Journal of Electroanalytical Chemistry, 368 (1994) 33-41 33

Mechanism of ionic and redox sensitivity of

Part 2. Experimental study of polypyrrole

Johan Bobacka, Zhiqiang Gao and Ari Ivaska

p-type conducting polymers

Laboratory of Analytical Chemistry, ibo Akademi University, SF-20500 Turku-ibo (Finland)

Andrzej Lewenstam l

Laboratory of Analytical Chemistry, AObo Akademi University, SF-20500 Turk&b0 (Finland) and Faculty of Material Science and Ceramics, University of Mining and Metallurgy, 30-059 Cracow (Poland)

(Received 6 January 1993; in revised form 12 July 1993)

Abstract

The potentiometric response of polypyrrole films is studied. The films are electrochemically deposited on platinum. The effects of film thickness, counter-anions and film conditioning on the response are studied. It is demonstrated that the response can vary from anionic to cationic, depending on the charge-compensating counter-anion (doping anion) incorporated during electropolymer- ization. Furthermore, the ionic response may be altered by redox couples in the solution, and under some conditions polypyrrole may also act as a pure redox electrode. The mixed ionic and redox sensitivities of polypyrrole are explained by considering both ion and electron transfer at the polymer 1 solution interface. The experimental results are related to a theoretical model developed for p-type conducting polymers

1. Introduction

During the last few years, the emphasis in research on conducting polymers has been on applications. Con- ducting polymers are also,of interest as sensor materi- als in the field of analytical chemistry [1,2]. Elec- tropolymerization of heteroaromatic compounds [3], and in particular polypyrrole (PPy) [41, offers new possibilities in sensor design. PPy is conveniently de- posited from aqueous solutions [5,61, although the poly- merization process is not known in detail because of its complicated mechanism. Furthermore, PPy is relatively stable in its oxidized doped state as long as overoxida- tion is avoided [71. Electropolymerization of pyrrole on various substrate materials can be used to obtain PPy directly in its oxidized and relatively stable form, and this certainly contributes to its potential usefulness as a sensor material, e.g. in an ion-selective electrode [2,8].

l To whom correspondence should be addressed at Abe Akademi University.

0022-0728/94/$7.00 SSDZ 0022-0728(93)03081-Y

The potentiometric response of PPy to monovalent anions, such as Cl- 19-121, Br- [lo], NO; [10,13,141, BFJ [12] and ClO; [15], has been reported. PPy electrodes have a fast response but unfortunately the selectivity is rather poor. It has been shown that the potentiometric response is determined by the PPy layer and not by the substrate (Pt) [14], which is of funda- mental importance when discussing the response of PPy. In most of the literature [lo-12,14,15] the slopes of the responses were found to be smaller than the theoretical value of -59 mV per decade at 25”C, indicating that the response is not purely anionic but obviously also includes some contribution from the cation [12,161. The cationic response of PPy was con- firmed with PPy synthesized with naphthalene- sulphonates as counter-ions [17]. These earlier results indicate that the potentiometric response of PPy may be influenced by anions and cations, depending on the synthesis conditions. Since PPy is electronically con- ducting, the potentiometric response may also be com- plicated by electron transfer at the PPy Isolution inter- face if redox couples are present in solution [18]. In

0 1994 - Elsevier Sequoia. All rights reserved

34 J. Bobacka et al. / Ionic and redox sensitiviv of p-type conducting polymers 2

view of the many factors involved in the potentiometric response of PPy, it is not surprising that there are some variations in the results reported in the literature. This also illustrates the need for a theoretical description of the response mechanism.

Some progress has been made in describing the mechanism of the potentiometric response of conduct- ing polymers. The simplest way is to use the Nemst equation which relates the electrode potential to the activity of either the anion or the cation by assuming that the oxidation level (doping level) of the polymer remains constant [19]. The influence of both anions and cations on the potential has been taken into ac- count by using a modification of the Nernst equation, again assuming that the oxidation level of the polymer is constant [16]. However, it is known that the oxida- tion level of a conducting polymer is closely related to its potential [20]. Therefore the possible variations in the oxidation level should be taken into account when describing the potential of a conducting polymer elec- trode. Indeed, the potential distribution across a con- ducting polymer membrane in contact with the elec- trolyte solution has been studied by considering the variations in the oxidation level of the polymer. How- ever, it was assumed that the anions were the only species transferred between polymer and solution. Thus the transfer of cations and electrons at the polymer I solution interface was neglected [21]. In order to clarify this situation, a more general theoretical model de- scribing the potentiometric response of p-type conduct- ing polymers was developed by us and presented in Part 1 of this series [22]. This model includes the transfer of anions, cations and electrons at the poly- mer I solution interface, and the possible changes in the oxidation level of the polymer are also taken into account.

In the present paper we report some experimental results for the potentiometric response of PPy. It is demonstrated that the open-circuit potential (rest po- tential 1161) of PPy may be determined by anions, cations and/or redox couples in solution. The mecha- nism of the potentiometric response is discussed and related to the theoretical model derived in Part 1 [22]. The equations derived are used to explain the PPy responses observed under different experimental con- ditions.

2. Experimental

2.1. Chemicals Pyrrole (Merck, > 98%) was purified by double

distillation and stored at low temperature protected

from light. Sodium dodecylsulphate (NaDS) (Fluka, MicroSelect), NaBF, (Fluka, purum), sodium p- toluenesulphonate (NaTOS) (Aldrich, 95%), and KCl, KI, KSCN, K,[Fe(CN),], K,[Fe(CN),l * 3H,O, CaCl, . 4H,O, indigo carmine (IC) and ammonia solution 25% (Merck, p.a.> were all used as received. Distilled deion- ized water was used to prepare the solutions.

2.2. Instrumentation Electrochemical polymerization of pyrrole was

mostly performed using a PAR model 273 potentio- stat-galvanostat. In some cases a PAR model 174A polarographic analyser or a model 2000 potentiostat- galvanostat connected to a model S-5720B frequency response analyser (NF Circuit Design Block Co. Ltd.) was employed. The polymerizations were performed in a single compartment three-electrode electrochemical cell. The working electrode was a Pt disc (area, 0.07 cm2) and the reference electrode was a Ag/AgC1/3 M KC1 electrode connected to the cell via a bridge filled with 0.1 M supporting electrolyte solution. A glassy carbon rod was used as the auxiliary electrode.

Potentiometric measurements were made using ei- ther an Orion Research expandable ion analyser (EA 940) or a Perkin Elmer 56 recorder. The PPy electrode was used in conjunction with the reference electrode described above.

Energy dispersive analysis of X-rays (EDAX) was performed using a Leica Cambridge Ltd. Stereoscan 360 scanning electron microscope.

2.3. Electrode preparation Before polymerization the Pt disc was polished with

0.3 pm alumina and rinsed with water. PPy was elec- trochemically polymerized on the Pt disc in aqueous supporting electrolyte solution containing 0.1 M monomer by cycling the potential between 0 and + 1 V at a scan rate of 20 mV s-l. It has been found that potential cycling for 45 min under these conditions produces an average film thickness of 7-8 pm when NaBF, is used as the electrolyte salt [12]. In this work polymerization times between 30 and 45 min were generally used. In some cases PPy was prepared gal- vanostatically using a current density of 1 mA cmp2 or potentiostatically at +0.8 V. The electrolyte concen- tration was 0.1 M, except for indigo carmine when a saturated solution (ca. 1 g per 100 ml) was used. The solution was purged with nitrogen before polymeriza- tion and nitrogen was passed over the solution during polymerization. The anion of the electrolyte salt used during polymerization, i.e. the charge-compensating counter-anion or doping anion, is given in parenthesis when discussing the different PPy electrodes. Thus PPy doped with the anion X- will be denoted by Ppy(X).

J. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2 35

2.4. Potentiometric measurements Before potentiometric measurements the PPy elec-

trodes were conditioned for at least 6 h in 10-i M solution of the salt to be tested. During measurements, the potential was read after 2 min in each solution in the concentration range 10-‘-10-4 M. The ionic strength of the solution was not adjusted to a constant level because of the low selectivity of the PPy elec- trode. Film conditioning and potentiometric measure- ments were performed at room temperature (24 + 1’0 and 0, was not removed from the solutions. Any deviation from this procedure is noted in the text.

2.5. ELMX measurements EDAX was performed in order to study the ion

exchange behaviour of PPy. Four PPy films were de- posited on indium-tin oxide (ITO) electrodes by cycling the potential between 0 and + 1 V for 45 min in 0.1 M NaBF, + 0.1 M monomer solution. After polymeriza- tion the potential was kept at +O.S V in order to ensure that the films were in their oxidized state, i.e. doped with BFC anion. The films were then condi- tioned in 0.1 M solutions of KCl, KI, KSCN and NaBF, for 10 days.

Two PPy films were prepared by galvanostatic poly- merization (j = 1 mA cm-’ for 30 mini using a satu- rated solution of indigo carmine as the supporting electrolyte. After polymerization these films were con- ditioned in 0.1 M solutions of KC1 and CaCl, for 10 days. The bulky indigo carmine was expected to be more strongly bound to the polymer film than the small BFC anion.

After conditioning all electrodes were rinsed in wa- ter and ethanol and then dried at ca. 105°C for 4 h before performing the EDAX. The analysis depth was approximately 1 pm.

3. Results and discussion

3.1. Effect of film thickness The polymerization time, and consequently the film

thickness, was found to influence the potentiometric response of PPy(BF,). Figure 1 shows that the slope of the response curve increases with polymerization time up to about 30 min after which it remains almost constant. Furthermore, the linearity of the response curve is improved when the film thickness is increased. However, because of the risk of overoxidation exces- sively long polymerization times should be avoided [ 121.

It can be seen in Fig. 1 that the slope is smaller than the theoretical value of -59 mV per decade as would be expected for a pure anionic response. The reduced slope may be due to coinsertion of cations (Na+) into the polymer, indicating that PPy(BF,) is sensitive not

J 0 10 20 30 40 50 60 70

Polymerization time / min

Fig. 1. The slope of the potentiometric response of PPy(BF,) in NaBF, solution showing the influence of the polymerization time when NaBF, was used as the electrolyte salt: o this work, A data from ref. 12. After polymerization the electrodes were conditioned in 0.1 M NaBF, solution prior to the potentiometric measurements.

only to the anion (BF;) but also to the cation. How- ever, owing to the negative slope anion transfer be- tween the polymer and the solution seems to dominate cation transfer. Such a situation can be predicted from eqn. (29) of ref. 22 when T_> 7+> 0 and from eqns. (351, (361, (39) and (40) of ref. 22 when S > y > 0. According to these equations the potentiometric slope may also be affected by redox couples in solution. Consequently the presence of dissolved oxygen may also contribute to the sub-Nernstian response ob- served. In other words, cation and electron transfer may compete with anion transfer at the polymer I solu- tion interface, resulting in the observed deviation from a purely anionic response (cf. Fig. 1 in Part 1 [221X

3.2. Effect of film conditioning

3.2.1. Final potential applied The final potential is defined as the potential ap-

plied to the polymer electrode (5 min) before the electrode is conditioned at open circuit. The final potential applied to the PPy(BF,) electrode influences the potential of the polymer electrode even after con- ditioning at open circuit for more than 8 h in 10-l M NaBF,. This can be seen in Fig. 2 where the potentio- metric responses of three similar PPy(BF,) electrodes are shown. The difference is in the final potential before conditioning. The potentiometric slopes of these electrodes are rather similar, between -41 and -44 mV per decade, but the main effect of the final poten-

36 .I. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2

tial is observed in the absolute potential values, i.e. the lines are shifted parallel to each other. A similar obser- vation has been made for PPy(C1) by Dong et al. [9]. The final potential applied affects the oxidation level (doping level) of PPy through electron transfer at the metal l polymer interface, causing the observed shift in the potential of the PPy electrode. This type of be- haviour is in agreement with eqn. (40) of ref. 22, where the ratio ([Z]/[Zd’])2/@+~) is determined by the oxi- dation level of the polymer which in turn is influenced by the applied potential.

3.2.2. Redox couples in solution In addition to the final potential applied to the

electrode, the potential of PPy is also affected by redox couples in the solution during open-circuit condition- ing. This can be seen in Fig. 3, which shows that the electrode potential is decreased in the presence of Fe(CN)i- in the conditioning solution but increased in the presence of Fe(CN)i-. For simplicity, the redox couple Fe(CN)i- and Fe(CN)i- will be denoted Fe(H) and Fe(II1) respectively in the remainder of this paper. The potentiometric responses shown in Fig. 3 were recorded in KC1 solutions in the absence of any redox couple. Since PPy is electronically conducting, electron transfer can take place between the PPy membrane and the Fe(III)/Fe(II) couple in solution during the

z . w

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300

250

Fig. 2. The potentiometric response of PMBF.,) in NaBF, solution showing the influence of the final potential applied before condition- ing. The polymerization time was 45 min and NaBF., was used as the electrolyte salt. After polymerization the electrodes were kept in the polymerization solution for 5 min at the following potentials: (1) +OS V; (2) 0.0 V; (3) -0.5 V. After this polarization the electrodes were conditioned in 0.1 M NaBF, prior to the potentiometric mea- surements. The potential was read after 6 min in each solution.

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-5 -4 -3 -2 -1 0

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Fig. 3. The potentiometric response of PPyfCl) in KC1 solution showing the influence of the presence of a redox couple ([Fe(CN),13-/4-) in the solution during conditioning of the polymer electrode. The polymerization time was 30 min and KC1 was used as the electrolyte salt. After polymerization the electrode was condi- tioned in the following solutions: (1) 10-l M KCl+ 10m3 M K,[Fe(CN),] for 20 min; (2) 10-l M KC1 for 20 h; (3) 10-r M KC1+10e3 M K,[Fe(CN),] for 20 min. Potentiometric responses were measured in KC1 solutions in the absence of any redox couple.

open-circuit conditioning [181, resulting in a change in the oxidation level of PPy. Therefore the potential of the PPy electrode is also influenced by redox couples in the conditioning solution. Thus the effect of the redox couples in solution is qualitatively similar to the effect of the final potential applied to the electrode, and eqn. (40) of ref. 22 can also be applied in this case. In one case, the oxidation level of the polymer is set by the potential of the metal substrate (electrochemical doping/dedoping). In the other case, the oxidation level of the polymer is set by the potential of the redox couples in the solution (chemical doping/dedoping). It is expected that both these electron transfer processes will be accompanied by transport of the charge com- pensating ions into/out of the polymer film in order to preserve electroneutrality, causing a metathesis of the polymer film [22].

The influence of 0, in the conditioning solution (10e4 M NaBF,) on the potentiometric response of a PPy(BF,) electrode is shown in Fig. 4. After polymer- ization one electrode was conditioned in an open beaker in contact with air, while another similar elec- trode was conditioned in a solution purged with nitro- gen. After a conditioning time of 6 h the potentiomet- ric responses of both electrodes were measured under nitrogen. As can be seen in Fig. 4, the film conditioned under air shows higher potentials than the electrode

.I. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2 37

conditioned under nitrogen, indicating that 0, is also able to increase the oxidation level and thus the poten- tial of a PPy electrode. It is well known that undoped PPy is oxidized by 0, [231. Our results show that doped PPy is also sensitive to 0,.

3.2.3. Electrolyte concentration The potential of the PPy electrode may be affected

not only by the applied potential and redox couples in the solution, but also by the concentration of the electrolyte used for conditioning the electrode. This is shown in Fig. 5 where the same electrode was condi- tioned first in 10-l M KC1 and then in lop4 M KCl. It is interesting to note that the potential values are higher for a high concentration of Cl- in the condition- ing solution. This observation can be explained by considering reaction (30) in ref. 22. The anionic re- sponse observed for PPy(C1) implies that 6 > y in this reaction. Therefore an increase in the electrolyte (KCl) concentration is expected to shift the equilibrium (30) to the right, i.e. to increase the ratio ([G’]/[kI’])2/(S+“) and thereby the electrode potential according to eqn. (40). In other words, the oxidation level of PPy in- creases with increasing electrolyte concentration.

It can also be seen in Fig. 5 that the slope of the electrode conditioned in lop4 M KC1 (-55 mV per decade) is greater than the slope of the electrode conditioned in 10-l M KC1 (-45 mV per decade).

250

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\ i

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\

01 I -5 -4 -3 -2 -1 0

Log k/M)

Fig. 4. The potentiometric response of PPy(BF,) in NaBF, solution showing the influence of oxygen (air) during conditioning. The poly- merization time was 33 min and NaBF, was used as the electrolyte salt. After polymerization the electrodes were conditioned for about 6 h in 10m4 M NaBF, solution under (1) air and (2) nitrogen. The potentiometric response was recorded under nitrogen. The elec- trodes were prepared under identical conditions.

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Fig. 5. The potentiometric response of Ppy(Cl) in KC1 solution showing the influence of the electrolyte concentration during condi- tioning. The polymerization time was 30 min and KCI was used as the electrolyte salt. After polymerization the electrode was condi- tioned in the following solutions: (1) 10-l M KC1 for 18 h; (2) 10e4 M KC1 for 4 h.

This type of behaviour can be explained by considering the possibility of co-ion (K+) insertion into the PPy film as described, for example, in scheme 4 and equi- librium (30) in ref. 22. The possibility of permselectivity failure in the case of electroactive polymers has been discussed in the literature [241. When co-ions are not totally excluded from the film, the experimentally ob- served sub-Nernstian anionic response is expected ac- cording to eqn. (29) of ref. 22, where 7_> T+> 0, and according to eqns. (36) and (40) when 6 > y > 0. When the PPy electrode is conditioned in a more dilute solution (lop4 M KCl) it can be assumed that the concentration of K+ (and Cl-) in the polymer film decreases and consequently the polymer becomes more permselective to the anions. This would mean that the ratio 7 Jr + in eqn. (29) and the ratio S/y in eqns. (36) and (40) increase with decreasing electrolyte con- centration in the conditioning solution [22]. Therefore a more negative slope is observed for the PPy film conditioned in 10d4 M KC1 compared with that condi- tioned in 10-l M KCl, as shown in Fig. 5. Furthermore it was also observed that the electrode conditioned in 10e4 M KC1 showed less hysteresis than the electrode conditioned in 10-l M KCl, as would be expected if co-ions influence the response.

3.3. Effect of the doping anion The effect of the type of the doping anion incorpo-

rated in the polymer membrane during polymerization

J. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2 38

z . W

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50

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2

\ 3-

4 /

v-5 -4 -3 -2 -1 0

Log (c/M) Fig. 6. The potentiometric response of PPy showing the influence of the doping anion. Electrodes (11, (2) and (3) were prepared by polymerizing with potential cycling for 45 min while electrode (4) was polymerized galvanostatically (j = 1 mA cm-* for 30 mitt). The following electrolyte salts were used: (1) NaTOS; (2) NaBF,; (3) NaDS; (4) indigo carmine. After polymerization, electrodes (11, (2) and (3) were conditioned in 10-l M NaBF, solution prior to the potentiometric measurements in NaBF, (potential read after 6 min in each solution). Electrode (4) was conditioned in 10-t M NaCl and the potentiometric response was measured in NaCl.

was studied using BF,-, p-toluene sulphonate (TO!!-) and dodecylsulphate CDS-). The potentiometric re- sponse was found to be strongly influenced by the anion, as can be seen in Fig. 6. PPy(TOS) and PPy(BFJ were found to give an anionic response to BF,- , while PPy(DS) shows almost the same potential in all the NaBF, solutions used in this study. When PPy was prepared using the bulky divalent indigo carmine (IC) as the doping anion, a cationic response to Na+ was obtained in NaCl solution. These results clearly demonstrate that the type of response obtained for PPy varies from anionic to cationic, depending on the dop- ing anion incorporated in the polymer membrane dur- ing polymerization. As will be shown below, the type of response observed for PPy can be related to the mobil- ity of the doping anion.

In order to obtain some information about the re- sponse mechanism, the ion exchange behaviour of PPy was studied using EDAX, as explained in Section 2.5. The EDAX spectra of the PPy(BF,) films conditioned in 0.1 M KCl, KI and KSCN showed clear signals for Cl, I and S respectively, but no signal for F was observed. However, the PPy(BF,) film conditioned in NaBF, showed a clear signal for F. These results indicate that BF; was spontaneously replaced by Cl-, I- and SCN- when the electrode was conditioned in

those solutions. It has been shown previously that TO!!- can be exchanged with NO; when PPy(TOS) films are conditioned in 1 M HNO, [HI. The anion exchange property of PHBFJ and PPy(TOS) towards small anions indicates that BF,- and TOY are mobile in PPy. This is in agreement with the anionic potentio- metric responses of PPy(BFJ and PPy(TOS) as shown in Fig. 6. Interaction between the anion and the posi- tive charge (polaron, bipolaron) on the polymer back- bone has been suggested as an explanation for the anionic potentiometric response of conducting polymer membranes [9]. However, since BFi is readily ex- changed with other ions, it seems that the interaction between PPy and small anions is rather non-specific. Therefore we cannot expect the PPy electrode to be very selective to anions, in agreement with previous results [9,12,15].

The EDAX spectra of Ppy(IC) films conditioned in 0.1 M KC1 and CaCl, showed clear signals for S (from the sulphonate groups of indigo carmine) but very small peaks for Cl. The S : Cl ratio was 40 and 25 for the films conditioned in KC1 and CaCl, respectively. This indicates that indigo carmine was retained in the films even after 10 days of conditioning, and therefore indigo carmine can be considered to be rather immo- bile in PPy. Conducting polymers containing immobi- lized dopants have been reported to show cation ex- change behaviour [2.5]. Furthermore, a cationic re- sponse to monovalent cations has been reported for PPy prepared using naphthalenesulphonates as doping anions [17]. The response was found to depend on the concentration of sulphonate groups in PPy. Further- more, it was suggested that not all the sulphonate groups of the divalent and trivalent anions in PPy are needed for charge compensation, so that the excess sulphonate groups may provide the cation exchange sites giving the cationic response [171. By analogy with these results, the cationic response observed for PPy(IC) can thus be related to the immobilized diva- lent doping anion. It is possible that the amount of negative charges incorporated by the indigo carmine during electropolymerization of pyrrole exceeds the amount of positive charges on oxidized PPy. Since indigo carmine is immobile in PPy, the excess negative charges must be compensated by mobile cations. The superstoichiometric entrapment of negative charge would thus make the polymer a cation exchanger, giving the experimentally observed cationic potentio- metric response. A cationic response was observed also for PPy doped with another bulky multivalent anion, Fe(CN)i-/4-, proving the generality of the concept.

It is interesting to note that PPy(DS) gives an an- ionic response with a very small slope, as can be seen in Fig. 6. The DS- anion is assumed to diffuse very

J. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2 39

slowly in PPy, but after prolonged immersion of PPy(DS) in KC1 some DS- may be exchanged by Cl- resulting in a polymer containing both DS- and Cl- [19]. In such a polymer the positive charges of oxidized PPy are compensated partly by DS- and partly by Cl-. Since the DS- anions have a rather low mobility and occupy some of the positive sites, they may actually reduce the effective anion exchange capacity of the polymer and the potentiometric response may be de- termined mainly by the mobile Cl- anions. Therefore an anionic response might be expected in KC1 solution, with the magnitude of the slope depending on the amount of co-ions (K+) inserted together with the Cl- during conditioning of the electrode in KC1 solution, as discussed above. These suggestions are supported by the results obtained with PPy synthesized with monova- lent naphthalenesulphonates as doping anions, where an anionic response to Cl- was observed in some cases n71.

The type of response observed, ranging from anionic to cationic, can thus be related to the relative mobili- ties of the anionic and cationic species in the polymer film, in agreement with the predictions of eqn. (29) of ref. 22. In this equation, the relative mobilities of the negatively and positively charged species are taken into account by T_ and T+ respectively. The experimentally observed anionic and cationic responses are also de- scribed by eqns. (36) and (40) of ref. 22, where the type of response is determined by 6 and y. Two extreme situations can be envisaged. First, when the anions are mobile in the polymer film and the cations are ex- cluded from the film, an anionic response is predicted from eqn. (29) where 7 _ B T+ and from eqns. (36) and (40) where S B y [22]. Second, if the film contains an excess of immobile anions and the excess negative charge is compensated by mobile cations, a cationic response is expected since T+x=- T_ and y B 6. All situations between these two extremes are also readily explained by eqns. (291, (36) and (40) of ref. 22.

3.4. Effect of redox couples in solution It was shown above that redox couples in the solu-

tion used for conditioning of the PPy electrode influ- ence the doping level and therefore the electrode po- tential. The slopes of the potentiometric response curves remained fairly constant as long as the potentio- metric responses were recorded in solutions without any redox couple added. However, if the potentiomet- ric response is measured in solutions containing redox couples, the slope is also changed as shown in Fig. 7. The potentiometric slope decreases with increasing total concentration of Fe(CN)z-/4- while keeping the ratio [Fe(III))I/[Fe(II)I constant, in this study equal to l/10. At a sufficiently high concentration of the redox

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iooc -5 -4 -3 -2 -1 0

Log (c/Ml Fig. 7. The potentiometric response of Ppy(C1) in KC1 solutions with different total concentrations of the redox couple Fe(CN)i-/4-. The polymerization time was 30 min and KC1 was used as the electrolyte salt. After polymerization, the potentiometric response was recorded

3 in presence of Fe(CN),- /4-. The ratio [Fe(III1]/Fe(II1] was kept constant at l/10, but the total concentration of the redox couple was varied: (1) no redox couple; (2) 10e5 M; (3) 10m4 M; (4) 10m3 M.

couple (lop3 M), the slope becomes virtually zero and the potential is stabilized to the value of the redox potential of the Fe(CN)i-/4- couple measured using a Pt electrode. This indicates that the potential of the polymer electrode may also be determined by the po- tential of the redox couple Fe(CN)i-/4- in the solu- tion. Similar results are obtained if the solution con- tains a reducing agent such as ammonia, as shown in Fig. 8.

The results obtained in this work indicate that the redox process (electron transfer) takes place in parallel with the ion-exchange process at the polymer mem- brane Isolution interface. At low concentrations of re- dox couples, the ion exchange (anion and cation) pro- cess may dominate giving an ionic response (e.g. see eqn. (40) of ref. 22). However, if there is a substantial concentration of a redox couple in the solution the potential of the PPy electrode will be determined en- tirely by the potential of the redox couple in the solution, as predicted by eqn. (39) of ref. 22. In this case PPy functions as a redox electrode [26]. Figure 9 shows the redox responses of a PPy electrode and a bare Pt electrode in 0.1 M Fe(CN)z-/4- solution with different [Fe(III)]/[Fe(II)] ratios. The two curves over- lap, in excellent agreement with eqn. (39) of ref. 22; the slopes are 59.7 mV per decade and 59.2 mV per decade for the PPy and Pt electrodes respectively. It should be pointed out, however, that although the redox responses of PPy and Pt are essentially the same,

40 J. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2

z . w

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Fig. 8. The potentiometric response of PPyfCI) in KC1 solution in (1) the absence and (2) the presence of 10e4 M ammonia. The polymer- ization time was 30 min and KC1 was used as the electrolyte salt.

there may be an important difference between the response mechanisms of these two electrodes. Theoret- ically, the redox sensitivity of PPy can be described by two different mechanisms of signal formation [22]. In the first case the potential of the polymer electrode is determined by the transfer of electrons at the polymer Isolution interface without a change in the redox composition of the polymer, i.e. no ion transfer

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0.01 0.1 1 10 100 lFe(lilll / IFeW

Fig. 9. Redox response of (1) PPyfCl) and (21 Pt in 10-l M Fe(CN)~-/4-. The PPy electrode was prepared potentiostatically at +0.8 V for 10 min and KC1 was used as the electrolyte salt. The electrode was allowed to equilibrate for ca. 14 min in each solution before the potential reading was taken. The redox response was recorded under nitrogen.

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z . 50 W

_501 -5 -4 -3 -2 -1 0

Log (c/Ml Fig. 10. The potentiometric response of PPyfCll in KC1 solutions containing different amounts of FefCNl~-/4-. The PPy electrode was prepared potentiostatically at +0.8 V for 10 min and KC1 was used as the electrolyte salt. After polymerization the electrode was equilibrated in a 10m4 M solution of KCI, also containing Fe(CN)z-/4-. After conditioning, the concentration of KC1 was increased and the potential was read 1 min after each addition of KCI. The ratio [Fe(III)]/[Fe(II)] was kept constant at l/10, but the total concentration of the redox couple was varied: (1) 0.5 x 10m3 M; (2) 1~10-~ M; (3) 2x1O-3 M; (41 5xlO-3 M.

occurs. In this case true metallic behaviour is expected, as for an inert metal like Pt. In the second case the potential of the polymer electrode results from a redox process that changes the doping level of the polymer film, i.e. both electron and ion transfer takes place at the polymer I solution interface. Such non-metallic be- haviour is perhaps more common in practical situa- tions, since ions are usually able to cross the polymer 1 solution interface. This non-metallic charac- ter of conducting polymers has recently been discussed by Doblhofer [27].

The interplay between the ionic and redox sensitivi- ties of PPy is illustrated further in Figure 10. In this experiment the Ppy(Cl) electrode was first allowed to equilibrate in 10e4 M KC1 containing 0.5 X 1O-3 M Fe(CN)i-/4- with [Fe(III)]/[Fe(II)] = l/10. At this point the redox couple determines the potential of the polymer electrode. After equilibration, the concentra- tion of KC1 is increased and an anionic response is observed (curve 1). The same procedure is repeated with higher total concentrations of the redox couple but always keeping the concentration ratio [Fe(III)]/ [Fe(U)], i.e. the redox potential (electron concentration in the solution), constant. As can be seen in Fig. 10, the anionic response is gradually suppressed as the total concentration of the redox couple is increased.

J. Bobacka et al. / Ionic and redox sensitivity of p-type conducting polymers 2 41

This experiment demonstrates that an ionic response can be observed, although the potential of the polymer is initially determined by the redox couple in the solu- tion, as long as the concentration of the redox couple is not too high. This behaviour can be explained by considering the kinetics of the processes involved. The immediate response is ionic (i.e. eqn. (40) of ref. 221, but a redox response (i.e. eqn. (39) of ref. 22) may be observed if complete equilibration through metathesis is allowed to establish. With increasing concentration of the redox couple in the solution, the rate of electron transfer is expected to increase and the ionic response is more effectively suppressed, as is observed experi- mentally in Fig. 10. 4

4. Conclusions 5

The experimental results presented in this paper show that the open-circuit potential of the PPy elec- trode is determined by anions, cations and/or redox

6 I

8

9 10

11

12

13

14

15

couples in solution. This means that anions, cations and electrons are able to interact with PPy, giving rise to a so-called mixed potentiometric response. In order to have a pure ionic response, the electron transfer at the polymer I solution interface should be negligible, i.e. the concentration of redox couples in the solution should be low. However, a pure redox response can be observed at a sufficiently high concentration of redox couples in solution. Although doped, PPy is electroni- cally conducting and redox sensitive, it differs from a metal in one vital aspect: the electron transfer at the PPy I solution interface is usually compensated by counter-ions entering or leaving the PPy film causing a metathesis of the polymer. In order to have metathesis, both electron and ion transfer have to take place simultaneously. Since this is frequently the case in practice, the use of PPy as an ion-selective electrode may be limited. However, PPy may be very useful as an intermediate layer in all-solid-state ion-selective elec- trodes [S]. If the ion transport in PPy is suppressed (no ionic response), only electron transfer at the polymer I solution interface is possible and therefore true metal- lic behaviour may be observed. Under such conditions PPy would act as an electron-selective electrode.

Finally, it can be concluded that both the ionic and redox responses observed for PPy are in agreement

with the theoretical model drived for p-type conducting polymers [221. Therefore it is believed that the theoret- ical model may serve as a base when describing the potentiometric response also for other conducting polymers.

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