On the improvement of stability of polypyrrole films in aqueous solutions

Synthetic Metals 157 (2007) 485–491

On the improvement of stability of polypyrrole films in aqueous solutions A. Alumaa, A. Hallik, V. Sammelselg, J. Tamm ∗ Institute of Physical Chemistry, University of Tartu, Tartu, Estonia Received 20 February 2007; received in revised form 14 May 2007; accepted 16 May 2007 Available online 2 July 2007

Abstract The stability of oxidised polypyrrole (PPy) doped with mobile anion (ClO4 − ) was studied in aqueous media. The influence of various components of the solution (dissolved O2 , indifferent electrolyte, organic compounds) on the stability of PPy was characterised by cyclic voltammetry and electron-probe X-ray microanalysis (EPMA). It was established that dissolved O2 favours the PPy overoxidation—the rate determining process of the electrochemical corrosion. The high concentration of the indifferent electrolyte on the other hand enhances the stability of the PPy in aqueous solutions to an essential extent. The more effective inhibitors of the PPy corrosion are the surface-active anions (dodecyl sulfate) and polar aromatic compounds (benzyl alcohol, benzonitrile). © 2007 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Stability; Degradation; Overoxidation; Adsorption; ␲-Electronic interaction

1. Introduction During the past decades, electrically conducting polymers, particularly polypyrrole (PPy), have attracted considerable attention. Many promising practical applications based on their novel electronic, electrochemical and optical properties have been proposed [1]. The stability of conducting polymers is a widely discussed topic [2–10]. Generally, the changes in PPy properties with time are caused by various processes as overoxidation [2–10], deprotonation [11] and deactivation by solvent as a reducing agent [12–14]. The overoxidative degradation is mainly characterised by splitting of conjugated double bonds and macromolecular crosslinking and/or by the attack of nucleophiles from the solution on the charged polymeric lattice. The dominating factor of this process is the solution composition. In weakly nucleophilic media the crosslinking mechanism will prevail over the nucleophilic one. With the increase of medium nucleophilicity, the contribution of the latter, and thus, the degree of the polymer functionalization rises [2]. The changes in the chemical content of the polymer are proved by spectroscopic studies [9,10]. The ∗

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model of the electrochemical behaviour of conducting polymers after an electrochemically stimulated conformational relaxation treatment was developed by Otero et al. [15] and also used for characterization of polypyrrole degradation by its molecular parameters [16]. Water as a solvent with a high value of donor number (DNbulk = 42)1 [17] is quite aggressive in respect to polypyrrole as a Lewis acid. Infrared and Raman spectroscopic studies prove that the overoxidation of PPy in aqueous media results in the formation of hydroxyl and carbonyl groups in the polymer chains [9], and in irreversible loss of conjugation and electrical conductivity, at which these processes are more expressed at higher pH [10,11,18]. According to Beck et al. [6–8], the corrosion of conducting polymers in aqueous media proceeds in two steps. The initial rapid electrochemical mechanism includes the rate determining overoxidation process

(1)

1 DN bulk is the measure of bonding ability of bulk solvent defined in reference of oxovanadium(IV)-acetylacetonate [16].

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coupled with cathodic dedoping of the polymer. A rather slow second process is caused by chemical attack of nucleophiles (water molecules) present in a swelled solid polymer at the radical cationic centres. The rate of overoxidation of PPy is higher at higher electrode potential and at higher pH of the solution. PPy dergrades remarkably already during short-term ageing (1 h) at potentials as low as +0.5 V versus SCE [9]. An improvement of PPy stability can be achieved by addition of hydroqinone into the solution [19]. More stable compared to the polymer doped with inorganic anions are PPy films doped with aromatic sulfonates [20,21] or dodecyl sulfate [22]. In Ref. [23] the influence of oxygen and carbon dioxide on the electrochemical stability of poly(3,4-ethylenedioxythiophene) and PPy were studied. The stability of conducting polymers will be a key factor in their practical applications, in particular in the case of their prolonged use in aggressive media (corrosion protection of metals, artificial muscles) [1]. While the influence of the solution pH on the stability of PPy is widely studied [11,18], then the effect of other factors has found only reserved attention. In this paper, the behaviour of PPy films soaked in aqueous solutions with different additions has been studied in order to elucidate the influence of various components of the solution (molecular oxygen, indifferent electrolyte and surface-active organic compound) on the stability of the electrogenerated PPy films. PPy/ClO4 − as the representative of PPy films doped with mobile inorganic anion of low nucleophilicity was studied. 2. Experimental The PPy films were electrogenerated at constant current density (2 mA cm−2 ) on platinum wire (0.1 cm2 ) in 0.1 M aqueous pyrrole solution containing 0.1 mol dm−3 of the supporting electrolyte (NaClO4 ). In all cases the polymerisation charge density was 2 C cm−2 (estimated film thickness ∼5 ␮m). The open-circuit ageing of the films (24 h) was performed at room temperature in water with the addition of electrolyte (NaClO4 ) or some organic compounds (benzene, n-hexanol, diisopropyl ketone, benzyl alcohol, benzonitrile, camphor, sodium dodecyl sulfate, tetrabutylammonium perchlorate) with or without de-aeration of the medium. The changes in the composition and electrochemical properties of PPy films were examined by cyclic voltammetry, electron-probe X-ray microanalysis (EPMA), scanning electron micrography (SEM). The JSM-840 (JEOL Ltd., Japan) equipment with EDS System Voyager (Noran, USA) was used [24]. Cyclic voltammograms (CV) were recorded on bipotentiostat AFCBP1 (Pine, USA) in a two-compartment cell. The saturated Ag/AgCl as reference electrode was used. The counter electrode was a Pt-wire forming a ring around the working electrode. CV were measured in 0.1 M NaClO4 after saturation of the solution with Ar (99.999%) for 10 min. The first cycle was started at the stationary potential Est of the PPy electrode in the measuring solution scanning towards negative direction, and thereupon, in reversed direction. The following cycles were taken beginning at 0.5 V after polarisation during 5 min. The surface tension of solutions of the organic compounds was determined by a Traube’s stalagmometer, calibrated by water. The surface tension was cal-

culated proceeding from the number of drops detached in air [25]. Pyrrole (Aldrich) used for synthesis of PPy films was purified by distillation over CaH2 under vacuum and kept refrigerated in the dark. Analytical grade salts, benzene (99.5%), n-hexanol (98%), diisoproyl ketone (98%), benzyl alcohol (98%), benzonitrile (98%), sodium dodecyl sulfate (>99%), tetrabutylammonium perchlorate (98%) and Milli Q+ water, were used for preparing the solutions. Camphor was additionally purified by sublimation. 3. Results and discussion 3.1. Effect of dissolved dioxygen on the ageing of PPy The ageing of PPy/ClO4 films in water causes essential changes in its electrochemical properties depending whether the medium is de-aerated or not (Fig. 1). The ageing of PPy in water is accompanied by the essential decrease of the stationary potential Est of the electrode, besides Est is greater in case of ageing in de-aerated medium (∼0.5 V). The reduction process of aged PPy is shifted to more negative potentials and in addition of anions also cations participate in the overall ionic charge transport to a great extent. Due to this, the aged PPy film is essentially less active in aqueous solution of large hydrated multicharged cations (La3+ ) than in solution of small monocharged cations. From the difference of the film reduction charge in 0.1 M NaClO4 and 0.1 M La(ClO4 )3 the quota of Na+ cation participation in the total electrochemical reduction of PPy aged in H2 O (air) was estimated to ∼40% in the first cycle, diminishing to ∼10% in the second cycle. It should be mentioned, that both anions and cations participate in the ionic charge transport also in the redox processes of the as-prepared PPy films [26], but the contribution of Na+ ions in reduction of PPy/ClO4 (5 ␮m) in 0.1 M NaClO4 solution is very low (<5%).

Fig. 1. Cyclic voltammograms of PPy/ClO4 films measured in 0.1 M NaClO4 : (1) as-prepared film; (2) film aged 24 h in de-aerated (with Ar) water; (3) film aged 24 h in water not purged with Ar.

A. Alumaa et al. / Synthetic Metals 157 (2007) 485–491 Table 1 The retention of doping level and electroactivity of PPy/ClO4 film (in % in respect to the as-prepared film) after 24 h ageing in water or aqueous solutions of NaClO4 and after following cycle in 0.1 M NaClO4 from Est to −1.0 V and back to 0.5 V Concentration of NaClO4 and conditions 0 (Ar) 0 (air) 0.1 M (Ar) 0.001 M (air) 0.1 M (air) 3 M (air) 0.1 M (air, 0.5 V) 3 M (air, 0.5 V)

Retention of doping level

Retention of electroactivity

24 h

24 h + 1 cycle

24 h

24 h + 1 cycle

36 42 73 51 70 84 62 82

80 65 88 70 78 87 67 86

59 81 78 83 84 87 83 86

92 86 98 88 88 92 87 91

The changes in the electrochemical activity of the film after its ageing in water are presented in Table 1 (the first two rows). The retention of the doping level and of electroactivity of the aged films were determined immediately after ageing and after the first cycle of the aged film in 0.1 M NaClO4 . The retention of the doping level was calculated from EPMA data expressed as the retention of ClO4 − ions in mol% and the retention of electroactivity from the reduction charge of the first and second CV of the aged in water PPy films. The determined parameters of the aged PPy films are expressed as percentage in respect to that for as-prepared PPy film. The 24 h ageing of PPy/ClO4 in water causes essential decrease of the doping level (Table 1, the first two rows), what considerably exceeds the decrease of the electroactivity of the PPy film, at which this difference is greater in medium not purged with Ar. The great differences between the values of retention of the doping level and electroactivity point to the remarkable part of electroactivity of the aged PPy, that is not related to the common dedopation of PPy/anion film. This may be caused by quinoide structure of the aged PPy film able to undergo partial redox transitions. As shown in Refs. [6–8], the anodic process of the fast electrochemical corrosion and following slow chemical corrosion process results in the pyrrole units functionalized with hydroxide and carbonyl groups and in following formation of quinoide structure of PPy chain with damaged long-range conjugation in the polymer. According to our results, dioxygen dissolved in water favours the irreversible formation of this kind of structure. As results from Table 1, the loss of the doping level of PPy films during ageing can be partly restored by film cycling in the solution of an electrolyte. In accordance with the electrochemical corrosion mechanism [6–8], it is possible to reactivate the PPy sites that lost their charge due to cathodic dedoping corrosion process during film ageing. In case of ageing in free from dioxygen medium, the percentage of doping restoration can exceed 40%. Although the essentially lower percentage of restoration in case of ageing in H2 O (air) points mainly to the participation of dioxygen in irreversible anodic overoxidation of PPy, it must be stressed that O2 also participates in cathodic corrosion processes as the cathodic reduction of O2 , occuring

487

in parallel to the cathodic PPy dedoping process. So, the above mentioned processes can also explain the higher retention of the doping level after long-term ageing in H2 O (air) in comparison with medium purged with Ar (Table 1). The chemical changes in the aged PPy films are accompanied by the alteration in the morphological structure of the polymer. According to SEM studies it seems that upon ageing in water the globules of PPy became smaller and are more densely packed as it described also in Refs. [11,27]. 3.2. Effect of the indifferent electrolyte on the ageing of PPy As seen from Table 1, the addition of NaClO4 as a supporting electrolyte with anions of very low nucleophilicity to the test solution hinders the decrease of doping level of PPy during its ageing in aqueous media. It is interesting that this influence becomes evident already at very low concentrations (10−3 M) of the salt, and appears at high electrolyte concentrations on a large scale both in dioxygen-free solutions and in solutions containing dissolved air. On the other hand, despite the remarkable changes in the shape of CV curves with ageing (Fig. 2), the influence of the electrolyte addition into the ageing solution on the change of electroactivity remains essentially lower. The protection effect of PPy/ClO4 by indifferent electrolyte is mainly determined by the concentration of the electrolyte anion in the solution and not by the nature of the cation used (Li+ , Na+ , Cs+ , Mg2+ ). The following reasoning can be used to explain the protection effect of the indifferent electrolyte on the corrosion of PPy: 1) decrease of the open-circuit potential of the ageing PPy/ClO4 electrode with the increase of the anion activity in the ageing solution [28,29]; 2) replacement of OH− and HCO3 − (from dissolved CO2 ) ions and water molecules (orientated with oxygen atom in direction of polymer) as nucleophilic particles in PPy pores with

Fig. 2. Cyclic voltammograms of PPy/ClO4 films measured in 0.1 M NaClO4 : (1) as-prepared film; (2) film aged 24 h in 3 M NaClO4 ; (3) film aged 24 h in 0.001 M NaClO4 .

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Fig. 3. The change of potential of PPy/ClO4 electrode during ageing: (1) in 0.001 M NaClO4 ; (2) in 0.1 M NaClO4 ; (3) in 3 M NaClO4 .

electrolyte ions by the process of rearrangement of double layer on the polymer/electrolyte solution boundary. The potential of the PPy modified electrode is determined mainly by the ratio of oxidised and reduced forms of PPy, and by the potential jump on the PPy/electrolye solution boundary, which in the case of PPy doped with mobile anions is expressed by the equation [28]:       RT RT aox − E = E0 + (2) ln ln aan F ared F as the simplification of general equation presented by Lewenstam et al. [29]. From Fig. 3, it becomes evident that in the beginning of ageing of PPy/ClO4 film (during the first 4 h) the electrode potential is the lower the higher is the electrolyte concentration of the ageing solution. At lower potentials, particularly at E < 0.4 V, the overoxydation as the rate determining process of the electrochemical corrosion of the PPy proceeds essentially slower [9]. More prolonged ageing causes the levelling of E of PPy/ClO4 electrodes in solutions with different electrolyte concentrations and finally the differences in E values are mainly determined by the doping levels of the aged PPy films, resulting in the lowest potential of the PPy electrode, aged in 0.001 M solution as the most corrosive media studied in this work. Therefore, the former reason for the protection effect of the indifferent electrolyte can be realised only in the beginning of the ageing process and, as proved by additional experiments (holding of PPy electrode at constant potential E = 0.5 V in the electrolyte solutions with different concentrations), is not principal. 3.3. Adsorption of organic compounds on the PPy film To understand the influence of adsorption of organic compounds on the degradation of PPy film, empirical studies of the adsorption of some organic compounds with different properties on the as-prepared PPy/ClO4 film were carried out. Benzene as

Fig. 4. Cyclic voltammograms of as-prepared PPy/ClO4 film measured: (1) in 0.1 M NaClO4 ; (2) in 0.1 M NaClO4 + 0.023 M n-hexanol; (3) in 0.1 M NaClO4 + 0.3 M benzyl alcohol.

a non-polar compound, n-hexanole and diisopropyl ketone as polar compounds, benzyl alcohol and bezonitrile as polar aromatic compounds, camphor as a polar compound which forms on the air/solution and Hg/solution interfaces forms a condensed film [30] and NaDDS and TBAClO4 as organic surface-active electrolytes were chosen for this study. The changes in CV curves of as-prepared PPy/ClO4 electrode measured 5 min after submersion in the 0.1 M NaClO4 solution with the addition of molecular organic compounds indicate the adsorption of the organic compounds on the polymer surface, at which its extent depends on the nature of the compound and the electrode potential (Fig. 4), and also, on the concentration of the additive. The adsorption capability is expressed by the inhibition of the reduction and reoxidation processes and by the increase of the extent of cation participation in the reduction process at higher negative potentials. The influence of benzene, as a non-polar compound with relatively low surface activity at the air/solution boundary (Fig. 5), upon the proceeding of the PPy redox process is very low. The maximum depression of the reduction current of the CV (at E = −0.3 V) measured in the solution containing 0.02 M benzene is essentially lower than the one of the hexanol solution with identical squeezing out effect (comparison at equal lowering of surface tension σ of water). So, the depression of the reduction current at E = −0.3 V in solution of benzene and n-hexanol (σ = 12 mN m−1 ) are 7 and 34%, respectively. Low surface activity of benzene at the PPy/solution boundary is related to its non-polarity in case of oxidised form of PPy as a highly polar medium (concentration of dopant ions in the as-prepared PPy/ClO4 film is very high extending up to 5 M), and to the lack of specific component in the adsorption energy of benzene in case of the reduced form of PPy. n-Hexanol as compound with high surface activity on the air/solution boundary (Fig. 5) has essential influence on the proceeding of the PPy reduction process (Fig. 4). This influence depends essentially on the electrode potential (polarity of the PPy medium). Thus, the depression of the reduction current

A. Alumaa et al. / Synthetic Metals 157 (2007) 485–491

Fig. 5. The decrease of surface tension of water depending on the concentration logarithm of the organic compound: (1) benzene; (2) n-hexanole; (3) diisopropyl ketone; (4) camphor; (5) benzyl alcohol; (6) benzonitrile; (7) TBAClO4 ; (8) sodium dodecyl sulfate.

increases by the potential lowering down to E = −0.3 V, in the solution of 0.05 M n-hexanol up to 65% of the current in the solution without the addition. The subsequent lowering of the electrode potential leads to the formation of the reduction current maximum (Fig. 4), related to the participation of cations in the reduction process. For that reason the height of this maximum grows with the increase of the electrolyte concentration in the measuring solution up to 1 M. In the solution of n-hexanol essentially inhibited is also the reoxidation process. Therefore, for the complete reoxidation of the PPy/ClO4 film its short-term (10 min) polarisation at higher positive potentials is necessary. The adsorption of polar organic compounds on PPy is influenced also by the nature of the functional group. The comparison of the CV curves measured in the solutions of n-hexanol and diisopropyl ketone at equal squeezing out effect of the adsorbate (σ = 30 mN m−1 ) points to the lower surface activity of diisopropyl ketone in oxidised PPy and to the higher surface activity in reduced PPy. In the solution of diisopropyl ketone at higher negative potentials (E < −0.5 V) the essential resistance of reduction process due to adsorbate was observed. The reduction process in this solution proceeds differently from the one occuring in the solution of n-hexanol, that is without the complementary current maximum characteristic to CV curves taken in the presence of n-hexanol. At the same time, in the solution containing diisopropyl ketone, the part of polymer film remains non-reduced (in 0.04 M solution in extent of 30%) during the reduction cycle. As follows from Fig. 4, the polar aromatic compound inhibits the redox process of PPy/ClO4 essentially more than the polar non-aromatic compound. At that the considerable depression of the reduction current takes place immediately in the beginning of the CV at quite high positive potentials. From comparison of CV curves measured in the solutions of benzyl alcohol and n-hexanol with equal squeezing out effect of the adsorbate (σ = 30 mN m−1 ), it becomes evident that at E = 0.4 V the rel-

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Fig. 6. Cyclic voltammograms of as-prepared PPy/ClO4 films measured: (1) in 0.1 M NaClO4 ; (2) in 0.1 M NaClO4 + 0.001 M sodium dodecyl sulfate.

ative depression of the reduction current in solution of benzyl alcohol is six times greater than in solution of n-hexanol (the reduction current measured in the solution of benzyl alcohol is ∼2 times lower). At the same time, at E = −0.3 V this relation of the reduction current depression is only 1.2. Similar enhanced surface-active effect in respect to the oxidised form of PPy/ClO4 has been established also in the case of benzonitrile in spite of its relatively low surface activity on the air/solution boundary (Fig. 5). Analogous specific influence was also observed in case of the adsorption of aromatic compounds on metallic electrodes (Hg, Bi) [31], and was explained as being the result of the ␲electronic donor–acceptor interaction between carbon atoms of the benzene ring and the positive charges on the electrode surface. Likely, this interaction takes place also in the case of the adsorption of polar aromatic compounds on the oxidised form of PPy. The influence of camphor on the kinetics of the PPy redox process is very weak. The little inhibition of the reduction process takes place only at E < −0.3 V. It seems that differently from air/solution and metal/solution interfaces [30] the formation of the condensed camphor film on the PPy surface does not occur. As follows from Fig. 6, already a very small addition of the surface-active anion (0.001 M dodecyl sulfate) in 0.1 M NaClO4 solution in spite of its low squeezing out effect (σ is only 6 mN m−1 ) inhibits the redox process of PPy to an essential extent. In contrast to the molecular surface-active compounds and particularly to the surface-active cation (TBA+ ), in the presence of dodecyl sulfate ions the depression of the reduction current preferably takes place just at higher positive potentials [32], it means, in the beginning of the CV scan. This effect is related with the adsorption of DDS− ions on the positively charged PPy surface. As is confirmed by EPMA analysis, bulky DDS− ions cannot penetrate into PPy/small dopant film and replace the original anion even at its respectively high concentrations. At the same time the strong influence of dodecyl sulfate ions on the PPy reoxidation process was observed. As seen from

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Table 2 The retention of doping level of PPy/ClO4 film after 24 h ageing in aqueous solutions of organic compounds and after following cycle in 0.1 M NaClO4 from Est to −1.1 V and back to 0.5 V Solutiona

H2 O 0.05 M n-hexanol 0.04 M diisopropyl ketone 0.3 M benzyl alcohol 0.03 M benzonitrile 0.001 M NaDDS 0.1 M NaClO4 0.1 M NaClO4 + 0.03 M benzonitrile

σ (mN m−1 )b

0 39.2 30.2 30.7 11.2 5.9 0 11.2

Retention of doping level 24 h

24 h + 1 cycle

42 60 51 73 71 86 70 85

65 91 69 90 86 91 78 90

a Solutions without de-aeration of the medium. Concentrations of molecular organic compounds are ∼80% of saturation concentration. b σ: lowering of surface tension of water caused by adsorption of organic compound, as a measure of squeezing out effect.

Fig. 6, the reoxidation process of PPy is almost completely inhibited, indicating that the adsorption layer of the DDS− ions on the reduced PPy surface is practically impenetrable to ClO4 − anions. The film redox-activity can be restored by decreasing the concentration of NaDDS in the measuring solution (0.1 M NaClO4 ) down to 10−5 M. From the viewpoint of the PPy corrosion inhibition such organic compounds are most interesting that are able to adsorb on the PPy surface at higher anodic potentials (E = 0.5–0.3 V), that is in the potential region of corrosion of PPy in its oxidised form. According to the adsorption studies described above, these substances are first of all the polar aromatic compounds (benzyl alcohol, benzonitrile) and the surface-active anions (DDS− ). 3.4. Influence of adsorption of organic compounds on the PPy corrosion As follows from Table 2, in case of 24 h ageing of PPy/ClO4 in solution of molecular organic compounds and NaDDS, the retention of PPy doping level is remarkably higher than the one in pure water due to the screening of PPy surface by organic molecules or anions against the attack of nucleophilic particles. As expected, the best protection effect was attained with NaDDS due to very high surface activity of the DDS− anion in respect to oxidised PPy already at very low adsorbate concentrations and low squeezing out effect. Remarkable effect was also observed with polar aromatic compounds, and in case of benzonitrile once again at relatively low squeezing out effect. Noteworthyly high is the retention of doping level after longterm ageing and following cycling of PPy film in the case of n-hexanol and benzyl alcohol. As was found in Refs. [12,13], alcohol was not fully chemically inactive agent in respect to oxidised PPy. In the vapour of methanol as reducing agent, the resistance of PPy film was found to increase. During longterm ageing of PPy in solutions of alcohols (benzyl alcohol, n-hexanol), probably beside the film corrosion to certain extent also the deactivation process of PPy by adsorbate molecules takes place, that may be restored in reoxidation cycle.

The organic molecules adsorbed during ageing in PPy film cannot be completely removed by rinsing or soaking in water. These molecules cause the additional kinetic resistance of both reduction and reoxidation process and can be removed only by multiple cycling. On the other hand, the surface-active anions (DDS− ) can be easily removed from film already by its rinsing with water. 4. Conclusions In aqueous medium the stability of PPy doped with a mobile anion (ClO4 − ) can be influenced by various additives. The influence of dioxygen dissolved in water is complicated. On the one hand, dioxygen favours the PPy overoxidation as the anodic rate determining process of the electrochemical corrosion of the polymer, and therefore, the irreversible changes in the chemical composition of the polymer. On the other hand, dioxygen also takes part in the cathodic corrosion process—the cathodic ionisation of dioxygen, that proceeds in parallel to the cathodic PPy undoping. The essential protection effect regarding PPy/ClO4 corrosion in aqueous medium, was achieved with the addition of indifferent electrolyte (NaClO4 ) and that due to the rearrangement of the double layer on the PPy/solution boundary (replacement of nucleophilic particles with anions of low nucleophilicity). The high retention of doping level (over 80%) of PPy/ClO4 after long-term (24 h) ageing in solution with high electrolyte concentration (c ≥ 3 M) was not decreased remarkably even by multiple prolonging of the exposure time. The corrosion of PPy can be inhibited also by organic compounds due to the screening of its surface against the attack of the nucleophilic particles by organic molecules. The more effective inhibitors for the oxidised PPy are the organic compounds that are able to adsorb on PPy surface at higher anodic potentials, such as the surface-active anions (DDS− ) and polar aromatic compounds (benzyl alcohol, benzonitrile). The high surface activity of polar aromatic compounds in respect to the oxidised PPy is the result of the ␲-electronic donor–acceptor interaction between carbon atoms of benzene ring and the positive charges on the polymer chain. Unfortunately, the use of organic compounds as the corrosion inhibitors of PPy is not perspective in all cases as they also inhibit the redox process of the polymer. The most perspective way to protect PPy in aqueous solutions against the corrosion is the use of indifferent electrolytes in the reaction medium with concentration as high as possible. In addition to the direct influence of the indifferent electrolyte, its influence is also related (particularly at celectrolyte > 1 M) to the reduction of the concentration of O2 in the solution. As shown, the last is favourable to the PPy overoxidation as the anode process of the electrochemical corrosion. Acknowledgement The financial support of the Estonian Science Foundation under Project No. 6651 is acknowledged.

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