A microelectrode study of the influence of pH and solution composition on the electrochemical behaviour of polyaniline films

271

J. Electroanal. Chem., 313 (1991) 271-289 Elsevier Sequoia S.A., Lausanne

JEC 01615

A microelectrode study of the influence of pH and solution composition on the electrochemical behaviour of polyaniline films M. Kalaji, Department (Received

L. Nyholm of Chemistry,

18 March

and L.M. Peter * University of Southampton,

Southampton

SO9 5NH (UK)

1991)

Abstract Platinum microelectrodes (diameter 10 am) have been used to study the influence of pH, ionic strength, isotopic substitution and different anions on the redox switching of electrochemically prepared polyaniline (PANI) films under first cycle and multicycle conditions. Microelectrodes reduce iR errors greatly and allow rapid scan cyclic voltammetry ( > 100 V/s) to be used in order to avoid degradation of the PANI in the region of the second oxidation peak. In all cases, the oxidation charge passed on the first cycle was found to be larger than on subsequent cycles (the first cycle effect). The multicycle switching charge was found to decrease with increasing pH, decreasing ionic strength and with increasing anion size, whereas the charge associated with the first positive scan was much less affected. More positive oxidation peaks were also obtained in DC1 and DzSO, than in HCl and H,SO,, respectively. It is concluded that the oxidation of PAN1 proceeds initially via rapid field driven proton egress, followed by equilibration of the oxidised phase by diffusion controlled movement of acid. Under multicycle conditions, part of the oxidized PAN1 film appears to remain inactive, but can be reduced if the potential is held at -0.2 V vs. SCE for several minutes. The loss of electroactitity under reduction conditions suggests that ion ingress is much more difficult than ion egress in the PAN1 system.

INTRODUCTION

Polyaniline (PANI) is a conducting polymer which has aroused great interest over the last few years due to its potential for application in batteries, electrochromic devices and sensors [l-5]. The material is particularly challenging from a theoretical point of view because its conductivity depends not only on oxidation state, but also on the degree of protonation. The electron and acid/base equilibria thought to be

* To whom correspondence

OO22-0728/91/$03.50

should

be addressed.

0 1991 - Elsevier Sequoia

S.A. All rights reserved

272

-4xH’

-4xH+

-4xe

III

I

-4xe-

v

A

-4xHA

-4xHA

II

Fig. 1. Proposed

Iv

reaction

scheme for PANI.

involved in the transformations are shown in Fig. 1. It can be seen that the redox processes involve electron and proton transfer, whereas the salt/base equilibria involve transfer of acid. The leucoemeraldine (I, II) and emeraldine (III, IV) forms of PAN1 are well characterised, but less information is available about the unstable fully oxidized form V. Since it is not known whether V is protonated to any extent in acid solutions, only the base form has been included in Fig. 1. Considerable efforts have been made to elucidate the influence of factors such as pH and solution composition on the electrochemical transformations of PAN1 films [5-301. Unfortunately, much of the literature is contradictory and a number of key questions remain unanswered. For example, it is not clear whether the predominant mechanism for the transport of protons and anions is migration or diffusion. Also, the timescale required to achieve salt/base equilibrium is unknown. Another unsolved problem is the origin of the hysteretic redox behaviour observed in the PAN1 system as well as in other conducting polymers [5,24,28,31-351. The physical properties of chemically prepared PAN1 have been examined extensively [28,36-381. In particular, the composition of the emeraldine form (III/IV) is now well established and there is wide agreement that the high conductivity of the emeraldine salt (IV) is due to resonance stabilisation of a structure in

213

which all nitrogen atoms are equivalent [29,36-411 (this corresponds to a 50% protonation level). The degree of protonation of chemically prepared emeraldine has been determined as a function of pH by elemental analysis of the anionic component [36-381, and the rather broad curves of proton content vs. pH have been attributed to electrostatic repulsion effects. Reiss [39,40] has given a statistical thermodynamic treatment of the protonation which takes into account strong repulsion between protons on nearest neighbour sites, and this leads to the conclusion that protonic doping will saturate at 50%. The theory was developed in order to explain the apparent observation of a pH dependent pK, for emeraldine. However, the spatial distribution of proton sites in partially protonated emeraldine has not been firmly established. The resonance stabilisation of the 50% protonated form of emeraldine may provide a sufficiently strong driving force for phase separation between protonated and non-protonated regions to occur. Indeed, there is evidence that emeraldine has a granular structure in which metallic islands of protonated emeraldine salt (IV) coexist with the unprotonated base form (III) [42]. If a two phase mixture is formed, the proton content of emeraldine can no longer be related in a simple way to a nominal pK, value. The consequences for electrochemical experiments of phase separation in the emeraldine state have not been considered. Usually, the redox behaviour of PAN1 and other conducting polymers is considered within the framework of the Nernst equation by assuming that the oxidised and reduced polymer components form a multicomponent single phase system. It has even been assumed that the oxidised and reduced components are part of the same chain [41]. The obvious deviation from ideal Nernstian response observed in cyclic voltammograms of many conducting polymers has been attributed to the heterogeneous nature of the polymer which leads to a distribution of standard potentials [33,34,35]. However, approaches based on the concept of a single phase in thermodynamic equilibrium are open to question and percolation models may have to be considered. The role of protons in the oxidation of leucoemeraldine (I/II) to emeraldine (III/IV) has been established from the pH dependence of the peak potentials measured by cyclic voltammetry [5,7,11,19,22,24,25.28]. However, the precise measurement of these shifts is complicated by two effects. Firstly, the electrochemical behaviour depends not only on pH but also on ionic strength. Secondly, a variable liquid junction potential can lead to errors in the analysis if a saturated calomel electrode is used, although the effects can be minimized by working with a sufficiently high constant ionic strength [43-451. These complicating factors have been identified by Horanyi and Inzelt [17], who used a reversible hydrogen electrode and constant ionic strength solutions to show that the pH dependence of the potential of the first oxidation and reduction waves corresponds to one proton per electron. There have been several attempts to measure directly the exchange of protons between PANI films and the electrolyte phase [12,26], and although the results are not in agreement, it is clear at least that protons are involved in the oxidation and reduction processes. On the other hand, the role of anions in the oxidation process

274

has been studied in more detail, for example using the quartz microbalance [7] as well as radiotracer methods [6,15,16] and.rotating ring disc voltammetry [46]. All of these methods confirm that anion ingress occurs during the first oxidation process (leucoemeraldine to emeraldine). However, it is also clear that the insertion process begins only after a considerable fraction of the oxidation charge has been passed. In fact, anion insertion appears to lag behind the electrochemical process, since it continues for a while even after the sweep direction has been reversed (see, for example refs. 7 and 16). Similar observations for other electroactive polymer films, such as polyvinylferrocene, polybithiophene and polythionine have also been described by Hillman et al. [47]. Since in the case of PAN1 the electron and anion fluxes are not balanced, electroneutrality must be satisfied by the movement of protons or other cations. Indeed, we have suggested that the very rapid switching observed with microelectrodes may indicate that the process involves almost exclusively proton movement at short times [31]. The evidence presented in the present report also supports this interpretation. The fact that no decrease in mass has been observed with the quartz microbalance during oxidation of leucoemeraldine to emeraldine [7] suggests that H+ rather than H,O+ is the mobile protonic species. There is no general agreement about the nature of the rate determining steps in the electrochemical oxidation and reduction of PANI. Several authors have assumed that the diffusion of counter ions is the rate determining step and have derived apparent diffusion coefficients from a modified Cottrell analysis of potentiostatic current transients [21,25,27]. This approach is almost certainly incorrect since potentiostatic transients at microelectrodes show maxima that suggest behaviour analogous to electrocrystallization [31], whereas the transients at macroelectrodes are iR controlled [31,48]. Aoki and Tezuka [49] have proposed a model for the oxidation of conducting polymers that is based on the propagation of conductive domains, and Aoki [50] has expanded this idea as the basis for Monte Carlo simulations of the conversion process which show that the conductive zone appeared to be fractal and that islands of reduced material may be trapped within the film. Aoki and Tezuka [51] have also demonstrated the propagation of a conductive zone in a polypyrrole film employing a potentiometric measurement at addressable microband array electrodes. The propagation model predicts the general features of the rising potentiostatic transients observed experimentally, but it may not be directly applicable to PANI since it is necessary to consider the local state of protonation as well as the degree of oxidation. In the present paper, we present the results of a microelectrode study of the influence of pH, ionic strength, different anions, and isotopic substitution on both the first cycle and the multicycle switching reactions of PANI. As was shown previously [31], the switching processes of PAN1 can be extraordinarily rapid and can only be resolved free of iR errors if microelectrodes are employed. At the same time, the use of microelectrodes and high sweep rates has the advantage that degradation associated with the second oxidation reaction of PANI is minimized. We focus particular attention on the differences between the first and subsequent repeated cycles of the voltammetric response. The first oxidation cycle appears to

21s

represent the response of completely reduced and equilibrated PAN1 while the multicycle response reflects the dynamic switching behaviour of films which contain residual oxidized species [31]. EXPERIMENTAL

Aniline (Aldrich, Gold Star) was distilled under reduced pressure prior to use. Sulphuric acid and hydrochloric solutions were prepared from Aristar grade (BDH) acids. All other chemicals were of analytical grade. Milli-Q Millipore water was distilled three times prior to use. The experiments were carried out using a three electrode potentiostat with a positive feedback iR compensation facility. Most experiments were carried out on a 10 pm or 5 pm diameter platinum microdisc electrode sealed in glass. A 500 pm platinum electrode was also used for comparison. Electrodes were polished on alumina (0.3 and 0.05 pm). All potentials were measured with respect to a saturated calomel electrode (SCE). A platinum wire served as counter electrode. Positive feedback iR compensation was generally used, and the maximum degree of compensation was determined in dummy cell measurements, as previously described [31]. All measurements were carried out inside a Faraday cage. PANI films were grown potentiodynamically by cycling the potential between -0.20 and +O.SO V in a solution of 0.1 mol dmp3 aniline in 1.0 mol dmd3 H,SO,, at a sweep rate of 20 mV s-‘. Film thicknesses were derived from the ratio between the switching charge and the film thickness established previously by ellipsometry [52]. After formation, the PAN1 films were washed with water and placed in the aniline free electrolyte solution. Films were then cycled at least 10 times prior to the start of the experiments in order to establish equilibrium with the electrolyte. First cycle responses were recorded after keeping the electrodes at - 0.20 V vs. SCE for 5 min to ensure that the film was in its reduced state. Multicycle steady state responses were normally reached after 2-5 cycles. In all electrolytes, the first cycle response was recovered when the film was held again for several minutes at -0.20 V. It is important to note that the films exhibited the same responses in a solution of 1.0 mol dm-3 H,SO, even after prolonged experiments in solutions of different pH and ionic strength. Moreover, the responses observed at different pH and ionic strength values did not depend on the order of the experiments, indicating that the differences in the responses were not due to electrolyte trapped within the film from the preceding experiment. RESULTS

AND DISCUSSION

The influence of pH at constant ionic strength The reaction scheme shown in Fig. 1 shows that electron transfer can be coupled to proton exchange and to acid/base equilibria. Provided that the polymer can be treated as a single phase and that it remains in equilibrium with the electrolyte, the equilibrium behaviour can be derived from the Nernst equation. Indeed, if these

276

assumptions are valid, the potential-pH behaviour can be represented by a Pourbaix diagram [30]. Wu-Song Huang et al. [41] measured the equilibrium potential of a PAN1 film in the emeraldine form as a function of the Hamnett acidity function over the range H,, = - 1.0 to 5.0 and found a slope of - 59 mV per H, unit over the range H,, = 2.0 to 5.0. At lower Ho values, the potential variation disappeared. These authors considered that the oxidised and reduced components involved in Nemstian equilibrium are the quinone diimine and amine segments of the same polymer chain. In their treatment of the results, Wu Song Huang et al. also assumed that the protonation equilibria involve exclusively the quinone diirnine segments of the chain, and for protonation of the two imine nitrogens they derived pK,, = 1.05 and PK,,~ = 2.55. Most of the potential-pH relationships to be found in the literature are based on cyclic voltammetry, in particular on the shifts in peak potential with pH. The interpretation of the voltammetric data differs from the discussion in the previous paragraph because the oxidised and reduced species are not amine and quinone diimine units on a single chain, but rather the distinct components shown in the scheme in Fig. 1. The problems associated with these measurements have been highlighted recently by Inzelt and Horanyi [17]. These authors showed that the first pair of voltammetric peaks shifts - 59 mV per pH unit .for HCI concentrations from 5 to 0.05 mol drne3 (it is important to note that Inzelt and Horanyi used solutions of constant ionic strength for these measurements). This slope is characteristic of a process involving 1 proton per electron, i.e. oxidation of II to IV. The second pair of peaks was found to shift -120 mV per pH unit over the pH range 1.6 to 4.8, indicating that two protons are transferred per electron, i.e., the dominant process is the conversion of IV to V. In the present work, the voltammetric behaviour of PAN1 films was investigated in HCl + KC1 mixtures of constant ionic strength. pH values higher than 3 were not investigated due to the poor buffering capacity of such solutions and the fact that the PAN1 films become electroinactive at pH 3-4 [11,22,25,53]. Figure 2 shows the effect of a variation in the pH at constant ionic strength on the shapes of the first and subsequent multicycle voltammograms recorded at 100 V s-’ at a 10 pm platinum electrode. The most striking effect is that the current on the first positive scan is much larger than on subsequent scans and is also relatively insensitive to the changes in pH. At the same time, it can be seen that the first anodic peak is displaced to more positive potentials than the subsequent coincident multicycle peaks. The trends evident in Fig. 2 were also observed in sulphuric acid + potassium sulphate solutions of constant ionic strength. There is clearly a large “first cycle effect” which needs to be taken into account when interpreting cyclic voltammetric and related data such as mass changes measured with the quartz microbalance. Orata and Buttry [7], for example, chose to use multicycle conditions in their microbalance studies of PANI, whereas Inzelt and Horanyi [16] restricted their radiotracer analysis to the first sweep measured after a delay period at the negative limit. These differences in experimental approach make a direct comparison difficult.

To.2A cm-2

1

l?T

0.1 A cm_2

O.OOSM HCL O.SM KU

I 0.4

1

-0.2

I

0

I

0.2 potential

I

I

0.k 0.6 / V vs SCE

0.6

0.8

I

0.0

Fig. 2. First cycle and multicycle cyclic voltammograms obtained for a 60 nm PANI film on a 10 pm platinum electrode at a sweep rate of 100 V s-’ in 0.50 mol dmm3 HCl, 0.05 mol dmm3 HCl+0.45 mol drnm3 KC1 and 0.005 mol drns3 HCl+0.50 mol dm-’ KCl, respectively.

The microelectrode responses at different pH values were compared with cyclic voltammograms recorded using the 500 pm diameter platinum electrode at a sweep rate of 10 mV SC’. The same general trends were observed, although the voltammograms at the larger electrode showed small additional peaks between 0.40 and 0.50 V that can be attributed to electroactive species formed by partial degradation of the film [7,46,54-571. The degradation was most marked at the lowest pH values. The advantages of using a microelectrode and high sweep rates to avoid degradation problems are illustrated by Fig. 3. In the case of the microelectrode voltammograms, it is possible to sweep many times through the second oxidation peak without any

278

I0.3

A ~rn-~

IOpm

Pt

100 vs-’

04

L

-0.2

I

0

I

0.2

0.4

Potential

0.6 /

I

J

08

1.0

V vs SCE

Fig. 3. Comparison of first cycle and multicycle cyclic voltammograms in (a) 1.0 mol dm-3 HCI recorded with a 60 nm thick PANI film on a 10 pm platinum electrode at a sweep rate of 100 V s-’ and (b) a 4 pm thick PAN1 film on a 500 pm platinum electrode at a sweep rate of 10 mV s-‘. Note the rapid degradation of the PANI film in (b).

., indication of, film degradation. By comparison, degradation is clearly severe at the low sweep rate in the case of the 500 pm electrode. The negative shift of both sets of peak potentials observed with increasing pH under slow multicycle conditions in the present work agreed with the results of Inzelt and Horanyi [17]. The pH dependence of the peaks recorded on the first cycle was less clear cut, and at high sweep rates even positive shifts were recorded. The loss of redox activity under multicycle conditions is confirmed by the values of the ratio of the multicycle charge Q,, to the charge Q, measured on the first cycle. For the case of the microelectrode measurements at 100 V s-‘, Q,,/Q, decreased from close to unity in 0.50 mol dmP3 HCl to 0.24 in 5 X lop3 mol drne3 HCl+ 0.50 mol dmm3 KCl. At the lower sweep rate of 10 mV s-’ used for the measurements on the 500 pm electrode, Q,,/Q, decreased from close to unity to

219

A

I

I

I

-0.2

-0.1

0

I

I

0.1

a

0.2

I

0.3

I

I

0.4

0.5

B 0.5 Acn~-~

I

I

0.5

0.6

0.7

1

0.8

I

0.9

I

1.0

potential/Vvs SCE Fig. 4. First cycle and multicycle cyclic voltammograms covering (A) the first and (B) the second redox reaction of PANI. The voltammograms were recorded with a 150 nm thick PAN1 film on a 10 km platinum electrode in 1 .O mol dm- 3 H2S04 at sweep rate of 100 V s-‘. The potential was kept at +0.50 V for five minutes prior to the start of the experiments.

0.59 for the same change in concentration. The results demonstrate clearly that the fraction of the film that is redox active under multicycle conditions decreases as the solution pH is increased. At the highest pH studied, less than 25% of the PANI film remains active after cycling once at 100 V s-‘. By contrast, the polymer remains almost completely active in 0.50 mol dmM3 HCl. The cyclic voltammograms shown in Figs. 2 and 3 cover both redox processes, so that the reduction behaviour could be complicated by the fact that the film has undergone two transformations, first to the conducting form and then to the insulating higher oxidation state. A comparison between the voltammograms in Fig. 2 and cyclic voltammograms recorded between -0.20 and + 0.50 V showed that this is indeed the case. At the highest pH values a larger fraction of the film remained electroactive when the scan was reversed at + 0.50 V rather than + 0.93 V. In 0.5 mol dmp3 HCl, on the other hand, no such dependence on the upper limit was found. In order to deconvolute the first and second oxidation processes and to establish whether acid/base equilibrium is achieved under dynamic conditions, cyclic voltammograms were recorded separately for the first and the second redox reaction using films that had been allowed to equilibrate for 5 min at a potential of +0.50 V (approximately halfway between the two redox couples). First, the reduction of the PANI films was investigated by scanning from +0.5 V to -0.2 V; the cyclic voltammograms obtained are illustrated in Fig. 4a. It can be seen that the effect of waiting at + 0.5 V is small and restricted to the first cathodic peak under multicycle conditions. The magnitude and position of the anodic peak are unaffected. The

280

second oxidation process was then studied in the same way. The cyclic voltammograms shown in Fig. 4b indicate clearly that the equilibration period also has very little effect on the following transition to the fully oxidised form of PANI. We conclude therefore that acid base equilibrium in the emeraldine form is established (at least at this pH) on a time scale comparable with that of the cyclic voltammetric experiments. It would be interesting to discover whether a first cycle effect exists for the transition from the fully oxidised state of PANI, but no attempt was made to investigate this by equilibrating the film at +0.93 V because rapid degradation occurs at this potential. These results suggest strongly that acid/base equilibrium is reached both on the first oxidation sweep as well as under multicycle conditions in 1 mol drne3 H,SO,, although this may not be the case at higher pH values. Since the acid/base equilibrium appears to be established rapidly, at least in solutions of low pH, we propose that the low value of Qmc/Qrc reflect the fact that the emeraldine base is reduced much more slowly than the emeraldine salt. This proposal explains why the electroinactive fraction increases with increasing pH. If, as seems possible, the emeraldine salt and base phases are segregated, reduction may be restricted to the conducting emeraldine regions, leaving behind “islands” of unreduced emeraldine base embedded in an insulating matrix of leucoemeraldine. If we accept for the moment that the emeraldine base is electroinactive on the negative scan, we are faced with the problem that it can apparently be oxidised without difficulty to the fully oxidized form (V) in the second oxidation peak on the first cycle. This point is illustrated by Fig. 2, which shows that the loss of the electroactivity occurs only on the reverse sweep. It could be argued that complete reduction of the emeraldine is slow because it results in islands of conducting material trapped within an insulating matrix. However, if this were the case, we should expect the second oxidation reaction to follow the same pattern since the fully oxidized form is also nonconducting. As this does not appear to be the case, we conclude that the reduction must be limited by some other phenomenon. In any case, the transition from the conducting to the nonconducting form only takes place when reduction of the film is almost complete [48], so that the formation of islands in an insulating matrix would account for only a small fraction of the “missing” charge. The marked differences in the electroactivity of PAN1 towards oxidation and reduction appear to point towards an inherent asymmetry in the ease with which ions can be transferred to and from the film. The oxidation of leucoemeraldine to emeraldine and subsequently of emeraldine to the fully oxidized form (V) can in principle be achieved solely by proton expulsion. On the other hand, the corresponding reduction steps must involve proton ingress. The loss of activity on the reverse sweep therefore appears to be linked to slow proton ingress. The reason for this is not clear at present, but it may reflect the field distribution within the partially oxidised PANI film or possibly structural rearrangements that accompany oxidation and reduction. It is worth noting that Hillman et al. [47] have shown that ingress of species into films of electroactive polymers such as polyvinylferrocene,

281

polybithiophene and polythionine is slower than egress of the same species and also that charged species, notably protons in hydrated systems, move faster than neutral species (e.g. the solvent) due to the influence of the field. The influence of anion concentration at constant pH The importance of maintaining constant ionic strength when investigating pH effects has already been emphasised in the preceding section. Here we consider the effects of holding the HCl concentration constant while varying the KC1 concentration. Previous work has established that the concentration and nature of the anion are dominant; cation effects seem to be less important [6]. Again, liquid junction potential errors can cause problems, as pointed out by Inzelt and Horanyi [17]. These authors used a Ag/AgCl electrode to eliminate this problem and studied the shifts in the two redox peaks as a function of log,, [Cl-] at pH values between about 2 and 4. They found that the first redox couple shifts negatively by 60 mV per decade of Cl- concentration, whereas the second peak shifts positively by 60 mV under the same conditions. These results show the involvement of one Cl- per electron in both the first and the second redox processes. It follows that the predominant transformations at pH 2 2 are leucoemeraldine base (I) to emeraldine salt (IV) and emeraldine salt to the fully oxidised unprotonated form (V). Figure 5 shows the effect on the microelectrode voltammograms of varying the KC1 concentration at a constant HCl concentration of 0.005 mol dmm3. The results confirm that the voltammetric response is sensitive not only to pH but also to the concentration of KCl. The changes are not due simply to increased solution resistance, since the iR compensation was adjusted to take this into account. Calculation of the changes in the liquid junction potential also showed that this effect could be neglected. It is clear that the fraction of the film that is redox active under multicycle conditions diminishes progressively as the concentration of KC1 is reduced. At the same time, the first cycle effect also becomes less pronounced until at the lowest concentration it is no longer evident in the charge data. Even in this limit, however, the potential of the first oxidation peak is still more positive on the first cycle. Similar, but less pronounced, effects were observed in a set of experiments carried out with the 500 pm electrode at 10 mV s-‘, although in this case the voltammograms were much better resolved and actually showed two peak couples. Again the first cycle effect was more evident in the high sweep rate data. Hence, at 100 V s-‘, the ratio Q&Q, was found to be 0.38 when the KC1 concentration was 1.0 mol drne3 while the corresponding ratio at a sweep rate of 10 mV/s was 0.65. The peak potential shifts observed at 100 V s-’ and 10 mV s-’ when the chloride concentration was changed were not always consistent, but the general trends follow the behaviour expected from the scheme in Fig. 1. At low sweep rates, the first multicycle anodic peak shifts to more negative potentials as the KC1 concentration is increased, whereas no shift was observed at 100 V s-‘. A negative shift is expected at low pH because the dominant oxidation process is the conversion of leucoemeraldine base (I) to emeraldine salt (IV). On the other hand, the second pair of peaks is expected to shift positively as the KC1 concentration is increased if

282

100

vs-’

UN

5mM HCI 0 1 M KCI

5mM HCI 1 OM KCI

cmw2

0.5A

1

-02

I

I

0

0.2

I

I

0.4

I

0.6

0.8

I

I

I

I

I

-0.2

0

02

04

I

I

0.6

0.8 1.0

s SCE

(CI

5mM HC\ IOmM KCI

1

/7

10

potential/V

-0.2

1

I

I

I

0

02

04

I

0.6

I

08

J

I

1

10

-02

0

1

0.2

I

0.4

\

0.6

1

0.8

1

1.0

potential/V =SCE Fig. 5. First cycle and multicycle cyclic voltammograms recorded with a 330 nm PAN1 film on a 10 pm platinum electrode at a sweep rate of 100 V s-’ in 0.005 mol dm-’ HCI + 1.0 mol dm-’ KCI, 0.005 mol dm-3 HCl+O.lO mol dm-3 KCI, 0.005 mol dm-3 HCI+O.OlO mol dme3 KCI, and 0.005 mol dmm3 HCl+ 0.001 mol dme3 KCI, respectively.

emeraldine salt is oxidised to the unprotonated form (V). Under multicycle conditions, a positive shift was indeed observed at 10 mV s-‘, whereas the oxidation peak was found to be almost independent of the KC1 concentration at 100 V sP’.

283

The influence

of anion size

As can be seen in Fig. 6, the electroactivity of a PAN1 film also depends on the nature of the anion. Differences, although less pronounced, were also noted when comparing the electrochemical behaviour of PAN1 films in H,SO, and HCl. The differences in behaviour cannot be dis.missed as being due to pH effects since the pH differences between the solutions are too small [58]. It appears that an increase in the size of the anion causes the multicycle peaks to decrease and also the oxidation peak potentials (first cycle and multicycle) to shift positively. In the case of p-toluene-sulphonic acid, the shift in peak potential is so large that not all of the film is oxidized on the first cycle and a steady state multicycle response is not attained until after several cycles. The electroactivity of PAN1 is clearly influenced by anion size and decreases in the series HSO; > Cl-> CF,COO-> p-CH@SO; . The fact that smaller responses and more anodic first oxidation peaks were seen in HCl than in H,S04 can probably be ascribed to the larger degree of hydration [59] of the chloride ion. The size effect may be related simply to the diffusion coefficient and mobility, but in the case of the largest ions, a pore size exclusion effect may also be important. This effect will hinder large anions from penetrating into the smallest pores so that complete oxidation can only be achieved if some rearrangement or swelling of the polymer structure takes place. The electroactivity also depends on the stability of the emeraldine salt, and we may assume that this will be lower for large anions as the result of short range repulsive interactions and the fact that the strength of an

1000v s-1 0.1Acme2

potential/

IM CF$OOH

V s

SCE

Fig. 6. First cycle and multicycle cyclic voltammograms recorded with a 10 nm thick PAN1 film on a 10 pm platinum electrode at a sweep rate of 1000 V SC’ in 1.0 mol dm-” H,SO,. 1.0 mol drnm3 trifluoroacetic acid and 1.0 mol dmm3 p-toluenesulphonic acid, respectively.

284

100 vs-l ---lM

I

I

I

-0.2

0

02

I

0.4

H2S04+H20

I

1

0.6

0.95A/cm2

-02

I

I

0

02

Fig. 7. First cycle and multicycle cyclic voltammograms recorded pm platinum electrode at a sweep rate of 100 V SC’ in (-----) ) 1.0 mol dmm3 D,SO, +D,O. (-

ion-pair decreases decreases.

as the ratio between

The effect of deuterium / hydrogen

the charge

I

0.4

J

06

with a 110 nm thick PANI film on a 5 1.0 mol dmm3 H,SO, +H,O and

and the size of the ions involved

substitution

The important role of protons in the redox reactions of PAN1 is well established. Since it appears that field assisted proton egress occurs during oxidation of leucoemeraldine, we have investigated the effects of isotopic substitution on the first pair of voltammetric peaks. Figure 7 shows that in D,SO, both the first cycle oxidation and the multicycle peak potential appear at more positive potentials than in H,SO,. A similar effect was also seen when comparing voltammograms obtained in DC1 and HCl. The significant influence of deuteration cannot be explained by the small increase in pH (about 0.3 pH units) of D,SO, compared with H,SO, (if it were a pH effect we would have expected more negative peak potentials in D,SO, than in H,SO,), and it seems unlikely that differences in anion solvation are responsible. This leaves a specific effect of deuteration on ion transport as the most likely explanation.

285

The quartz microbalance measurements reported by Orata and Buttry [7] indicate that a significant fraction of the leucoemeraldine form can be oxidised with~ul measurable muss change. This means that the proton must be expelled as HC and not H,O+. It seems feasible that proton transport could occur preferentially along polymer chains, so that the deuteration effect on the first oxidation peak could reflect the enhanced mobility of H+ compared with D+ (by way of comparison, the mobility of D+ in ice is about 6 times less than that of H+ [60]). There is some evidence [61] that proton transport in other materials takes place by tunnelling between nitrogen atoms, and it is possible that this mechanism assists proton transport along the PAN1 chains. Discussion It is clear from the results presented here that the multicycle switching charge depends on pH and anion concentration. The key feature is the fact that the first cycle and multicycle voltammograms differ in this respect because the current response on the first positive sweep is almost independent of pH. This difference between the first and subsequent positive cycles was also evident though less pronounced at 10 mV SC’. It is clear that part of the PANI film is not reduced under multicycle conditions, although it can be reduced if the potential is held for a few minutes at the negative limit. We propose that the experimental observations can be rationalized by assuming the oxidation of the leucoemeraldine forms I and II is accompanied initially by rapid proton egress driven by the electric field in the insulating phase (the transport number of protons is expected to be much higher than that of anions). As the conversion of the leucoemeraldine phase proceeds, the field distribution will become complicated by the growth of conducting regions, and eventually the field will collapse. Nevertheless, the conversion of leucoemeraldine into the emeraldine phase is evidently very rapid and complete, as shown by the behaviour on the first scan. The larger overpotential observed on the first scan is probably due to the need to nucleate and grow emeraldine centres which then expand with protons being ejected by the electric field as growth progresses. We may speculate that proton migration could involve either movement along the polymer chain by transfer between nitrogen atoms or through a structured layer of absorbed water so that the transport number of the proton is close to unity. According to this model, field driven anion ingress is negligible at short times or during the early part of the cyclic voltammogram. However, as the oxidation process proceeds, emeraldine and emeraldine base will be formed initially in a ratio which is determined by the degree of protonation of the starting material. Since the effective pK, of leucoemeraldine is believed to be lower than that of emeraldine, equilibrium must be re-established by the movement of acid (i.e. protons and anions) into the film. This process will be diffusion controlled if electroneutrality has been established by proton movement, and it may therefore lag behind the electron and proton transfer processes to an extent which depends on pH and anion concentration. This would explain the time lag in anion movement that has been seen in quartz microbalance [7] and radiotracer [16] studies

286

of PANI. Further time resolved studies of anion ingress are needed to establish whether it is an entirely diffusion controlled process or whether anion migration is also important. Unfortunately, quartz microbalance and radiotracer methods may not have the required time resolution, especially if the PAN1 film is thin since the diffusional time constant is of the order of 1*/D. The formation of both emeraldine base and emeraldine salt could lead to phase segregation by proton and anion diffusion within the film, although the timescale of such an effect is not known. It is possible that the appearance of nucleation-like behaviour in potential step transients at microelectrodes [31] is related to phase segregation phenomena as well as to electrochemical transformation. On the other hand, the reduction of the emeraldine phase is known to be extremely fast in strongly acid solutions and no nucleation-like behaviour is observed. This suggests that the conducting emeraldine salt (IV) is reduced very rapidly to leucoemeraldine base (I) as a result of expulsion of anions from the film. By contrast, the movement of protons into the film is needed to facilitate the reduction of the emeraldine base (III) to the leucoemeraldine base (I). If we accept that ion egress is rapid whereas ion ingress is slow, this mechanism explains not only why the reduction is very rapid at low pH values but also why the fraction of reducible material falls as the pH is increased. An extension of the model to the second redox reaction is straightforward. We assume that the second oxidation is accompanied by a fast expulsion of protons as a result of the oxidation of the emeraldine forms (III) and (Iv) to the fully oxidized form (V). However, although electroneutrality has been established by proton expulsion, the fully oxidized film contains too much acid which must diffuse out into the solution. Since the potential scan is reversed shortly after the second

+4xA

-4xe

-8xH * -4xA-

-4xe.

-4xH’-4xe.

. eX

Fig. 8. Kinetic scheme acid/base equilibration

for the oxidation processes.

of PANI

Fast processes Slow processes

showing

rapid

ion egress

and slow ion ingress

and

287

oxidation peak, it is not surprising that the apparent mass of the film is higher [7] for the fully oxidized film than for the reduced film. Unlike the two oxidation reactions of PANI, the reduction of the fully oxidized form (V) relies on the ingress of protons and anions. Since the mobility of protons is higher than that of anions, the reaction probably proceeds by the reduction of the fully oxidized form (V) to emeraldine base (III) followed by the diffusion of acid into the film in order to establish the equilibrium between emeraldine base (III) and emeraldine salt (IV) determined by the bulk pH. The fact that the reduction of (V) is slow due to the requirement for ion ingress explains why the electroactive fraction of the film is larger when the scan is reversed at +OSO V under multicycling conditions. The proposed model is summarised schematically in Fig. 8. It explains why the first cycle charges are almost the same at all pH values up to the point of scan reversal and also why the reduction charge is smaller than the oxidation charge on the first cycle. In addition, it also provides a satisfactory explanation of the observed dependence of the cyclic voltammograms on pH, KC1 concentration and sweep rate. CONCLUSIONS

The electroactivity of PAN1 has been rationalized in terms of a model in which it is assumed that the oxidation of the basic and protonated forms of leucoemeraldine as well as the further oxidation of the emeraldine forms is accompanied by a field driven expulsion of protons and a slower ingress of acid, primarily under diffusion control. The reduction of the emeraldine and the fully oxidized forms of PAN1 is evidently much slower, suggesting that ion ingress is considerably less easy than proton egress. The experiments suggest that proton transport is the dominant process at high sweep rates. The rate of oxidation of leucoemeraldine is slower when deuterated acids are used, which is consistent with a mechanism involving proton movement along the polymer chain. However, the rate of oxidation is also decreased when larger anions are used, indicating that the ease of anion ingress into the film also controls the oxidation process, probably at a later stage. The results show that the redox behaviour of PAN1 is probably linked to rapid field driven ion egress and slower ingress of ionic and neutral species by diffusion. However, further time-resolved non-electrochemical measurements are required to separate and quantify the proton and anion contributions to the redox processes in PANI. ACKNOWLEDGEMENTS

L.N gratefully acknowledges financial support from the Swedish Natural Science Research Council. Part of the work was also supported by the Science and Engineering Research Council.

288 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

S. Chao and M. Wrighton, J. Am. Chem. Sot., 109 (1987) 5526. E. Jones, 0. Chyan and M. Wrighton, J. Am. Chem. Sot., 109 (1987) 6627. E. Paul, A. Ricco and M. Wrighton, J. Phys. Chem., 89 (1985) 1441. H. Sbinohara, T. Chiba and M. Aizawa, Sensors Actuators, 13 (1989) 79. J. Heinze in E. Steckhan (Ed.), Topics in Current Chemistry, Vol. 152, Springer-Verlag, Heidelberg, 1990, pp. l-47. V.E. Kazarinov, V.N. Andreev, M.A. Spytsin and A.V. Shlepakov, Electrocbim. Acta, 35 (1990) 899. D. Orata and D.A. Buttry, J. Am. Chem. Sot., 109 (1987) 3574. E.M. Genies, J.F. Penneau and E. Vieil, J. Electroanal. Chem., 283 (1990) 205. M. Lapkowski and E.M. Genies, J. Electroanal. Chem., 279 (1990) 157. E.M. Genies and M. Lapkowski, J. Electroanal. Chem., 220 (1987) 67. T. Hirai, S. Kuwabata and H. Yoneyama, J. Chem. Sot. Faraday Trans. 1, 85 (1989) 969. M. Lapkowski and E.M. Genies, J. Electroanal. Chem., 284 (1990) 127. K. Okabayashi, F. Goto, K. Abe and T. Yosbida, J. Electrochem. Sot., 136 (1989) 1986. T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 177 (1984) 293. G. Inzelt and G. Horanyi, J. Electrochem. Sot., 136 (1989) 1747. G. Horanyi and G. Inzelt, Electrochim. Acta, 33 (1988) 947. G. Inzelt and G. Horanyi, Electrochim. Acta, 35 (1990) 27. J. Ye and R.P. Baldwin, Anal. Chem., 60 (1988) 1979. W.W. Focke, G.E. Wnek and Y. Wei, J. Phys. Chem., 91 (1987) 5813. G. Inzelt, Electrochim. Acta, 34 (1989) 83. J.C. LaCroix and A.F. Diaz, J. Electrochem. Sot., 135 (1988) 1457. W.S. Huang, B.D. Humphrey and A.G. MacDiarmid, J. Chem. Sot. Faraday Trans. 1, 82 (1986) 2385. CD. Batich, H.A. Laitinen and H.C. Zhou, J. Electrochem. Sot., 137 (1990) 883. G.P. Evans, in H. Gerischer (Ed.), Advances in Electrochemical Science and Engineering, Vol. 1, VCH, Weinheim, 1990, p. 1. W.E. Rudzinski, L. Lozano and M. Walker, J. Electrochem. Sot., 137 (1990) 3132. K. Shimazu, K. Murakoshi and H. Kita, J. Electroanal. Chem., 277 (1990) 347. J.C. LaCroix and A.F. Diaz, Makromol. Chem. Macromol. Symp., 8 (1987) 17. E.M. Genies, A. Boyle, M. Lapkowski and C. Tsintavis, Synth. Met., 36 (1990) 139. J.P. Travers, F. Genoud, C. Menardo and M. Nechtschein, Synth. Met., 35 (1990) 159. R.J. Cushman, P.M. McManus and S.C. Yang, J. Electroanal. Chem., 291 (1986) 335. M. Kalaji, L.M. Peter, L.M. Abrantes and J.C. Mesquita, J. Electroanal. Chem., 274 (1989) 289. B. Villeret and M. Nechtschein, Phys. Rev. Lett., 63 (1989) 1285. SW. Feldberg and I. Rubinstein, J. Electroanal. Chem., 240 (1988) 1. J. Heinze, M. Storzbach and J. Mortensen, Ber. Bunsenges. Phys. Chem., 91 (1987) 960. J. Heinze, R. Bilger and K. Meerholz, Ber. Bunsenges. Phys. Chem., 92 (1988) 1266. J.C. Chiang and A.G. MacDiarmid, Synth. Met., 13 (1986) 193. A.G. MacDiarmid, J.C. Chiang, W. Huang, B.D. Humphrey and N.L.D. Somasiri, Mol. Cryst. Liq. Cryst., 125 (1985) 309. A.G. MacDiarmid, J.C. Chiang, A.F. Richter, N.L.D. Somasiri and A.J. Epstein, in L. Alcacer (Ed.), Conducting Polymers, Reidel, Dordrecht, 1987, p. 105. H. Reiss, J. Phys. Chem., 92 (1988) 3657. H. Reiss, Synth. Met., 30 (1989) 257. W.S. Huang, A.G. MacDiarmid and A.J. Epstein, J. Chem. Sot. Chem. Commun., (1987) 1784. A.J. Epstein and A.G. MacDiarmid, Mol. Cryst. Liq. Cryst., 160 (1988) 165. F.J.C. Rossotti and H. Rossotti, The Determination of Stability Constants and other Equilibrium Constants in Solution, McGraw-Hill, New York, 1961. H. Irving, in IS. Longmuir (Ed.), Advances in Polarography, Vol. 1, Pergamon Press, London, 1960, p. 42. A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980.

289 46 T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 177 (1984) 281. 47 A.R. Hillman, D.C. Loveday, M.J. Swann, R.M. Eales, A. Hamnett, S.J. Higgins, S. Bruckenstein and C.P. Wilde, Faraday Discuss. Chem. Sot., 88 (1989) 151. 48 J. LaCroix, K. Kanazawa and A. Diaz, J. Electrochem. Sot., 136 (1989) 1308. 49 K. Aoki and V. Tezuka, J. Electroanal. Chem., 267 (1989) 55. 50 K. Aoki, J. Electroanal. Chem., 292 (1990) 63. 51 Y. Tezuka and K. Aoki, J. Electroanal. Chem., 273 (1989) 161. 52 R. Greef, M. Kalaji and L.M. Peter, Faraday Discuss. Chem. Sot., 88 (1989) 277. 53 A.F. Diaz and J.A. Logan, J. Electroanal. Chem., 111 (1980) 111. 54 Y.B. Shim, M.S. Won and SM. Park, J. Electrochem. Sot., 137 (1990) 538. 55 R. Jiang and S. Dong, J. Chem. Sot. Faraday Trans. 1, 85 (1989) 1585. 56 D.E. Stilwell and S.M. Park, J. Electrochem. Sot., 136 (1989) 688. 57 D.E. Stilwell and S.M. Park, J. Electrochem. Sot., 135 (1988) 2254. 58 A. Albert and E.P. Serjeant, The Determination of Ionization Constants, Chapman and Hall, London, 1984. 59 R.E. Verrall, in F. Franks (Ed.), Water - A Comprehensive Treatise, Vol. 3, Plenum Press, New York, 1973, Ch. 5. 60 F. Franks, in F. Franks (Ed.), Water - A Comprehensive Treatise, Vol. 1, Plenum Press, New York, 1972, Ch. 4. 61 N.S. Isaacs, Physical Organic Chemistry, Longman, Harlow, 1987.