) and 30 mmol dm�3 SiMo12 and 0.1 mol dm�3 EDOT(together with 10 mg of PyBA in 1 dm�3 water) (c). Electrolyte: 0.5 mol dm�3 H2SO4.Scan rate: 50 mV s�1.

Fig. 1. Voltammetric generation of (A) a PEDOT/PyBA/SiMo12 coating and (B) a firstcycle coating deposited on glassy carbon from modification solutions (as describedin Experimental section). Scan rates: (A) 0.5 V s�1.

Fig. 3. (A) Cyclic voltammetric responses of the PEDOT/PyBA/SiMo12 coating on glassycarbon as recorded at different scan rates: (a) 0.002, (b) 0.004, (c) 0.006, (d) 0.008, (e)0.01, 0.02 (f) and 0.05 V s�1. (B) The inset: the dependence of the oxidation peakcurrent (at ca. 0.25 V) on the scan rate. The other conditions as in Fig. 2.

L. Adamczyk et al. / Materials Chemistry and Physics xxx (2014) 1e7 3

The result is consistent with the fact that the organic polymer(PEDOT) layers are usually generated on the electrode surfaceduring positive potential scans, whereas PyBA and SiMo12 poly-anions are simultaneously attracted during potential cycling andthe growth of the coating. The structures of polymer, PyBA andinorganic polyanions are expected to interact electrostatically witheach other because the oxidized PEDOT is positively charged, whilethe polyoxometallate and carboxylate groups of PyBA are anionic.Regardless of the nature of the interactions between the PEDOTand PyBA components, the PEDOT/PyBA units cannot be consid-ered as simple mixtures of these components [32]. The lattercharacteristics should lead to an increased stability and higherdensity of the composite coating. Regardless of whether PEDOT iselectrodeposited either in a dispersed granular form or as a truelayer in a composite coating, the fact that all the voltammetricpeak currents (being characteristic of the components) increasesteadily in Fig. 1A suggests a fairly homogeneous distribution ofSiMo12 polyanions within the coating.

When the composite SiMo12-PEDOT/PyBA coating (Fig. 2A-a)was scanned in the potential range from �0.2 to 0.8 V only in thesupporting electrolyte, the voltammetric response was character-ized by three sets of well-defined peaks. For comparison, we givethe response of a simple PEDOT/PyBA coating (Fig. 2A-b) that wasgenerated as a composite coating (Fig. 2A-a), with the exceptionthat instead of SiMo12, the equivalent amount of H2SO4 (in moles)was added to the modification solution. The data in Fig. 2A-a andA-b confirm the relatively low contribution of PEDOT/PyBA to theoverall electrochemical charging of the SiMo12-PEDOT/PyBA/coating. The diagram of the response of the SiMo12 coatingdeposited on glassy carbon has been presented in Fig. 2A-c. As itcan be seen from the attached figure, the SiMo12 cathodicand anodic peaks have a shape similar to that of the peaks origi-nating from the PEDOT/PyBA/SiMo12 coating; these peaks arecharacteristic for the SiMo12 Keggin structure. Fig. 2B presentscomparison of voltamperometric response of three types of coat-ings deposited on glassy carbon frommodifying solution of variousconcentration 30 mmol dm�3 SiMo12 and 0.068 mol dm�3 EDOT(together with 10 mg of PyBA in 1 dm�3 water) (Fig. 2B-a),15 mmol dm�3 SiMo12 and 0.068 mol dm�3 EDOT (together with10 mg of PyBA in 1 dm�3 water) (Fig. 2B-b) and 30 mmol dm�3

SiMo12 and 0.1 mol dm�3 EDOT (together with 10 mg of PyBA in1 dm�3 water) (Fig. 2B-c). The best properties revealed the PEDOT/PyBA/SiMo12 coating.


In other words, the voltammetric behaviour of the hybridcoating is dominated by the redox characteristics of poly-oxometallate. To avoid any interference that might be caused by thehydrogen evolution reaction at negative potentials, we limited ourexaminations to the threemost positive sets of voltammetric peaks.In the light of literature reports [35], the above redox reactionsshould be interpreted in terms of three consecutive two-electronprocesses that can be described as follows:

SiMo12VIO40

4� þ 2e� þ 2Hþ ¼ H2SiMo10VIMo2

VO404� (1)

H2SiMo10VIMo2

VO404� þ 2e� þ 2Hþ ¼ H4SiMo8

VIMo4VO40

4�

(2)


Fig. 4. SEM images of (A) a PEDOT, (B) a PEDOT/PyBA and (C) a PEDOT/PyBA/SiMo12-coatings on glassy carbon.


H4SiMo8VIMo4

VO404� þ 2e� þ 2Hþ ¼ H6SiMo6

VIMo6VO40

4�

(3)

Despite the fact that the coating was fairly thick (ca. 0.9 nm), theabove observations are consistent with the surface-type behaviour

Table 1Elemental analysis of bare stainless steel and the PEDOT/PyBA/SiMo12 composite coatin

Chemical elements

C Cr Si Ni

Bare stainless steel (at %) 0.17 12.60 0.34 0.25Stainless steel covered (at %) 44.28 e 0.58 e


of the system and the good dynamics of charge propagation in thecomposite system.

Fig. 3A shows the cyclic voltammetric responses of the SiMo12-PEDOT/PyBA coating, as recorded at different scan rates (from 0.02to 50.0 V s�1). When the peak currents are plotted versus the scanrate, the relationship is practically linear (effectively with a zerointercept) up to about 50.0 V s�1. Despite the fact that the coatingthickness (ca. 1 mm) exceeds by far the monolayer level, the systemstill exhibits the surface-type characteristics. This behaviour im-plies a good dynamics of charge propagation in the compositesystem.

3.2. Microscopic and spectrochemical properties

Using SEM (Fig. 4), we examined and compared the morphol-ogies of the PEDOT (A) band composite PEDOT/PyBA (B) andPEDOT/PyBA/SiMo12 (C) coatings (all deposited on glassy carbon).While the structures are granular in both cases, the actual grainsizes and distributions for PEDOT (Fig. 4A) and PEDOT/PyBA aredifferent in comparison to those of the PEDOT/PyYBA/SiMo12. Thesize of PEDOT/PyBA granules (Fig. 4B) seems to be smaller (10e50 nm) in comparison to cells occurring in the PEDOT/PyBA/SiMo12coating. These granules are characterized by a polygonal structure,a flat surface with a high degree of development and an averagesize of 40 mm. It is difficult to explain the difference in terms of theability of SiMo12 to occupy the spaces between PEDOT/PyBAgranules. It is more likely that the polyanions containing SiMo12units connected to the negatively charged PEDOT/PyBA nano-structures to form larger sub-microstructures. X-ray microanalysisof surfaces of the PEDOT/PyBA/SiMo12 composite coating and of thebare steel indicates the presence of carbon, oxygen, silicon, iron andmolybdenum, respectively (Table 1).

To obtain a better insight into the nature of interactions betweenPyBA and PEDOT within the composite coatings, we performed theex-situ FTIR measurements (by reflectance) of the glassy carbonelectrode surfaces as modified with PEDOT, Fig. 5(a), and PEDOT/PyBA, Fig. 5(c). Comparison of Fig. 5(b, c) with the reflectancespectrum of PyBA (as deposited on glassy carbon (Fig. 5b)) was alsomade. In the latter case, the PyBA coating was fabricated in amanner analogous to that of the composite coating, except thatonly PyBA component was present in the modified mixture.

Fig. 5a shows the FTIR spectrum of doped PEDOT, respectively.The peaks 1480e1583 cm�1 are attributed to the asymmetric Ca]

Cb stretching, and the peak at 1440 cm�1 is assigned to the sym-metric Ca]Cb stretching. The peak at 1363 cm�1 is ascribed to theCbeCb stretching, and peaks originated from the stretching in thealkylenedioxy group are observed at 1053e1208 cm�1. Interest-ingly, the absorption peak at 1640 cm�1 has been associated withthe doping level of PEDOT. Chevrot and co-workers assigned thepeak at 1640 cm�1 to the C]C bond, whose position depended onthe doping level of the polymer [36].

The system (Fig. 5b) is characterized by two well-defined peaksin the range of frequencies from 1400 to 1600 cm�1, being typicallyattributed to the pyrrole ring stretching modes [37]. A number ofbands typical of polypyrrole are visible in the film spectrum,including several bands between 900 and 1200 cm�1 and 1466,

gs.

Mn Cu V P Fe Mo O

0.30 0.13 0.04 0.02 86.20 e e

e e e e 0.14 6.80 48.20


Fig. 5. FTIR spectra of: (a) a PEDOT, (b) a PyBA and (c) a PEDOT/PyBA coatingsdeposited on glassy carbon.

Fig. 6. FTIR spectra of: (a) a SiMo12 in KBr and (b) PEDOT/PyBA/SiMo12-coatingsdeposited on glassy carbon.


1541 cm�1 bands [38]. Finally, the eCO stretching mode of thecarboxylic group has been observed at 1675 cm�1 in the spectrum(Fig. 5b). This frequency is characteristic of the undissociated group.

In contrast to PEDOT [20] or polyaniline [39], no sizeable cur-rents or peaks were observed during the voltammetric potentialcycling of PyBA. The PyBA formed a much thinner and practicallynon-electroactive coating on the electrode surface. It was alsonoticeable from the comparison of the infrared spectra of PyBA inKBr (for the sake of conciseness not shown here) to those of PyBAdeposited as an ultrathin coating on glassy carbon (Fig. 5b) that thePyBA had undergone some polymerization (around the pyrrolering) after voltammetric potential cycling. The spectrum of thePEDOT/PyBA coating (Fig. 5c) cannot be regarded as a simple su-perposition of the PEDOT (Fig. 5a) and PyBA (Fig. 5b) spectra. Forexample, the characteristic bands of PEDOTare not only shifted, butalso significantly altered in the incorporation of PyBA. Also, newbands (e.g., at 628 cm�1) are visible in the PEDOT/PyBA spectrum.


On the other hand, the characteristic band of PyBA at 2152 cm�1

disappears in the formation of the composite coating. Furthermore,the PEDOT and PEDOT/PyBA samples seem to differ in the oxidizedstate. Indeed, the latter spectrum contains so-called doping-induced bands in the range of 900e1300 cm�1 [40]. Thus, morespecific (than simple electrostatic) chemical interactions betweenPyBA and PEDOT can be expected within the composite coating.

Fig. 6a presents the spectrum of the heteropolyacidH4SiMo12O40 in KBr. The peaks characteristic of the Keggin struc-ture have been indicated in the figure (inside the ellipse), theabsorbance values for these peaks are given in Table 2. The presenceof SiMo12 within the composite coating was also evident from theex-situ FTIR (Fig. 6b) examination (by reflectance) of the glassycarbon electrode surfaces as modified with a thin (B) PEDOT/PyBA/SiMo12 coating.

It is reasonable to expect that the bands from 600 to 1041 cm�1

are originated from the SiMo12 (Fig. 6b). Since they were somewhatshifted in comparison to the data for the SiMo12 in KBr (namely tothe bands in Fig. 6a) at 1002, 959 and 760 cm�1 characteristic ofMo]O(terminal oxygen), MoeO(terminal oxygen) and MoeOeMostretching vibrations and [41e44], a strong interaction between thepolyanions and the polymer backbone can be postulated. Whereas,the sets at 615 and 916 cm�1 are characteristic of OeSieO and SieO.The band around 1620 cm�1 for SiMo12 is due to oxonium ions(H3Oþ) or, more likely, to the presence of “neutral” water.


Table 2Infrared bands of silicomolybdic acid.

Infrared bands of silicomolybdic acid

Wavenumbers (cm�1) Attribution of the band

1002 ns Mo]O terminal959 nas MoeO terminal916 nas Si]O760 nas MoeOeMo615 d OeSieO


Comparison of the spectra presented in Figs. 5aec and 6b clearlyindicates the presence of PEDOT and PyBA in the coating. Thecharacteristic sets of spectra shown in Fig. 5(aec) (marked with thevertical lines) for PEDOT/PyBA are very little shifted.

3.3. Corrosion test

In order to be able to comment on the protective properties ofthe PEDOT/PyBA/SiMo12 coating against local corrosion, potentio-dynamic curves were recorded in a solution of 0.5 mol dm�3

K2SO4 þ 0.5 mol dm�3 KCl at pH ¼ 2. To compare the protectiveproperties of the PEDOT/PyBA/SiMo12 coating, a PEDOT/PyBAdeposited on stainless steel coating is also shown in the figure. Asindicated by our previous studies reported in publications [45,46]and Fig. 7b, this coating does not have sufficient anticorrosiveproperties.

Such electrokinetic potential curves were considered for barestainless steel (Fig. 7a), PEDOT/PyBA (Fig. 7b) and PEDOT/PyBA/SiMo12 (Fig. 7c) coated steel, respectively. The addition of chlorideions to the corrosion solution causes a dramatic increase in anodiccurrents at E ¼ 0.09 V. The passivation of the steel is preventedunder such conditions. The PEDOT/PyBA/SiMo12 coating depositedon steel protects the substrate against corrosion and results inshifting the corrosion potential by approximately 0.32 V. Indeed, itcan be noticed that the corrosion potential has shifted from �0.67to �0.35 V following the application of the PEDOT/PyBA/SiMo12-coatings on the steel specimen.

As can be seen in Fig. 7 (curve c), the deposited coating inhibitsthe anodic processes. The pit nucleation potentials amount to0.09 V for the uncoated steel (a), and 0.39 V (b) for the steel with thePEDOT/PyBA/SiMo12 coating, respectively. So, the thermodynamicsusceptibility to the formation of pits is very different; the besteffectiveness in anticorrosion protection was exhibited by the

Fig. 7. Potentiodynamic curves for bare stainless steel (a) and for stainless steelspecimens coated with PEDOT/PyBA (b) and PEDOT/PyBA/SiMo12 (c).


PEDOT/PyBA/SiMo12 coating, when the SiMo12 concentration in themodifying solutionwas equal to 0.03mol dm�3. After taking out thespecimen from the corrosion solution, no pits were observed on itssurface. These observations enable one to conclude that compositecoatings composed of a conducting polymer with the addition ofPyBA and SiMo12 inhibit the access of aggressive anions to thesubstrate and greatly improve the effectiveness of anticorrosionprotection. Introducing negative-charge anions to the positivelycharged conducting polymer matrix leads to the neutralization ofthe anion-exchange properties of the coating. A definite improve-ment in the adhesion of the coating was also found.

Fig. 8 presents images of stainless steel, respectively, coated anduncoated with a coating after making a potentiokinetic test in asolution of 0.5 K2SO4 þ 0.5 KCl (pH ¼ 2). Numerous pits originatedfrom localized corrosion are visible on the stainless steel surface(Fig. 8A), whereas the steel covered with the coating is homoge-neous and does not show any changes. In Fig. 8C, a specimen afterbeing tested in a corrosion solution is shown, where part of thecoating has been removed from the steel in order to examinewhether any pits have formed under the coating. It can be noticedthat the coating protects the steel against pitting corrosion, as no

Fig. 8. Images of bare (A) and coated steel (B), respectively, as taken after potentio-kinetic tests. In (C), steel covered with a coating and fragments of the steel surface afterthe removal of the coating are shown.


Fig. 9. Open circuit potential vs. exposure times for bare stainless steel (a) and forstainless steel covered with PEDOT/PyBA (b), PEDOT/PyBA/SiMo12 (c).


pits are observed on the steel under the coating. Chloride anions arelargely blocked off after introducing negatively charged hetero-polyacid ions.

The steel covered with the PEDOT/PyBA/SiMo12 coating imme-diately after being immersed shows a stationary potential from thepassive range (Fig. 9). After 8.5 h, the coating begins to swell, butafter 10 h the potential decreases down to the values correspondingto active dissolution. Only after 10 h were sparse pits observedusing a converted microscope (for the sake of simplicity, the mi-crographs are not shown here). As can be seen from Fig. 9a, theuncoated steel, after being immersed in the sulphate solutioncontaining Cl� ions, immediately exhibits a potential of ca.�0.67 V,so it undergoes active dissolution.

4. Conclusions

The formation of a composite PEDOT/PyBA/SiMo12 coating oncarbon and steel substrates was demonstrated in the paper. PEDOT/PyBA serves as a stable and conductive matrix for the fast outer-sphere electron-transfer polyoxometallate redox sites. The com-posite coatings are also expected to be porous enough to permit theunimpeded flux of protons. The experiments were performedsuccessfully in this medium due to the stability of the PEDOT/PyBAand because of the fact that the counterion (Hþ) flux and theconcomitant protonation of polyoxometallate clusters facilitate thesystem’s reversible redox chemistry. The existence of the Kegginanions was confirmed by the SEM, FTIR and electrochemicalpreparation patterns. The PEDOT/PyBA/SiMo12 coating was char-acterized by a very good physicochemical stability.

The coatings exhibited very good adhesion to the substrate andensured fairly effective protection against pitting corrosion in astrongly acid medium containing chloride anions. An importantissue to note was that the PEDOT/PyBA composite served as a stableand dense host matrix for large silicium heteropolymolybdatesanions. Upon their incorporation within the composite coatings,the access of pitting-causing anions (chlorides) to the surface ofstainless steel is largely blocked due to the existence of electrostaticrepulsion effects.


Acknowledgements

The authors gratefully acknowledge the support offered by theNational Science Centre (Poland) under the Project No. 2011/03/B/ST4/02413.

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