Electrochemical study of poly(phas) in acetonitrile and water + acetonitrile electrolytes

183

J. Electrounnl. Chem., 238 (1987) 183-195 Elsevier Sequoia S.A., Lausanne - Printed

in The Netherlands

ELECTROCHEMICAL STUDY OF POLY(PHAS) WATER + ACETONITRILE ELECTROLYTES

P. AUDEBERT

IN ACETONITRILE

AND

and G. BIDAN

Cenire d’Eiudes Nu&aires de Grenoble, DPpartement de Recherche Fondameniale, Laborarorres de Chrmie/Equrpe Electrochlmre MoYculawe, 85 X, 38041 Grenoble Cedex (France) (Received

24th March

1987; in revised form 6th July 1987)

ABSTRACT The behaviour of electrodes coated with either monomeric or polymeric anthraquinone (AQ) deposits has been investigated in aqueous media. The basal compound PHAS ([l-(pyrrol-1-yl)-hex-6-yl]-9,10anthraquinone-2-sulphonate), consisting of an anthraqumone linked to a pyrrole moiety via an alkyl linkage, is electroactive either as air-dried monomer layers or when it IS electropolymerized on different electrodes at various pHs. However, platinum-coated electrodes were found to work only at basic pH (7-14). whereas glassy carbon electrodes work in the entire pH range (O-14). with the redox potential of the AQ/AQH, couple depending linearly on ambient pH. Unlike the monomer layers, polymer films are electroactive only if 30% acetonitrile is added to the aqueous electrolyte.

INTRODUCTION

In earlier work [la,b] we reported the electrochemical preparation of various polypyrrole films bearing covalently attached anthraquinone (AQ) units and their behaviour in DMSO electrolytes. In such aprotic conditions, the bonded anthraquinones can be reduced by two successive one-electron transfers to their radical anion (AQ)‘-) and dianion (AQ’-) forms according to the following scheme: +1

AQ

+

e-

e-

fl

AQ’-

+

AQ2-

Moreover, complexation of the AQ2- species with lithium cations has been pointed out in these studies. This paper presents further investigations on this kind of polymer. The present study will be limited to the case of poly(PHAS), since the other related polymers were found to exhibit very similar characteristics [lb]. In the first part, the electrochemical behaviour of these polymer films in anhydrous acetonitrile with either TEAFB or LiClO, as the supporting salt will be described briefly. Using TEAFB (tetraethylammonium fluoroborate), comparable results are 0022-0728/87/$03.50

0 1987 Elsevier Sequoia

S.A.

184

obtained in DMSO and acetonitrile, but in this latter solvent a slightly different behaviour of the polymer films is noticed when LiClO, is used. In fact, the lithium complexation of the anionic quinonic species appears to be stronger in acetonitrile than in DMSO and moreover affects the AQ’- as well as the AQ*- species. This would be expected from the previous results concerning the electrochemistry of quinones in acetonitrile in the presence of alkaline cations which show a strong complexation of the latter with both charged quinonic species [2]. The next step in our research interest was to examine the behaviour of the films in protic media and especially in water. Unfortunately, all the films were found to be electrochemically inactive in pure aqueous electrolytes, which led us to replace them by different water + acetonitrile mixtures. The second part of this paper is therefore devoted to the study of some poly(PHAS) films in such electrolytes, with either TEAFB or tetraethylammonium hydroxide (TEAOH) as the supporting salt, in relation to both the water content and the pH. In contrast to their inactivity in pure water, the films were found to be active in as much as 70% of water in acetonitrile in the O-14 pH range, with both the shapes and the potentials of the voltammetric peaks being pH-dependent. This study accords with studies of the monomer either adsorbed in a thin layer on the electrode or incorporated into a carbon paste electrode (CPE) [3] in pure aqueous electrolytes, since PHAS is insoluble in water as well as in water + organic solvent mixtures. In addition, a test for the catalytic activity of poly(PHAS)-coated electrodes in dry TEAFB acetonitrile towards the dioxygen reduction is provided EXPERIMENTAL

The synthesis of PHAS has been described previously [l], as well as its electropolymerization in a two-compartment cell from a 2 X 10m3 M monomer solution in 0.1 M LiClO, in CH,CN. The platinum electrode used had a diameter of 2 mm and the glassy carbon, one of 3 mm. Films were grown at a constant potential of + 0.8 V vs. an Ag/10e2 A4 Ag+ reference. For the experiments described in Section (II), PHAS layers were obtained by syringing 1 ~1 of a lo-* M PHAS solution on the electrode surface, which led to an average surface concentration P of 3 X 10m7 mol cm-2 in the case of the platinum electrode and 1.3 x 1O-7 mol cmm2 in the case of the glassy carbon electrode. The carbon paste electrode (CPE) has been described recently [3], and the paste was prepared by mixing together 15 mg of furnace black (Degussa), 2 mg of monomer and a few drops of electrolyte. The preparative electroreduction of PHAS was performed on a stirred solution of 2.5 X low4 M of PHAS in 50 ml of anhydrous 0.1 M TEAFB acetonitrile, passing 40 C (80% of the theoretical charge) at a constant current of 15 mA between 0 and 30 C, and 10 mA between 30 and 40 C. The working electrode was a 10 cm2 platinum grid and its potential never fell below - 1.9 v. Pure acetonitrile electrolytes were prepared with Merck (HPLC grade); the acetonitrile was dried for 2 days over activated neutral alumina (Woelm, Akt. grade

185

I). Water mixtures were prepared with the same acetonitrile, but undried, and the water was from a Millipore Mili Q system. Buffered solutions were prepared after Merck titrisol preparation, and mixed with 0.1 M TEAFB acetonitrile when required for polymer studies. For strongly basic solutions, 1 M or 0.1 M tetraethylammonium hydroxide solutions were used, and perchloric acid at the same concentrations for strongly acidic solutions. The electrolyte salts TEAFB (Fhtka) and LiClO, (G. Frederick Chem. Co.) were dried for 2 days in vacuum at 80 and 150°C respectively. RESULTS

AND DISCUSSION

(I) Electrochemical acetonitrile The electrochemical cantly in dry TEAFB

AC

:j I ,';Aa-

behaviour

response acetonitrile

d_:’

a

c

b

of PHAS

2”O

and the poly(PHAS)

films

in anhydrous

of the poly(PHAS) films does not differ signififrom that in dry DMSO [lb]. Provided that the

Cycle

b 60’”

”

c 3001k

”

Fig. 1. Cyclic voltammograms of a poly(PHAS) film in 0.1 M TEAFB acetonitrile. The fiim was prepared by passing 3.3x10-* C cme2 at a potential of +0.8 V in 2X lo-’ M PHAS solution in 0.1 M LiClO, acetonitrile. Scan rate: 50 mV s-l. Fig. 2. Full tines: Cyclic voltammograms of poly(PHAS) films in 0.1 M LiClO, acetonitrile. The film was prepared by passing (a) 8x10-’ C cmd2 and (b) 0.33 C cme2 at a controlled potential of 0.8 V of 2 x 10K3 M PHAS solution in 0.1 M LiClO, acetonitrile. Scan rate: 100 mV s-’ (a) and 50 mV s-l (b). (a) x = 0.1 pA; (b) x = 2 pA. Dashed line: Cyclic voltammogram of the monomer PHAS (2 x 10e3 M) in the same electrolyte at 50 mV s-l; x = 8 pA.

186

electrolyte salt does not contain alkali cations, two well-defined one-electron transfers are observed, leading to the formation of the radical anion AQ’- and the dianion AQ2- species, respectively, inside the film (Fig. 1). The only difference concerns the AQ2- formation redox potential, which is slightly less negative in ion-pairing [4] is stronger in this acetonitrile, probably showing that AQ2- -N(Et): solvent than in DMSO. Lithium

effects

When TEAFB is replaced by LiClO, as the electrolyte salt, the voltammograms of both the monomers and polymers (Fig. 2) change drastically, since very strong complexation by lithium cations occurs not only with AQ2- species, as observed previously, but also with the generally less complexed AQ’- species. A solution of PHAS monomer exhibits voltammograms with two peaks shifted towards positive potentials and closer together (Fig. 2) if one compares it with the behaviour of PHAS in DMSO. Similar behaviour observed with the benzoquinone (Q) has been attributed to the precipitation on the electrode of the [Q’--Li+] complex [2b], an explanation which has been confirmed by the observation of a blue film on the electrode appearing when chloranil is reduced under analogous conditions [2c]. On the voltammograms of the poly(PHAS) films, two separated peaks are also observed, which are more shifted towards positive potentials if compared with the monomer behaviour. Such behaviour is not unexpected since it should be considered that no precipitation is liable to occur inside the film because of the bonded character of the AQ units, which allows two-electron transfers to be observed, as in TEAFB acetonitrile. The large positive shifts of the peaks are attributable to the complexation of both charged species with the Li+ cations. The AQ2- formation peak, which has never been previously observed in the presence of Li+ cations in acetonitrile, is situated at a potential showing that these latter species are still more complexed than the AQ’- under such conditions. However, we have no clear explanation of the form of the peak corresponding to the second reduction transfer. Moreover, it should be noted that all these results are valid only with thin poly(PHAS) films (O-20 pm). (With poly(PHAS), films with thicknesses in the range O-20 nm (see ref. lb) correspond approximately to films prepared by passing O-l.3 X 1O-3 C cme2.) With thick films, the voltammograms show an additional peak in a potential range (Fig. 2) dependent on the scan rate, and the dianion couple ceases to be observed. Since adsorbed protons have been shown to play an important role in the electrochemistry of substituted polypyrroles [lb,5], it is probable that in this case the behaviour of the films becomes very complex and involves at least the addition of polypyrrolic protons on anionic quinonic species as well as the lithium cation complexation cited previously. Stability

As noted previously [lb] in DMSO, slow protonation occurs in poly(PHAS) films subjected to prolonged cycling, which leads to the almost complete disappearance of the AQ/AQ’system, when the film is cycled only over this system (i.e. between

187

Fig. 3. Cyclic voltammogram of a solution of 9,10-dihydro (PHAS) prepared by electrochemical preparative reduction of 2.5 ~10~~ M of PHAS in 50 cm3 of 0.1 M TEAFB CH,CN (80% of the theoretical charge passed) followed by addition of 5 X 10m4 M perchloric acid. Fig. 4. Cyclic voltammograms of a PHAS layer deposlted on a Pt electrode. The layer was obtained by air-drying 1 ~1 of lo-* M PHAS solution in dry CH,Cl, (r = 3 x lo-’ mol cm-*). Electrolyte: 0.1 M TEAOH H,O. Scan rate: 50 mV s-‘. (a) First cycle; (b) second cycle; (c) third cycle.

- 0.5 and 0.3 V). As in DMSO, such protonation is hindered by previous dipping of the film in collidine. In addition, if the passivated film is cycled over a wide oxidation potential range (up to + 0.5 V), a large reoxidation peak is again observed around O-O.1 V, followed by the restoration of about 80% of the initial electroactivity of the film [lb]. If the film is cycled repetitively over the same potential range, no such phenomenon is observed and the film activity decreases by 10% over 300 cycles (Fig. 1); this is probably caused by effective degradation of the AQ units. In our previous paper [lb], we attributed the reoxidation peak observed on cycling passivated films to the reoxidation of protonated AQ species. To ascertain the identity of the oxidized species, we attempted the electrochemical preparation of dihydro(PHAS). Anthraquinols are known to be unstable species [6] which are reoxidized very easily (by oxygen, for example) to the original anthraquinone. Consequently, the reduction of PHAS to dihydro(PHAS) (DHPHAS) was performed in dry TEAFB acetonitrile in a dry box in two steps. First, the dianion was prepared electrochemically before two equivalents of perchloric acid were added. The initially deep violet solution turned to a yellowish fluorescent green. The cyclic voltammograms showed an oxidation peak (P, ) at 0.15 V and two reduction peaks (P, and P3) at -1.12 and -1.66 V attributable to the PHAS formed (Fig. 3). The situation of P2 and P3 as well as that of P, allows this latter to be attributed to AQH, oxidation in acetonitrile. Since P, occurs at the same potential as the reoxidation peak observed in long-time cycled films [lb], AQHz units have probably

188 to be invoked also in this case. Their production mechanism certainly implies the dismutation of AQH’- radicals issued from the protonation of AQ’- radical anions by adsorbed polypyrrolic protons, since the films were cycled only over the region of AQ’- existence.

(II) Electrochemistry

in water and water + acetonitrile

PHAS behaviour Pyrrole-anthraquinone monomers are extremely insoluble as soon as water is present in the electrolyte solution. Therefore we had to study PHAS in its solid state, and for this we chose first to perform cyclic voltammetric experiments on thin PHAS layers deposited by the evaporation of a PHAS solution on a platinum electrode. The results appeared somewhat disappointing since electroactivity appeared only in the pH range lo-14 (Fig. 4) and no electrochemical signal was recovered from the quinonic activity at lower pHs. Such behaviour was surprising when one compares this with previous literature results [7], so two additional series of experiments were performed with carbon electrodes in order to determine whether the electrode nature had a major influence. In a first series of experiments, PHAS crystals were crushed with graphite and inserted in a carbon paste electrode (CPE) that we used previously, and in a second series, we repeated on a glassy carbon electrode the experiments done first on the platinum electrode. The results of both types of experiment were much more in accordance with those expected: the anthraquinones were reduced in a single two-electron step between pH 0 and 13 (Fig. 5); the redox potentials E, were determined after cyclic voltammograms with both types of electrode; and the two

Fig. 5. Cyclic voltammogram recorded at pH 1 of (a) a PHAS layer (preparation 4) on a glassy carbon electrode (sweep rate 50 mV s-‘) and (b) PHAS included rate 5 mV s-l).

as in the legend of Fig. m carbon paste (sweep

189

0 I. 0 6. 0 5. 04

0

Fig. 6. Dependence of redox potentials on pH of adsorbed PHAS layers on glassy carbon (A), PHAS in carbon paste (0) poly(PHAS) on glassy carbon (0) and poly(PHAS) on platinum (0). (*) First transfer potential. Composition of medium: 70% water + 30% acetonitrile.

series of measurements showed good agreement. E, plots vs. pH are shown in Fig. 6 and exhibit a linear dependence with a slope of 0.55 V, which is in agreement with a 2 e-, 2 H+ system. Perhaps as a consequence of the too large thickness of the deposits, the peaks obtained for PHAS layers are sometimes unsymmetrical, and quantitative data are difficult to extract from these experiments, as well as from the carbon paste experiments, for the reasons explained previously [3]. However, they allowed a good quantitative comparison with the poly(PHAS) behaviour. In addition, this shows that the apparent inactivity of PHAS on platinum electrodes at pHs < 10 was not due to the substrate itself, but to the platinum surface which altogether lowers the anthraquinone reduction potential (see Fig. 6 and the following discussion) while it raises those of H+ and H,O, thus placing the quinonic electroactivity beyond the electrolyte limit. A deviation of the plots obtained with the CPE is noticeable in Fig. 6 in the pH range l-6 . We calculated the redox potentials of PHAS applying the formula E, = (E,, + I&)/2, in agreement with previous studies. However, it is possible that this calculation leads to slightly erroneous values in the case of slow irreversible reactions, which is generally the case for quinones in this pH range. Poly(PHAS) behaviour Poly(PHAS) films were found to be totally inactive in pure water electrolytes, whatever the pH of the medium and the nature of the electrode. Since PHAS layers are active under identical conditions, this is certainly due to the hydrophobic character of the poly(alkylpyrrole) sustaining skeleton, as has been shown to occur with the poly(alkylviny1) skeleton [8]. Therefore, we tried to allow the wetting of our

190

i/rA I

Fig. 7. Cyclic vohammograms of different poly(PHAS) films (depostted on platmum at 3.3XlOK’ C cme2) in different deoxygenated 0.1 M TEAFB water+acetonitrile mixtures of the following water contents: (a) 50%, (b) 70% (c) 80%. (d) 100%. Sweep rate: 50 mV s-r. Fig. 8. Cyclic voltammograms of different poly(PHAS) films deposited using 3.3 X 1O-2 C cm-l on a platinum electrode at different pHs; (a) pH 7, (b) pH 9.5, (c) pH 13, (d) pH 14. Sweep rate: 50 mV s-t. Composition of medium: 70% water + 30% acetonitnle.

films, by adding to the water various amounts of acetonitrile. Figure 7 shows the different voltammograms of identical poly(PHAS) films in 0.1 M TEAFB aqueous electrolytes containing 50 to 0% of acetonitrile. It can be noted that the films remain fully electroactive in electrolytes containing up to 70% of water, while cycling currents fall progressively to zero when the percentage of water increases. It was chosen to make the study in 30% acetonitrile + water electrolytes, and conclusions were drawn implicitly supposing that the acetonitrile content did not affect the relative proton activity in water, i.e. that the pH of such electrolytes could be considered to be the same as that in pure water. Electrochemistry

in relation to pH

Just as we noticed with platinum adsorbed PHAS layers, no electroactivity of the anthraquinones was observed for poly(PHAS) films deposited on platinum at acidic pHs. Unlike the PHAS layer, poly(PHAS) showed some electroactivity in the pH range 7-10, the voltammetric curves, however, exhibiting a more variable shape

191

b

0

,,,\_

0 0

5

10

PH

10

PH

a

e

d

TV?

-1

-0.5

0

-1

-0 5

0

-

-0

0

I

i

V

0

0

,,,,,,,,,, 5

Fig. 9. Cyclic voltammograms of different poly(PHAS) films deposited using 1.5X10-* C cm-* on a glassy carbon electrode at different pHs: (a) pH 1, (b) pH 3 (n = 10 PA), (c) pH 5, (d) pH 7, (e) pH 9, (f) pH 13 (n = 25 pA). Sweep rate: 50 mV s-l. Fig. 10. Dependence on pH of (a) the cyclic voltammogram cathodic peak currents rpc and (b) the differences AE, between the anodic and cathodic peak potentials for identically prepared poly(PHAS) films. Sweep rate: 50 mV s-t (films deposited on a carbon electrode at 1.5 X lo-* C cm-*). Some points have been deduced from the curves of Fig. 9.

(Fig. 8). A plausible explanation of this pH 7-10 electroactivity would be that the polymer film, more uniform than the monomer layer, isolates more of the platinum surface from the outside electrolyte, and thus impedes a too large H,O reduction even in this pH range. When they are deposited on a glassy carbon electrode, the poly(PHAS) films are electroactive in the pH range O-14, with a single-step transfer in the O-13 range (Fig. 9). The redox potentials obtained after cyclic voltammetry are listed in Table 1 and plotted vs. pH in Fig. 6. They are in very good agreement with the redox potentials of the monomer and exhibit the same slope of 0.55 V, in agreement with a 2 e-, 2 H+ mechanism. Figure 10 represents the dependence on pH of both the cathodic peak current (i,,) and the differences between the cathodic and anodic

192 TABLE

1

Various potential data for PHAS monomer and poly(PHAS). E,, refers to the quantity (E,, + E,)/2, respectively represent the anodic and cathodic peak potentials of the cyclic where E,, and E, voltammetry of the considered species at 50 mV s- I. Potential values given vs. an Ag/10w2 M Ag+ reference in acetonitrile PH

1 2 3 4 5 7 9 9.25 11.6 13

Adsorbed

PHAS layer on GC

E,/mV

Es/mV

- 520 -690 -135 -755 - 780 - 860 - 850 - 870 - 960 - 930

- 330 -310 -410 - 420 -440 - 720 -790 -810 -890 - 950

PHAS E,/mV in carbon paste

Poly(PHAS) on GC

-400 -500a -530 -585 -610 - 670 -800 - 830 - 870 -940

- 310 - 350 - 420 - 450 - 510 -700 -800 -800 - 870 - 920

Es/mV

a Peaks were ill defined.

peak potentials (A&). Although the accuracy of the data is often low with such measurements, the two curves are in accordance and show that the kinetics should pass through a minimum between pH 2 and 3, since the i, plots show a minimum at pH 2 and the AEp plots a maximum at pH 3-4. Such results are in relative agreement with Laviron’s analysis, although the latter predicts (according to different reaction paths occurring in relation to the pHs along the classical nine-member square scheme) a minimum at the slightly higher pH 3.7 for the kinetics of the first electrochemical reduction of benzoquinone, and at pH 5.5 for the second one [9]. This small discrepancy may be due to some lack of precision in our measurements, but may also result from the differences existing between benzoquinone and anthraquinone sulphonate, concerning the potentials as well as the pK, values. When poly(PHAS) is deposited on a platinum electrode (Fig. 8), similar voltammograms are recorded in the pH range 9-13, to those on a carbon electrode (Fig. 9). However, it should be noted that the anthraquinone reduction on platinum was found to take place at approximately 60-70 mV below that on carbon, as shown in Fig. 6. It is interesting to remark that at pH 14 a broad peak is observed when the polymer is deposited on a carbon electrode (similar to Fig. 9f) while two clearly separated peaks are observed with the platinum electrode (Fig. 8d). Accordingly, the electrochemical reaction appears to follow an ee step at slightly higher pH values when the polymer is deposited on carbon rather than on platinum. However, such behaviour is perhaps induced not only by the nature of the electrode, but also by differences in the polymer drafting on the two electrodes. In addition, it may appear surprising that no clear one proton-two electron mechanism (eHe or eeH, for example) appears in Fig. 6 whatever the substrate or the nature of the electrode. Since such mechanisms are liable to occur, they should arise in a short pH range,

193

1

0

a

I

b

20j4A

Fig. 11. (a) Cyclic voltammograms on a platinum electrode of 0.1 M TJZAFB acetonitrile. (- - -) Argon-saturated; () O,-saturated. (b) Same feature but on a poly(PHAS)-coated electrode (film thickness 3.3 X 10e2 C cmm2). The sweep rate was 20 mV s-’ for all experiments.

namely between pH 11 and 13, where the shape of the curve in Fig. 6 is not sufficiently well defined to be determining, but this leaves relatively little room for doubt and tends to show that the pKs of anthraquinols are relatively close together, which is the case of the pKs of quinols *. In the case of anthraquinols, it is presumed that slightly higher values are to be found, as is corroborated by the values of 11 and 13 which should limit the region of AQH existence.

Cataljxis We tested the ability of poly(PHAS) films to catalyse the dioxygen reduction in acetonitrile and water + acetonitrile. Figure lla represents the voltammograms obtained on a bare platinum electrode in the absence and presence of a saturated solution of oxygen. Figure llb represents the same voltammograms obtained this time on a poly(PHAS)-coated electrode. Although the reduction currents reach comparable intensities, the reduction wave exhibits a more reversible character and occurs at about 150 mV less negative on the coated electrode, enlightening the redox catalytic activity:

._ d‘;-1 e-

A0

02

AQ'_

02

,

In water-containing electrolytes, no equivalent effect could be observed on the carbon electrode, as shown in Fig. 12 which shows the two separate waves of the anthraquinone reduction and the 0, reduction in this case.

l

The pK, and pK, values for the QH2 e QH- e Q2- system were calculated to be 9.85 and 11.4 [lo].

194

*n

%-&q=b

110 PA Fig. 12. (a) Cyclic voltammograms on a glassy carbon electrode of 0.1 M HCJO, in 70% H,O+30% O,-saturated. (b) Same feature on a poly(PHAS)-coated CH,CN. (- - -) Argon-saturated; ( -) electrode (film thickness 1.5 x lo-’ C cm-*). The sweep rate was 30 mV s-’ for all experiments.

CONCLUSION

In this paper we have detailed the main features of the behaviour of poly(PHAS) films in acetonitrile or water + acetonitrile mixtures. The electrochemical response of the films does not differ significantly in acetonitrile from that in DMSO [l], but in acetonitrile the complexing effect of Lif is greatly enhanced and it is extended to AQ’- in this case. Like the poly(vinyl-anthraquinone) films [8], poly(PHAS) films are inactive in pure water, showing that the hydrophobic character of the substituents predominates over the hydrophilic character of the polypyrrole skeleton. However, the films recover their activity when enough (30% at least) acetonitrile is added to the electrolyte solution. Under such conditions, the films were found to be electroactive in a wide pH range when deposited on a glassy carbon electrode, but only in the basic pH range when deposited on a platinum electrode. The comparative study of the monomer in adsorbed layers or in carbon paste shows that the quinonic electroactivity remains very similar when dealing with either the monomer or the polymer. The redox potential vs. pH dependence was found to exhibit a slope of 0.55 V in all cases, in agreement with the existence of a 2 e-, 2 H+ mechanism in the pH range O-11, and a value of 0 above pH 13, significant of a 2 e- mechanism. In addition, the catalytic efficiency of the poly(PHAS) films towards the reduction of solvated dioxygen was demonstrated in acetonitrile although it appears to fail when water is the main electrolyte constituent. Since no desorption of the polymer occurs at any pH, we believe that this type of electrode modification may be useful in most respects. ACKNOWLEDGEMENTS

The authors Clectrochimiques

wish to thank et generateurs

CNRS, PIRSEM tlectrochimiques)

and AFME (ATP Preparations for partial financial support.

REFERENCES 1 (a) P. Audebert, G. Bidan and M. Lapkowski, Electroanal. Chem., 219 (1987) 165.

J. Chem. Sot., Chem. Commun.,

(1986) 887; (b) J.

195 2 (a) E.M. Peover and J.D. Davies, J. Electroanal. Chem., 6 (1963) 46; (b) B.R. Eggins, J. Chem. Sot., Chem. Commun., (1967) 1267, see also The Chemistry of Quinonoid Compounds, Part II, Wiley Interscience, Bristol, 1974, Ch. 14; (c) A. Desbene-Monvemay. A. Cherigui, P.C. Lacaze and J.E. Dubois. J. Electroanal. Chem., 169 (1984) 157. 3 P. Audebert and G. Bidan, Synth. Met., 14 (1980) 71. 4 C. Riissel and W. Jaenicke, J. Electroanal Chem., 199 (1986) 139. 5 G. Bidan and D. Limosin, Ann. Phys. (Paris), 11 (1986) 5. 6 S. Coffey (Ed.), Rodd’s Chemistry of Carbon Compounds, Part III H, Elsevier, Amsterdam, 1979, p. 36. 7 C. Degrand and L.L. Miller, J. Electroanal. Chem., 117 (1981) 267, 164 (1984) 213: L.L. Miller, B. Zinger and C. Degrand, ibid., 178 (1984) 87. 8 P.M. Hoang, S. Holdcroft and B.L. Funt, J. Electrochem. Sot., 132 (1985) 2129. 9 E. Laviron, J. Electroanal. Chem., 164 (1984) 213. 10 J.H. BaxendaIe and H.R. Hardy, Trans. Faraday Sot., 49 (1953) 1140.