A spectroelectrochemical study of the electrodeposition of polythiophene films

403

J. Elecfrounal. Chem., 243 (1988) 403-417 Elsevier Sequoia S.A., Lausanne - Printed

A SPECI’ROELECIROCHEMICAL OF POLYTHIOPHENE FILMS

A. ROBERT

HILLMAN

School of Chemistry (Received

and ELIZABETH

in The Netherlands

STUDY OF THE ELECTRODEPOSITION

F. MALLEN

University of Bristol, Cantocks Close, Bristol BS8 ITS (Great Britain)

14th August

1987; in revised form 3rd November

1987)

ABSTRACT The growth of polythiophene films has been studied using time resolved in situ transmission spectroscopy in the wavelength range 350-820 nm. Several distinct stages to the polymerisation process have been identified. In the first of these, a small quantity of intermediates, which we suggest might be short chain oligomers, was observed. At slightly longer times, which a previous electrochemically based analysis associated with the expansion of growth sites, the absorbance shifted to longer wavelengths. Finally, features normally considered to be characteristic of the “metallic” form of the polymer predominate. Correlation of spectroscopic and electrochemical data showed these clearly, together with more subtle changes occurring in thicker films which were not apparent from the electrochemical data alone. Absorbance vs. charge plots were used to estimate optical parameters for growing films: whilst we found no evidence for a potential dependence of these parameters in the range 1.80-1.95 V, the values were different from those obtained at high potentials.

INTRODUCTION

Polythiophene is one of a range of conducting polymers which has been the subject of considerable recent interest. A number of applications based on the electronic and optical properties of thiophene- and pyrrole-based polymers have been proposed. These include batteries [l] and various electronic devices (diodes [2], triodes [3], transistors [4] and ion gates [5]), in the first case, and display devices [6] in the second case. The fact that films of these materials can be produced electrochemically has been claimed to be an advantage for several reasons: one step polymerisation/deposition, uniformity of coating, simultaneous doping and (at least in principle) control over the process via the electrode potential. Recently, we have studied the nucleation/growth process of polythiophene films on gold electrodes [7], our objectives being to obtain a detailed understanding of the way in which these films are formed and to see whether their ultimate properties could be controlled, in particular via the electrode potential. The general approach followed 0022-0728/88/$03.50

0 1988 Elsevier Sequoia

S.A.

404

that used by Pletcher for pyrrole-based systems [8,9], adapted from model treatments of metal [lo] and metal oxide [ll] electrodeposition. On the basis of current responses to potential steps, we proposed a model for the nucleation and growth of polythiophene films and made estimates of some of the kinetic parameters as functions of polymerisation potential [7]. However, in common with many electrochemical experiments, much of the deductive process was indirect. Very simply, we were able to quantify the consumption of monomer but not quantify or identify the species produced. We have therefore looked to spectroscopic techniques which have been widely used in this respect [12-141. In this paper we describe the use of time-resolved in situ UV-visible spectroscopy to monitor the growth process. Our general objective was to test our electrochemically elucidated nucleation/ growth model. More specifically, we wished, firstly, to observe intermediates, secondly, to see whether film optical (and by inference other) properties were dependent on deposition conditions and, thirdly, to look for the onset of metallic behaviour. Additionally, we have found that under certain conditions, film deposition failed: whilst the electrochemical response was characteristic, it gave no indication as to the process(es) occurring. It seemed possible that UV-visible spectroscopy might be helpful here. All the measurements reported here were carried out in transmission mode. Consequently the experiments “see” surface-bound and solution-phase species, but do not distinguish between them. This is in contrast to ellipsometric data [15,16], which detects only surface-bound species. The apparent disadvantage of the technique we use here can thus be turned to advantage by combining the two sets of data. This comparison will be the subject of a forthcoming report; here we report the results of the UV-visible element of this work. EXPERIMENTAL

The electrochemical instrumentation has been described in detail elsewhere [7]. Briefly, the potential was generated by a purpose-built variable height/variable duration pulse unit and controlled by an Oxford Electrodes potentiostat. Current transients were recorded on a Gould OS4020 digital storage oscilloscope and output to a Bryans 60000 series X-Y-t recorder. UV-visible spectra were acquired using a Hewlett-Packard HP8451A diode array spectrophotometer. Application of the potential pulse to the working electrode was triggered from the keyboard of the spectrophotometer. Because there is a time lapse between issuing the command and the opening of the spectrophotometer shutter, a delay was introduced in the command to the pulse unit. The correct value of this delay was determined by comparing the applied electrode potential with the output from a photodiode positioned by the working electrode. This element of synchronisation, which was found to be reproducible to ca. 10 ms, is important in the correlation of transient optical and electrochemical responses described below. The CH,CN solvent (Analar) was refluxed over CaH,, distilled and stored over molecular sieves. 0.1 mol dmF3 tetraethylammonium tetrafluoroborate, TEAT,

405

(puriss. Fluka) was used as background electrolyte. Monomeric thiophene (Aldrich) was added in the required amounts via a syringe. The working electrode was Pt or Au sputtered on previously ultrasonically cleaned glass (for measurements in the visible region only) or quartz (for measurements extending into the UV region). Since all measurements were made in transmission, a compromise had to be made between Pt/Au absorbance and electrode resistance. Films of overall resistance > 30 G were rejected; typical values were lo-20 Qt.Silicone rubber cement was used to mask off all but ca. 1 cm2 of the electrode. This ensured that the distribution of potential (as a consequence of ohmic drop) over the exposed area was not more than 10 mV, which is of the order of the level of reproducibility associated with the nucleation studies [7]. All potentials were measured and are quoted with respect to SCE. The counter electrode was a large Pt foil placed opposite the working electrode with a hole cut to allow passage of the light beam. This configuration gave even films by eye. The cell consisted of a 4 cm x 1 cm quartz fluorescence cell, with a teflon lid machined to take the three electrodes and argon (pre-dried) via a teflon needle. Whilst all solutions were purged with argon, the gas stream was directed over the solution during measurements to maintain quiescent solution conditions. Molecular sieves were used to minimise the water content; however, since these were not “glove-box” experiments, some water is inevitably present. Measurements were made at room temperature (20 o C). The potential profile consisted of a double potential step from 0 V to the polymerisation potential, EpO,,for a time t,, then back to 0 V. The spectrophotometer was programmed to take spectra at chosen intervals of time (usually 1 s) over selected wavelength regions (usually 350-820 nm at 2 nm intervals): these are given with the relevant data. Since it is a single beam instrument, a separate reference measurement is required. The programme was written so that this was taken immediately prior to issue of the command to apply the potential step. In the assessment and treatment of data, several points are worthy of mention. Firstly, in those cases where the working electrode was Au, there is the question of electrode dissolution to be answered. In our previous work [7] we deduced that this was not the dominant contributor to the current at any stage. In this work we have found that it contributes slightly to the overall current at short times: this is evidenced by a very small decrease in absorbance in the first one or two (dependent on E,,,) spectra. We have been able to correct for this by running a “blank” experiment without thiophene present, so that Au dissolution is the only contributor to the current, and scaling the changes in absorbance at 820 nm in a real experiment to this change. We estimate that this procedure results in absorbance data accurate to ca. 50% for the first one or two points, as will be evident from the data quoted below at these times. Thereafter, the problem is minimal because, firstly, Au dissolution is prevented by the overlying film and, secondly, because the absorbance changes associated with oligomer/polymer formation dominate. A second point we wish to make concerns quality control of growth transients and the resulting films. A simple test of whether film growth is occurring in a manner and at a rate consistent with our previous measurements, is to use the current plateau value at

406

long times (i,,,) to extract a value for the rate constant k,, which can then be compared with the value previously determined at that potential. In all cases, with the exception of the “failed” transients described below, the agreement was good. The other check we operated to ensure satisfactory growth was to perform a cyclic voltammogram directly after film deposition. In any case where either criterion was not satisfied the data were rejected. RESULTS

AND DISCUSSION

Qualitative observations

Spectra taken during the course of a polymerisation run are shown in Figs. l-3. The timescales, which are given with each figure, have been selected to highlight the features we now discuss. The potential used is intermediate in the range we have investigated (1.68 G E&V < 2.00) and again, for illustrative purposes, was chosen to show the types of behaviour seen at different times. As we found in our purely electrochemical work, increasing the value of E pO, caused the observed responses to be compressed in the time domain, so that different features illustrated in Figs. l-3

1

t

w

I c

WAVELENGTH

/ nm

Fig. 1. Spectra obtained at early times during the course of thiophene polymerisation. The polymerisation potential was 1.80 V; the solution contained 50 mM monomer in 0.1 M TEAT+ CH,CN. The electrode was a thin Au film on glass. Successive spectra were acquired at 1 s intervals: the lowest curve is that at t=ls,thehighestat r==3s.

UAVELENGTH /rum

Fig. 2. Spectra obtained at intermediate times: the lowest curve corresponds to f = 4 s, the highest curve to t = 8 s. These spectra constitute the remainder of the experiment shown in Fig. 1.

became progressively less distinct. The i-t responses were, within experimental error, the same as described previously [7] but, to aid comprehension, we include the current transient for Figs. l-3 (with typical times for spectral acquisition marked) in Fig. 4. Looking first at Fig. 1, we see evidence for a peak at 475( rf:10) nm and a clear peak at 58O(f 5) nm. The first peak is at the limit of instrumental sensitivity, so quantitation is not possible, but every run producing a good polythiophene film does show this feature so we believe it to be real. The second feature at 580 nm grows as a roughly symmetrical band with time for between approximately 2 and 6 s, the timescale decreasing with Ewl. Reference to Fig. 4 shows these features to be associated with the very early stages of growth when, on the basis of our electrochemical data [7], we believe the polymer growth sites are still distinct, rather than extensively overlapped. This data alone does not enable us to say whether the species observed are in the solution near the electrode or on the electrode surface: ellipsometric experiments [16,17] resolve this ambiguity. At slightly longer times, ranging from 2-7 s at high EPO, to 6-15 s at low EpO,, the comparatively symmetrical peak at 580 mn is replaced by a feature which shows roughly the same shape on the short wavelength side, but which becomes increasingly flat on the longer wavelength side, as shown in Fig. 2. We associate this increasing absorbance at lower energies with the evolution of longer chains. Note

408

WAVELENGTH

/M

Fig. 3. Spectra obtained at longer times. These were obtained during the course of an experiment identical to that described by Figs. 1 and 2, except that data were acquired at 2 s intervals and for a much longer period of time, 90 s. For clarity of presentation, we display only every 3rd spectrum, commencing with that at 11 s (lowest curve) and ending with that at 71 s (top curve). For the plots in Figs. 5 and 6 we used data from all the spectra.

that scales of Figs. l-3 are rather different. Whilst the small features of Fig. 1 are increasingly swamped by the absorption band at long wavelengths, we are frequently able to discern them (after suitable amplification of the absorbance axis) at these times, so presumably there are still small (steady-state?) amounts of these species present. As marked in Fig. 4, these changes occur during the latter part of the rising portion and the early part of the plateau region of the current transient. Finally, at longer times, corresponding to the plateau region of the current transient, the absorbance profile becomes more pronounced at long wavelengths and we are only able to see the high energy end of the feature with our instrument. This is the band extending out into the near-IR described by other workers studying comparatively thick films, ca. 0.1-10 pm (e.g. refs. 18 and 19) and associated with the metallic behaviour of the oxidised form of the polymer [20]. Semi-quantitative considerations We first try to obtain some estimate of the nature and concentration of the oligomers observed in experiments typified by the data in Fig. 1. The peak at 580 nm has a half width at half maximum (hwhm) of ca. 100 mn, which is considerably broader than one might expect for a solution species, e.g. the monomer has a peak

409

0

2

4

6

/I 811

25

10

50

15

>

%

Fig. 4. Current transient for polymerisation of thiophene, showing the times at which different spectral features are observed. The plotted curve is composed of data from two experiments, those of Figs. 1 and 2 (for t = 1-8 s) and Fig. 3 (for t = lo-90 s), respectively. Note that the join between the traces indicates that the level of reproducibility is sufficiently good to permit comparison of the optical data between experiments under nominally identical polymerisation conditions, for example at short/long times at high/low time resolution. The regions covered by Figs. 1-3 are indicated by the schematic spectra shown in the insets.

at 232 nm with hwhm 23 nm. Undoubtedly, if this were a surface attached species, one might expect some broadening [21], but we also suspect that in fact there are several absorbing species present with similar absorption maxima. The observed feature is then the envelope of overlapping bands, the peak characterising the dominant chromophore. We have tried to estimate the length of the chains at this point, using literature data for authentic oligomeric thiophene species [22]. Empirically it is found that a logarithmic plot of peak wavelength vs. number of linked thiophene rings, nr, is linear for values of nr up to 5, at which point the peak wavelength is 418 nm. Extrapolation to 580 nm gives nr - 12, i.e. the predominant chromophore contains about 12 thiophene units. Note here that we have been careful to use the word chromophore, rather than oligomer, since longer chains with broken conjugation would give the same result. We also wish to make it plain that this is only a very approximate value for two reasons: firstly, the points on the “calibration” plot become more closely spaced as nT increases (about 17 nm apart at this point) and, secondly, these data are for neutral exclusively a--(~’ linked species. Protonation is not considered a problem [23]. We expected to detect much shorter chains, which would absorb in the region 300-500 nm but, with the exception of the very small peak at 475 run, we find no evidence for these species. We have no explanation as to why there should be an apparent preference for the observed chromophore, but the important deduction is that reaction proceeds via a common intermediate across the potential range we have employed. We now make an order of magnitude comparison of monomer consumed and intermediate observed, using the experiment of Fig. 1 as a typical case. Coulometri-

410

tally we find 14 nmol cme2 of monomer are consumed during the first 4 s. This corresponds to 5% of that available via diffusion. The absorbance change at 580 nm is 0.008. We assume rough equality of extinction coefficients with those of authentic oligomers [22] (inspection of the latter data shows that precise identification of the intermediate(s) does not in fact alter the argument at this crude level). This leads to 0.16 nmol cm- 2 of intermediates, equivalent to a mean concentration of 1.6 X lo-’ mol dme3 within the diffusion layer, or 0.05% of the available monomer. The difference, a factor of 100, in the populations of monomer consumed and intermediate produced arises for two reasons: firstly, there are several (we estimate ca. 12) monomer units per detected chromophore and, secondly, the absorbance used accounts only for the predominant oligomer, and the peak width implies the presence of several species. At longer times (Figs. 2 and 3) the data is more reliable. Here we use the optical response, Abs, as a measure of the polymer produced Abs = cc,1 = CT

0)

where z is the extinction coefficient, cr is the concentration of film chromophores, I is the (mean) film thickness and I?/nmol cme2 is the number of immobilised chromophores. The charge passed, q, is a measure of the quantity of monomer consumed. Assuming that all the monomer consumed ends up on the electrode surface (a point we return to in a later section) r is related to q by I? = q/nFA

(2) where n is the number of electrons involved in polymerisation/oxidation of each thiophene unit and A is the electrode area. Combination of eqns. (1) and (2) gives Abs = cq/nFA

(3) so that plots of Abs vs. q should allow extraction of c as a function of wavelength. Furthermore, deviations from linearity, changes in slope or the appearance of intercepts in plots of eqn. (3) should highlight complications such as commencement/cessation of generation of a particular chromophore or changes in optical properties. Figures 5 and 6 show sample data obtained at wavelengths of 580 nm and 750 nm during the course of a polymerisation at 1.80 V. The insets show the behaviour at short times. We draw attention to several features on these plots. The obvious one is that we do not obtain a single straight line through the origin: the process therefore involves more than one chromophore and that observed first does not appear immediately. At other potentials we see similar features: a positive intercept on the charge axis followed by up to four linear sections, the first having a slightly smaller slope than the third and both of these having significantly smaller slopes than the second and fourth sections. At the largest values of I$,,r the polymerisation is sufficiently rapid that the time resolution (1 s for acquisition of a full spectrum) does not allow us to discern the first short linear section. For those potentials at which we can obtain enough points to draw an acceptable linear section at short times, we find that the intercepts on the charge axis are 0.5

411

005

II OOL-

/ 0 03.

0.7!

/

0°2. /,

/ //

,’

I

o-5

J Iv

/

AbS

O-Z!

t

Fig. 5. Plot of absorbance at 580 nm vs. charge during the course of thiophene polymerisation under the conditions of Figs. l-3. Data in the main part of the plot were taken at 2 s interval, whilst those in the inset were taken at 1 s intervals.

mC cmW2. At the time corresponding to this intercept, the current is in the lower half of the rising portion (see Fig. 4), where we have previously stated [7] that the nuclei are still discrete. The break to the second linear section occurred typically after 5 mC cmm2. At the times corresponding to these charges, the current is typically 85-98% of the plateau current, iIII [7]. The significance of these larger

412

Abs

100

50

150

qh-li2 Fig. 6. Analogous

data to Fig. 5 at 750 nm.

values, compared to the intercepts, is that the change in spectral features occurs when the growing nuclei are extensively overlapped [7]. Equation (3) was used to estimate extinction coefficients, c,-e,,, for the four

413

180

185

190

195

Epol/V

Fig. 7. Plot of ~,,t at 750 nm vs. I?,,. The full line is the result of a least mean squares analysis, which yields a slope of -900 (+1400) (M cm V-l), i.e. a variation of 135 (k210) over the 0.15 V interval studied. The dashed line is the mean value quoted in Table 1.

linear sections at two wavelengths (580 nm and 750 nm) as a function of potential. As the example of Fig. 7, for elII at 750 nm, shows, we find no evidence for a systematic trend with potential in this range (see below for behaviour at higher potentials). The data are summarised by the mean values of c+iV in Table 1. Comparing first the values of E at the two wavelengths for each linear section, we find that for the first region they are larger at 580 nm, but thereafter they are greater at 750 nm. Our supposition is that at early times only oligomers are present, the absorbance at 750 nm being the “tail” of their absorption envelope. In the final section, polymer is the dominant chromophore, so absorption at longer wavelengths is greater. We have no detailed explanation for the second and third breaks in the Abs vs. q plots. They both occur during the period of time when the current is roughly constant, so there is no electrochemical evidence there for any marked change in the growth process. All we can surmise is that these smaller optical changes are associated with more subtle structural changes occurring when the “metallic” band starts to appear. Finally, we compare our values of c obtained during growth with literature optical density data obtained after growth. Kaneto et al. [24] obtained values for a (= ccr) of 4.0 X lo4 and 5.5 X lo4 cm-’ at 580 nm and 750 run, respectively. Using an ellipsometrically determined value of ca. 12 M for cr [26] in a solvated oxidised polythiophene film, these give values for z of 3300 and 4600 M-’ cm-‘, respectively. We attribute this to a difference in conditions: Kaneto et al. employed much

414 TABLE 1 Mean values in the potential range 1X0-1.95 V of extinction coefficients at 580 nm and 750 nm for polythiophene at different stages in the polymerisation process Wavelength/nm

e/M-t

cm-’ a

I

II

III

IV

580 750

900 770

1370 1520

900 1040

1210 1430

a Typical standard deviation for quoted values across the potential range 1.80-1.95 V is 20%; within an individual run at any given potential, typical standard deviation on slope of a linear section is 5% or better, as exemplified by Figs. 5 and 6.

higher monomer concentrations (ca. 0.4 mol dmP3) and voltages (lo-20 V). In the first case, Roncali et al. [25] state that monomer concentration affects polymer properties markedly. In the second case, ellipsometric data [16,17] shows a change in optical properties at values of Fro, = 2.1 V. Our data are similar to recent data on the poly(3-methylthiophene) system [27] obtained under similar conditions. In this latter work, the authors showed a plot analogous to Figs. 5 and 6 but for data at 600 nm, showing the same qualitative features and an intercept of about 2 mC cme2. Failure of film deposition procedure

On occasions we found that, as shown by subsequent cyclic voltammetry, electroactive polymer was not deposited. The current transient did not have the general shape shown in Fig. 4: instead we only saw an initial spike followed by a continuously falling current and recovered only a minimal charge on stepping the potential back to 0 V. In extreme cases, where the current fell to low values over about 5 s, we saw no peaks in the visible region. Our presumption here is that the product of monomer oxidation is intercepted before it can form oligomer. In less extreme cases, where the fall in current was less pronounced, we saw data of the type shown in Fig. 8. A clear peak at 570 nrn is seen, the height of which increases with time, but at a slower rate than seen in Fig. 1 for a “successful” transient. No significant absorbance is found in the 650-820 nm wavelength region. Whilst ohmic potential losses caused by Au dissolution must be considered, we discount this possibility for two reasons. Firstly, loss of a resistively significant amount of material would result in a large increase in transmission, which we do not see. Secondly, a purely electrochemical experiment using a bulk Au electrode gives the same i-t behaviour. The similarity of the peak position to that observed during the initial part of a successful polymerisation leads us to suggest that the same intermediate(s) are produced, but that they are intercepted by a non-thiophene derived species. Since failure was invariably associated with relaxation of the solvent drying procedure, the prime candidate is water. The importance of water in polymerisation of aromatic heterocycles has also been reported by others [28]. At least a fraction of the products of this undesirable reaction is probably deposited

415 I

'mA

4

0

1

2

3

4

+/,

i .!I04

.003

.002

YAVELENCTH

/mu

Fig. 8. Data for a “failed” polymerisation. The polymerisation potential was 1.74 V and the monomer concentration was 50 mM. The electrode area was 2.5 cm’. (a) Current vs. time transient. (b) Spectrum at t=4s.

onto the electrode as a thin insulating film. Coulometric considerations show that such a film ( - 2 nm thick) would require only - 20% of the monomer consumed in the experiment of Fig. 8 to follow this route.

416 CONCLUSION

Consistent with a previous electrochemical study, the growth of polythiophene films is found to occur in several distinct stages. Each of these stages is characterised by a different slope on an absorbance/charge plot. At early times, typically up to 6 s, we were able to observe two peaks, at 470 nm and 580 nm, due to the formation of a small quantity of intermediates. On the basis of literature data for authentic thiophene oligomers, we suggest that these might be short chain oligomers, containing approximately 7 and 12 monomer units. Crude estimates of the quantity of monomer consumed and oligomer produced (based on charge passed and literature extinction coefficients) are in fair agreement. The same intermediates, regardless of their identity, are observed at all the potentials studied here. At slightly longer times, corresponding to the rising portion of the current-time transient, the absorbance at wavelengths greater than - 600 nm increases markedly. This is associated with a break in the absorbance/charge plot. We have previously associated this phase of growth with the expansion of nucleation sites, so this is consistent with the production of longer chains. We have quantified this to some extent via absorbance/charge plots, the slopes of which (extinction coefficients) change from being greater at 580 nm (short times) to being greater at 750 nm (long times). At long times, the “metallic” absorption band extending into the near-IR described by numerous literature reports [18,19,24] predominates. Even here, however, the situation is not quite as simple as it is often portrayed, for we see two additional distinct linear sections in the absorbance/charge plots. There is no obvious qualitative change in either the electrochemical data (current) or the spectra which offer any insight as to the origin of this more subtle effect. Within the potential range for which we were able to obtain quantitative information, 1.80-1.95 V, we found no evidence for variation with potential of the optical parameters for any of the individual growth sections at any wavelength. However, the values obtained were different from those which have been obtained at significantly higher potentials. Comparison of the current-time behaviour and the spectral features shows that “metallic” characteristics appear comparatively suddenly and only when, on the basis of our previous nucleation and growth study, we believe overlap of the growth centres is essentially complete. It does not seem that “metallic” behaviour is approached progressively as the polymer chains grow and interact with each other. One must then move from the molecular description we have employed at short and intermediate times to one based on band theory, as is conventional for descriptions of the electronic properties of bulk materials. ACKNOWLEDGEMENTS

We thank the SERC for an equipment grant and one of us (E.F.M.) thanks the SERC for a studentship.

417 REFERENCES 1 P.J. Nigrey, D. MacInnes, D.P. Naims, A.G. MacDiarrnid and A.J. Heeger, J. Electrochem. Sot., 128 (1981) 1651. 2 H.D. Abruna, P. Denisevich, M. Umana, T.J. Meyer and R.W. Murray, J. Am. Chem. Sot., 103 (1981) 1. 3 P.G. Pickup and R.W. Murray, J. Electrcchem. Sot., 131 (1984) 833. 4 E.W. Paul, A.J. Ricco and M.S. Wrighton, J. Phys. Chem., 89 (1985) 1441. 5 P. Burgmayer and R.W. Murray, J. Electroanal. Chem., 147 (1983) 339. 6 K. Kaneto, K. Yoshino and Y. Inuishi, Jpn. J. Appl. Phys., 22 (1983) L412. 7 A.R. Hillman and E.F. Mallen, J. Electroanal. Chem., 220 (1987) 351. 8 S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 229. 9 S. Asavapiriyanont, G.K. Chandler, G.A. Gunawardena and D. Pletcher, J. Electroanal. Chem., 177 (1984) 245. 10 M. Fleischmann and H.R. Thirsk in P. Delahay (Ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 3, Wiley-Interscience, New York, 1963, p. 123. 11 Southampton Electrochemistry Group, Instrumental Methods in Electrochemistry, Ellis Horwood Ltd, Chichester, 1985, p. 283. 12 W.R. Heineman, F.M. Hawkridge and H.N. Blount in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 1. 13 A.J. Bard and L.R. Faulkner, Instrumental Methods, Fundamentals and Applications, Wiley Interscience, New York, 1980, p. 577. 14 Southampton Electrochemistry Group, ref. 11, p. 317. 15 R. Greef in R.E. White, J.O’M. Bock&, B.E. Conway and E. Yeager (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 8, Plenum Press, New York, 1984, Ch. 5. 16 A. Hamnett and A.R. Wman, Ber. Bunsenges. Phys. Chem., 91 (1987) 329. 17 A.R. Hillman, A. Hamnett and P.A. Christensen, Proceedings of the 171st Electrochemical Society Meeting, Philadelphia, 1987, Abstr. 343. 18 C. Taliani, R. Danieli, R. Zamboni, P. Ostoja and W. Porzio, Synth. Met., 18 (1987) 177. 19 Y. Furakawa, M. Akimoto and I. Harada, Synth. Met., 18 (1987) 151. 20 G.K. Chandler and D. Pletcher, Specialist Periodical Report on Electrochemistry, Vol. 10, Royal Society of Chemistry, London, 1984, Ch. 3. 21 A.R. Hillman in R. Linford (Ed.), Electrochemical Science and Technology of Polymers, Vol. 1, Elsevier Applied Science Publishers, London, 1987, Ch. 5. 22 J.W. Sease and L. Zechmeister, J. Am. Chem. Sot., 69 (1947) 270. 23 A. Albert, Heterocyclic Chemistry, 2nd edn., Athlone Press, London, 1968, p. 263. 24 K. Kaneto, K. Yoshino and Y. Inuishi, Solid State Commun., 146 (1983) 389. 25 J. Roncali, M. Lemaire, R. Garreau and F. Gamier, Synth. Met., 18 (1987) 139. 26 A. Hamnett and A.R. Hillman, in preparation. 27 P. Lang, F. Chao, M. Costa and F. Gamier, in press. 28 A.J. Downard and D. Pletcher, J. Electroanal. Chem., 206 (1986) 147.