Spectroscopic investigations of polymer-modified electrodes containing cobalt phthalocyanine: application to the study of oxygen reduction at such electrodes

JOURNAL OF

ELSEVIER

Journal of Electroanalytical Chemistry 386 (1995) 173-182

Spectroscopic investigations of polymer-modified electrodes containing cobalt phthalocyanine: application to the study of oxygen reduction at such electrodes C. Coutanceau, A. Rakotondrainibe, P. Crouigneau, J.M. L6ger, C. Lamy Laboratoire de Chimie 1, Electrochimie et Interactions, URA CNRS no 350, Unit~ersitd de Poitiers, 40 at'enue du Recteur Pineau, 86022 Poitiers Cddex, France

Received 5 October 1994

Abstract

Polymer films such as polypyrrole (PPy), polypyrrole modified by cobalt tetrasulphonated phthalocyanine (PPy-CoTsPc), polyaniline and polyaniline modified by cobalt tetrasulphonated phthalocyanine were electrosynthesized at gold electrodes by cyclic voltammetry. A UV-visible differential reflectance spectroscopic investigation of the behaviour of these electrodes as a function of their potential was carried out in acid electrolyte. It was shown that the insertion of cobalt phthalocyanine takes place into such polymer films and that it is possible to follow the variation in the UV-visible spectra of cobalt phthalocyanine with the electrode potential. The cobalt tetrasulphonated phthalocyanine appears as the CoIHTsPc species at higher potentials and becomes the CoHTsPc species when the potential decreases. An electron spin resonance investigation was performed at PPy and PPy-CoTsPc electrodes in deoxygenated and oxygensaturated solutions. It was shown that the ColUTsPc species is effective in the electroreduction of oxygen and that this species is more stable in oxygen-saturated medium than in deoxygenated medium, certainly because of its stabilization under the following form: ComTsPc-O2. In the case of the PPy-CoTsPc film, the polypyrrole matrix undergoes strong interactions with oxygen species, and more probably with hydrogen peroxide.

Kevwords: Polymer-modified electrodes; Cobalt phthalocyanine; Oxygen reduction

1. Introduction

It is possible to grow polypyrrole (PPy) [1,2] or polyaniline (PAni) [3,4] films on various electrode surfaces in organic as well as in aqueous solutions. These polymer films are interesting electron conducting polymer electrodes, since the properties of the films can be switched reversibly between the oxidized (conductor) and the neutral (insulator) states. In aqueous solution, it is possible to incorporate macrocyclic catalysts into these polymers such as iron [5,6] or cobalt [7] tetrasulphonated phthalocyanines (FeTsPc and CoTsPc) by using these anionic macrocycles as counterions during the m o n o m e r electropolymerization. This kind of electrode preparation allows the electrocatalytic material to be dispersed, at the molecular level, into the conducting matrix and electrodes to be obtained having a 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSI)I 0 0 2 2 - 0 7 2 8 ( 9 4 ) t ) 3 8 0 9 - 0

good activity and stability with respect to the electroreduction of oxygen [6,7]. A spectroelectrochemical study by in situ UV-visible differential reflectance spectroscopy ( U V D R S ) of the growth of polypyrrole and of the insertion of FeTsPc into the polymer matrix has already been performed by E1 Hourch et al. [6]. These researchers showed that this spectroscopic technique is suitable for following the growth of the polymer film and for studying the incorporation of FeTsPc since they were able to distinguish the U V D R S spectra of the metal phthalocyanine from that of the polymer matrix. On the contrary, Saunders et al. [8] investigated polypyrrole films containing cobalt phthalocyanine ( C o T s P c - P P y ) by electron spin resonance (ESR) spectroscopy and they observed complex E S R signals, one narrow signal which they attributed to the polypyrrole matrix and one wide

174

C. Coutanceau et aL /Journal of Electroanalytical Chemistry 386 (1995) 173-182

signal which they attributed to the cobalt(II) tetrasulphonated phthalocyanine. Moreover, He and coworkers [9,10] showed that the ESR signal of the iron phthalocyanine deposited on graphite or gold electrodes varied with the applied potential. In a previous paper [7], we demonstrated the good electrocatalytic activity and stability towards oxygen reduction of a polypyrrole electrode containing cobalt tetrasulphonated phthalocyanine. We showed that the reduction wave of oxygen at such electrodes was, in fact, the combination of two reduction waves. Concerning the first reduction wave, occurring at higher potentials, on the basis of the evaluation of the Tafel slopes and the calculation of the number of exchanged electrons by the oxygen molecule using the KouteckyLevich law, we proposed the following mechanisms for the oxygen reduction at a P P y - C o T s P c electrode. In the potential range where the Tafel slope is close to - 6 0 mV d e c a d e - l , i.e. in the low current density region, ColITsPc

+ O 2

'

CollITsPc-O2

rate-determining step ComTsPc-O~-+ H++ e-

~c o n T s P c - O 2 H

fast step CoUTsPc-O2H + H++ e-

, CollTsPc + H202

fast step In the potential range where the Tafel slope is close to - 1 2 0 mV decade -1, i.e. in the high current density region, ConTsPc + 0 2

~C o l I t T s P c - 0 2

fast step ComTsPc-O2 + H ++ e

~C o U T s P c - O e H

rate-determining step ColITsPc-O2 H + H + + e -

~ CoIITsPc + H 2 0 2

fast step These mechanisms are in agreement with those proposed by Zagal et al. [11] for the oxygen reduction at cobalt and iron tetrasulphonated phthalocyanines adsorbed at vitreous carbon electrodes. Concerning the second reduction wave, at lower potentials, we proposed that the following mechanism takes place at the polypyrrole matrix: H20 2 + 2e-

,2OH-

P A n i - C o T s P c electrodes as a function of the potential. Moreover, since the valence of the central metal ion of the catalyst is expected to change during the electroreduction of oxygen, an ESR study of the oxygen reduction at this kind of electrode is presented to support the above-cited mechanism, or at least to determine the reaction intermediates.

2. Experimental 2.1. UV-t~isible differential reflectance spectroscopy

A Harrick RSS-C rapid scan spectrometer with two Hamamatsu R955 photomultipliers was used for the UV-visible reflectance spectroscopic measurements. The output signals were processed in a Nicolet 370 digital signal averager and stored on a hard disk of a compatible microcomputer. The spectrometer was calibrated with a holmium oxide ( H o 2 0 3) standard filter. Each spectrum was obtained after 100 scans (27 ms scan-l). Details of the UV-visible reflectance spectroscopic experiments are given elsewhere [12]. The reflectance absorbance spectra were plotted as three-dimensional (3D) diagrams (absorbance, wavelength, potential) with the help of home-made software developed to collect and process the data [13]. The electrochemical cell was constructed with two quartz windows to allow the light beam to be reflected on the working electrode surface. The incident angle of the light beam is close to 45 °. The working electrode was a polycrystalline gold disk with a geometrical surface area of about 0.8 cm 2, polished before each electropolymerization with fine alumina (down to 0.3/zm). The reference electrode was an HglHg2SO4ISO42electrode (MSE) and the counterelectrode was a high surface area vitreous carbon plate. The electrochemical measurements were carried out using a classical experimental set-up, consisting of a Wenking LT 87 potentiostat, a Wenking MVS 87 waveform generator and an X - Y recorder (Linseis LY 1700). The voltammetric experiments were performed in 0.5 M H2SO 4 aqueous solutions. The voltammograms were recorded with a potential scan rate of 2 mV s between 0.85 and - 0 . 1 5 V (reversible hydrogen electrode (RHE)) at 20°C. Before each experiment, the electrolyte was either deoxygenated by bubbling ultrapure nitrogen for 20-30 min or oxygen saturated by bubbling oxygen for 20-30 min.

and in acid media, 2 O H - + 2H +

~2 H 2 0

In this paper we will present a spectroelectrochemical study by U V D R S of the behaviour of the PPy and PAni films as well as that of the P P y - C o T s P c and

2.2. Electron spin resonance spectroscopy

A Varian E3 X band spectrometer (wavelength of 3 cm) with a 100 kHz magnetic field modulation frequency was used for the ESR measurements.

175

C. Coutanceau et aL / Journal of Electroanalytical Chemisto' 386 (1995) 173-182 2.3. Electrode preparation

~--

Gas

Counter electrode (platinum)

li Reference electrode (MSE)

Fi~. 1. Scheme of the electrochemical cell used for the FSR measurements.

The electrochemical cell was constructed with a synthetic quartz tube in order to avoid the presence of paramagnetic impurities. This tube is made thinner in its central section which crosses the resonance cavity to form a calibrated vat of volume 0.50 mm 3 wherein the working electrode is located. Fig. 1 shows the scheme of the electrochemical cell used for the ESR experiments. The working electrode was a 1 cm 2 gold plate, on which the active film is deposited, connected to a gold wire. This electrode is mounted on a water-tight electrode holder in order to protect the wire from all contacts with the electrolyte. This electrode holder is terminated by a tube connected to the Luggin capillary to allow the measurement and the control of the applied potential with respect to the reference electrode. The counterelectrode was a platinized platinum wire and the reference electrode was an Hg IHg2SO 4 ISO~ electrode. The electrochemical set-up was the same as above. Each ESR spectrum of the A u ] P P y and A u ] P P y CoTsPc electrodes was taken at a fixed given potential. The potentials were varied between 0.8 and 0.0 V (RHE).

Cobalt tetrasulphonated phthalocyanine was synthesized using the method of Weber and Bush [14] and purified by dialysis. The characterization was made by UV-visible absorbance spectroscopy using an Ultraspec LKB spectrometer controlled by a microcomputer. The UV-visible spectra (Fig. 2) display a main band located at 663 nm for the dimeric form of the phthalocyanine (curve a in Fig. 2) and at 670 nm for the monomeric form (curve c in Fig. 2). These results are in agreement with those of Gruen and Blagrove

[15]. The polypyrrole electrodes ( A u l P P y and A u l P P y CoTsPc) were prepared by cyclic voltammetry from a 1 M H2SO 4 aqueous solution containing 0.1 M pyrrole freshly distilled under vacuum for the A u l P P y electrode and with 10 3 M CoTsPc added for the Au I P P y - C o T s P c electrode. The polymerizations were performed at 50 mV s ~ between -(I.2 and 0.9 V (RHE), at 2I)°C, for 10 voltammetric cycles for the UV-visible spectroscopic experiments and 2(1 voltammetric cycles for the ESR experiments. T h e polyaniline e l e c t r o d e s (Au I P A n i and Au IP A n i - C o T s P c electrodes) were prepared from a 1 M H2SO 4 aqueous solution containing 0.01 M aniline freshly distilled under vacuum for A u t P A n i and with 10 -2 M CoTsPc added for A u [ P A n i - C o T s P c . The polymerizations were performed for 10 voltammetric cycles at 50 mV s -~ between - 0 . 0 5 and 1,25 V (RHE). Before each polymerization, the solutions were deoxygenated by bubbling ultrapure nitrogen for 20-30 rain.

a.U.

1.5

1.0 m

b

0.5

0.0 55O

6OO

65O

700

750

k/nm

Fig. 2. UV-visible absorption spectra of 10 5 M CoTsPc in aqueous solutions; curve a, water (dimeric form); curve b, 209~ ethanol-80¢Ji water solution (monomeric form); curve c, ILl M NaOH oxygen saturated solution (monomeric form).

C. Coutanceau et aL / Journal of Electroanalytical Chemistry 386 (1995) 173-182

176

j / m A c m "2

j / m A cm" 2

2

0.4 0.2

0.0 -0.2 -0.2

-

~ 0.0

r

0.4

0.2

EN

0.6

T

1

0.8

1.0

In all voltammetric experiments, although the reference electrode was an Hg IHgzSO 4 ISO 2 electrode, all the potentials are quoted in the R H E scale.

3. Results

3.1. Electrochemical results Fig. 3 shows the voltammogram obtained with a polypyrrole film deposited at a gold electrode, recorded in a 0.5 M H z S O 4 deoxygenated solution. This voltammogram is characterized by a reduction peak located at 0.15 V (RHE). According to Geni~s and Pernault [16], the redox process of the polypyrrole film is constituted by three steps, two of which are electrochemical steps and the other is a chemical step:

ppy.+ . 2PPy +

' PPy + + e , ppy2++ e, ppy + ppy 2+

first electron transfer

differential

0.0

0.2

0.4

0.6

0.8

1.0

E/V (RHE)

Fig. 4. Voltammogram of a polypyrrole film containing CoTsPc, as grown at a gold electrode, in 0.5 M H2SO 4 deoxygenated aqueous solution; t, = 2 mV s - l, J2 = 2500 rev min 1, T = 20°C.

solution during potential cycling between 0.85 and - 0 . 1 5 V ( R H E ) (v = 2 mV s 1) for a polypyrrole film grown at a gold electrode. Two main absorption bands appear. When the first spectrum of Fig. 5, i.e. that recorded at 0.9 V (RHE), is kept as reference and subtracted from the other spectra, the two bands can be easily seen as shown in Fig. 6, representing the two-dimensional (2D) spectra of the polypyrrole film for different potentials. The first band is located at about 378 nm and the second at about 550 nm. These two absorption bands are more important when the potential decreases, indicating that the polypyrrole film absorbs more when reduced (in the insulator state). The absorption bands at about 378 and 550 nm have already been found by Geni6s and Pernault [16] and E1 Hourch et al. [6]. When the polypyrrole film is prepared in the presence of CoTsPc, it can be expected that the insertion

second electron transfer

/

dismutation reaction

In Fig. 4 is represented the voltammogram obtained with a polypyrrole film containing cobalt tetrasulphonated phthalocyanine. The shape of this voltammogram is very similar to that of the polypyrrole alone. However, an oxidation peak appears in this voltammogram, located at 0.15 V (RHE). This oxidation peak has already been observed by E1 Hourch et al. [5] for a polypyrrole film containing iron phthalocyanine. This peak is characteristic of the inserted phthalocyanine and corresponds to the macrocyclic skeleton oxidation process. This indicates unambiguously that the insertion of CoTsPc into the polymer did occur.

3.2. UV-visible measurements

-0.2

(RHE)

Fig. 3. Voltammogram of a polypyrrole film, as grown at a gold electrode, in 0.5 M H2SO 4 deoxygenated aqueous solution; (v = 2 mV s - i, ~ = 2500 rev min 1, T = 20°C.

PPy.

I

'

reflectance spectroscopic

Fig. 5 shows the 3D UV-visible reflectance spectra recorded in an N2-saturated 0.5 M H 2 S O 4 aqueous

/ " ' ~ % ? <~

la .u.

o,

A b

1. 100

p tl a

O. 200

~

=~

e

-0.700240

490 Wave l e n g t h

740

t~

pot ent e lt

o.g

/rim

Fig. 5. UVDRS 3D spectra of a polypyrrole film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; E a = 0 . 9 V (RHE), E c = - 0 . 1 V (RHE), v = 2 mV s -1, T = 20°C.

C. Coutanceau et al. /Journal of Electroanalytical Chemistry..t86 (1995) 173-182

177

.... ?i

"

'

-0. l i

p 0 t e

a u. A b : 400 s

rt

/a.u. A b 8 0 P

/

,:,~, L \ ~ . ~ - ~ /

t

i

0.4

1.000

~

b a 0.400

a

1 /V

(RHE)

~

e

b

~

0.600

240

0.9

-0. 600

240

4~0

490 ~avelength

C e

/

~

tent

?

O. 200

a n

./"0.4

74C

/nm

Fig. 8. UVDRS 3D spectra of a PPy-CoTsPc film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; E , = 0 . 9 V (RHE), E c = 0.1 V (RHE), r = 2 mV s i T = 20°C.

740

Wavelength /nm Fig. 6. U V D R S 2D spectra of a polypyrrole film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; E , = 0 . 9 V (RHE), E c = 0.1 V (RHE), t , = 2 mV s -1, T = 20~C.

of cobalt phthalocyanine does occur. This is, in fact, observed when comparing the electrocatalytic activity towards oxygen reduction of a polypyrrole film prepared without CoTsPc in the solution (Fig. 7, curve a) and of a polypyrrole film p r e p a r e d in the presence of 10 -3 M CoTsPc (Fig. 7, curve b) and when comparing the voltammograms obtained with PPy and P P y CoTsPc electrodes in acidic medium without oxygen. The increase in activity for the film p r e p a r e d in the

presence of CoTsPc indicates that the insertion of the catalyst occurred. However, when the UV-visible differential reflectance spectra of a P P y - C o T s P c film are recorded under the same experimental conditions as above, no great change appears in the spectra (Fig. 8). In fact, an important loss of absorbance occurs from 500 nm to 700 nm (Fig. 9). In this figure, the 2D spectra of a P P y - C o T s P c film, recorded under the same conditions as in Fig. 6, are shown. However, no indication about the insertion of CoTsPc and about the behaviour of this species with the electrode potential can be seen.

j / m A c m -2 0

.

.

.

.

i

i

•

i

J

-01 -I

e n

-2

/a .u.

t

A b s o

-3

0.4

i

b -0.2

i a i /v

(RHE)

-z

-4

.~

P o t

i

0.0

i

i

0.2

0.4 E/V

i

0.6

(].9

i

0.8

.0

(RHE)

Fig. 7. Oxygen reduction waves obtained with a gold PPy electrode (curve a) and a gold PPy-CoTsPc electrode (curve b) in a 0.5 M H 2SO4 oxygen-saturated aqueous solution; u = 2 mV s - I ~Q= 2500 r e v m i n t T=20°(7.

230

485 Wavelength

74~0 /rim

Fig. 9. U V D R S 2D spectra of a PPy-CoTsPc film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; E~,-0.9 V (RHE), E ~ . = - 0 . 1 V (RHE), u = 2 mV s i, T = 20°C.

178

C. Coutanceau et al. /Journal of Electroanalytical Chemistry 386 (1995) 173-182

Table 1 Wavelength of the main absorption bands as a function of the electrode potential for PAni and PAni-CoTsPc films A/nm

/a

u.

-8 :

PAni

PAni-CoTsPc

Upper potential (0.85 v (RHE))

690 345

Lower potential (-0.15 V (RHE))

345

Intermediate potential (0.3-0.4 V (RHE))

440 345

690 580 44O 345 345 420 670 430 345 620

i 100 5 oA o a

t la:

/V

:Hr:l ',

O. t 5 0

c e 0 800

2 40

,93 wavelen~tq

/40 /qm

Fig. 10. UVDRS 3D spectra of a PAni film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; E,=0.9 V (RHE), E c = - 0 . 1 V (RHE), u = 2 mV s l, T = 20°C.

This c o u l d be d u e to t h e g r e a t o p a c i t y o f t h e polypyrrole film c o n t a i n i n g the c o b a l t p h t h a l o c y a n i n e . O n the c o n t r a r y , w h e n t h e c o m p a r i s o n is m a d e between the UV-visible reflectance spectra of a PAni film (Fig. 10) a n d o f a P A n i - C o T s P c film (Fig. 11), g r e a t d i f f e r e n c e s a p p e a r . T a b l e 1 shows t h e m a i n b a n d s o b t a i n e d for t h e P A n i a n d t h e P A n i - C o T s P c films as a function of potential. It a p p e a r s t h a t in e a c h case s o m e b a n d s a r e d u e to t h e p o l y a n i l i n e m a t r i x for the P A n i - C o T s P c film. A t high p o t e n t i a l s , the a b s o r p t i o n b a n d s l o c a t e d at 690 a n d 345 n m exist for b o t h t h e P A n i a n d t h e P A n i C o T s P c films. This i n d i c a t e s t h a t t h e s e b a n d s a r e d u e to p o l y a n i l i n e . It is t h e s a m e for t h e b a n d l o c a t e d at 345 n m for t h e r e d u c e d state o f the p o l y a n i l i n e film a n d for t h e b a n d s at 4 3 0 - 4 4 0 n m a n d 345 n m at

i n t e r m e d i a t e p o t e n t i a l s . H o w e v e r , in t h e case of the P A n i - C o T s P c film, an a b s o r p t i o n b a n d exists which did not exist in t h e case o f a P A n i film alone. This a b s o r p t i o n p e a k is l o c a t e d at 580 n m for the high p o t e n t i a l a n d shifts up to 670 n m for low p o t e n t i a l s (Fig. 12). This a b s o r p t i o n p e a k is d u e to t h e C o T s P c species i n s e r t e d into t h e polymer. Nevin et al. [17] s h o w e d t h a t the s p e c t r u m o f t h e cobalt(III) tetrasulphonated phthalocyanine, electrochemically g e n e r a t e d at p H 2, was c h a r a c t e r i z e d by a m a i n a b s o r p t i o n b a n d l o c a t e d at a b o u t 640 n m a n d a s h o u l d e r at a b o u t 600 nm. T h e s e r e s e a r c h e r s s h o w e d t h a t the s h o u l d e r l o c a t e d at a b o u t 600 n m b e c a m e the m a i n a b s o r p t i o n b a n d w h e n t h e p h t h a l o c y a n i n e conc e n t r a t i o n was d e c r e a s e d . It s e e m s also t h a t the b a n d l o c a t e d at 640 n m is c h a r a c t e r i s t i c o f t h e d i m e r i c form o f the c o n I T s P c species w h e r e a s the b a n d l o c a t e d at 600 n m is c h a r a c t e r i s t i c o f the m o n o m e r i c f o r m o f this c o m p o u n d . It a p p e a r s that, for high p o t e n t i a l s , c o b a l t p h t h a l o c y a n i n e exists u n d e r the C o m T s P c species. T h e n , w h e n t h e p o t e n t i a l d e c r e a s e s , the m a i n a b s o r p -

0 700

/ /a

J

t~ D O 708 s c r a s

C

8 C~O

/

/

: a..

/

\ /V

rf

II4HE

t

67O

nm

%

ia

0.180

c

o

]~ ,,J]'

nm

+

0

-~

/

V,,

58O

o4 o

-0080

J

"

-0340

//'1

e

-0.600 240

4gO ~avelergtr

,J 240

i 290

i ,540

~ 390

440

490

540

i 590

640

i 690

7zlO

740 'n~

Fig. 11. UVDRS 3D spectra of a PAni-CoTsPc film as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solution; Ea=0.9 V (RHE), E c = - 0 . 1 V (RHE), v = 2 mV s 1, T = 20°C.

Wavelength/nm

Fig. 12. UVDRS 2D spectra of PAni-CoTsPc films as grown at a gold electrode recorded in a 0.5 M H2SO 4 deoxygenated aqueous solutions (T = 20°C): curve a, E = 0.9 V (RHE); curve b, E = 0.0 V (RHE).

C. Coutanceau et al. /Journal of Electroanalytical Chemistry 386 (1995) 173 182

Height/an 75 -

25

--

0 -2.5

-

-5

I

3340

I

3345

I

3350

I

3355

3360

3365

H/Gauss Fig. 13. E S R s p e c t r u m of a PPy film as grown at a gold e l e c t r o d e r e c o r d e d in a (I.1 M H 2 S O 4 d e o x y g e n a t e d solution; T = 20°C.

tion band shifts towards higher wavelengths, to reach 670 nm for the more negative potentials, indicating that cobalt phthalocyanine becomes the monomeric c o n T s P c species [15]. 3.3. Electron spin resonance measurements

L pp( A

H p p )2

where Lpp is the peak-to-peak height and AHppthe peak-to-peak linewidth of the signal, is dependent on the applied potential. Fig. 14 shows the variation in the intensity of the ESR signal obtained with a polypyrrole film as a function of the electrode potential, recorded in 0.5 M H2SO 4. It appears that the intensity of the

(AHpp) 2 l.pp / a.u.

3

\.

//

,\\ -,.q. \. -tl.

0 0.0

[ 0.2

i 0.4

I

0.6

signal increases in a regular manner from 0.8 to 0.15 V ( R H E ) where it reaches a maximum value and decreases at more negative potentials. When comparing these ESR results with the electrochemical behaviour of the polypyrrole film, an obvious relation appears between the redox state of the polypyrrole film and the intensity of the ESR signal. In the potential range where the polypyrrole film is completely oxidized, the intensity of the ESR signal is low. When the electrode potential is decreased, the polypyrrole film starts to undergo a partial reduction which seems to be accompanied by an increase in the intensity of the ESR signal. The maximum of the reduction peak of the polypyrrole electrode is located at 0.15 V (RHE), a potential corresponding to the maximum of the intensity of the ESR signal of the polypyrrole film, i.e. to the maximum of spins. According to Genibs and Pernault [16], the two redox process of the polypyrrole film give two voltammetric peaks which are very difficult to separate, even at a low potential sweep rate. The increase in the intensity of the ESR signal of the polypyrrole film can then be attributed to the formation of the radical polarons P P y + from the initial bipolaron ppy2+, following the reaction of reduction according to the following equation: ppy2++ e

Polypyrrole films give E S R spectra (Fig. 13) with a narrow and symmetrical signal. The intensity of the ESR signal, defined as the expression

i 0.8

E/V ( R H E ) Fig. 14. Intensity of the E S R signal vs. the e l e c t r o d e p o t e n t i a l r e c o r d e d in a 0.1 M H 2 S O a d e o x y g e n a t e d a q u e o u s solution for PPy films as grown at a gold e l e c t r o d e ; T = 20°C.

179

, PPy+

After this, another one-electron reduction of the polaron occurs to give the neutral PPy. Normally, the completely oxidized form of the polypyrrole film must be diamagnetic. However, it exhibits an ESR signal for more positive potentials. The answer to this problem can be found in the chemical dismutation equilibrium undergone by the polypyrrole film when oxidized [16], and thus by the formation of radicals. In the case of a P P y - C o T s P c film, the ESR spectrum is more complex since it appears to be the combination of two different signals as shown in Fig. 15. The E S R spectrum obtained is constituted by one narrow signal and one wide signal both centred at the same H 0 value. Saunders et al. [8] have already observed a similar behaviour for a P P y - C o T s P c film, and they attributed the narrow signal to the polypyrrole matrix and the wide signal to the CoTsPc compound, more precisely to the Co H species of the macrocycle. The ESR spectra of the polypyrrole-CoTsPc films obtained for each potential studied were deconvoluted into two signals (Fig. 15), one wide and the other narrower, by using the equation of the ESR signal in which the shape is represented by the deriwttive of a lorentzian function [18]. Concerning the narrower signal, i.e. the signal due to the polypyrrole matrix, the variation in its intensity as a function of the applied potential is shown in Fig. 16 for different electrolytes: in 0.1 M H 2 S O 4 deoxy-

C. Coutanceau et al. /Journal of Electroanalytical Chemistry 386 (1995) 173-182

180 Height/a.u.

(AHpp) 2

Lpp / i . u .

250

1510-

200

50

150

-5 I

100 0.0

-10-

I

0.2

0.4

0,6

I

0.8

E/V (RHE) -15 3360

I

[

I

I

3370

3380

3390

3400

3410

H/Gauss Fig. 15. E S R spectra of a P P y - C o T s P c film as grown at a gold electrode recorded in a 0.1 M H z S O 4 deoxygenated solution (T = 20°C): ( - - - - - - ) , deconvoluted peaks; ( ) deconvoluted curve; ( . . . . . . ), experimental curve.

genated solution (curve a), in the same oxygensaturated solution (curve b), and in 0.1 M H 2 S O 4 solution containing 10 -3 M H 2 0 2 (curve c). It appears first that the shape of the curve representing the intensity of the polypyrrole ESR signal in 0.1 M H 2 S O 4 deoxygenated electrolyte as a function of the electrode potential (Fig. 16, curve a) is very close to that obtained before for the polypyrrole film alone under the same conditions. When the electrolyte is oxygen saturated, a great change appears in the curve of the ESR signal intensity vs. potential (Fig. 16, curve b); the intensity of the signal reaches a maximum value for a potential of 0.4 V (RHE). We can then deduce that interactions between the polypyrrole matrix and oxygen or the hydrogen peroxide formed at the cobalt phthalo-

(AHpF,) 2 Lit p /

a.u,

15

I0

b 0

J 0.0

I 0.2

~

[ 0.4

~

I 0.6

0.8

E/V (RHE) Fig. 16. Intensity of the ESR signals vs. the electrode potential recorded in a 0.1 M H 2 S O 4 solution for the PPy signal of P P y CoTsPc films as grown at a gold electrode ( T = 20°C): curve a, deoxygenated solution; curve b, oxygen-saturated solution; curve c, deoxygenated solution containing 10 3 M H 2 0 2.

Fig. 17. Intensity of the ESR signals vs. the electrode potential recorded in a 0.1 M H 2 S O 4 solution for the CoTsPc signal of a P P y - C o T s P c films as grown at a gold electrode (T = 20°C): curve a, oxygen-saturated solution; curve b, deoxygenated solution.

cyanine occur. Curve c in Fig. 16 shows the variation in the ESR signal intensity of the polypyrrole matrix as a function of the potential in a 0.1 M H2SO 4 solution containing 10 -3 M H 2 0 e. The shape of this curve is very similar to that obtained with an oxygen-saturated electrolyte. It seems then that an interaction between the hydrogen peroxide and the polypyrrole matrix takes place. The maximum value of the signal intensity is reached for a potential of 0.35 V (RHE), i.e. a 50 mV shift with respect to the result obtained in an oxygensaturated solution. This shift can be due to the slow rate of diffusion of H 2 0 e through the polypyrrole film [19]; under the conditions of curve b of Fig. 16, the peroxide formed by the two-electron reduction reaction of oxygen at cobalt phthalocyanine occurs within the polypyrrole matrix, so that diffusion of the peroxide species does not affect the rate of the whole process of the peroxide reduction; otherwise, in the case of curve c of Fig. 16, the peroxide in the electrolyte solution has to diffuse through the polymer matrix before the two-electron reduction process into water can occur; in this case, the rate of the peroxide diffusion through the polymer film can affect the reduction process and may explain the 50 mV shift towards lower potential of the maximum value of the ESR signal intensity. Fig. 17 shows the variation as a function of the electrode potential of the intensity of the ESR signal obtained for the wider line which is due to CoHTsPc, in a 0.5 M H 2 S O 4 oxygenated solution (curve a) and in a deoxygenated solution (curve b). In each case, the value of the intensity decreases when the electrode potential decreases; after that, the value of the intensity of the ESR signal increases from 0.45 V (RHE) in the deoxygenated solution (Fig. 17, curve b) and from 0.25 V ( R H E ) in the oxygen-saturated solution (Fig. 17, curve a). It is well known that the cobalt(II) phthalo-

C. Coutanceau et al. /Journal of Electroanalytical Chemistry" 386 (1995) 173-182

cyanine is a paramagnetic species with an unpaired electron [20], while the con~TsPc species is diamagnetic [21]. Then, the c o n T s P c species gives a signal in ESR spectroscopy, and the value of the intensity of the ESR signal must decrease if the amount of this species decreases. The UV-visible study allows us to suppose that, at higher potentials, the presence of the ColnTsPc is effective. Therefore, the decrease in the intensity of the ESR signal can be explained by the formation of the C o m T s P c species from the initial ConTsPc. The fact that a decrease in the intensity of the ESR signal continues to occur in an oxygen-saturated solution for potentials lower than in a deoxygenated solution could be due to the formation of the C o m T s P c 0 2 species, the high electronegativity of the oxygen allowing this reaction intermediate to be stabilized.

4. Discussion

The spectroscopic results that we obtained for the polypyrrole electrode containing cobalt tetrasulphonated phthalocyanine allows us first to confirm the fact that the reduction reaction of oxygen at such electrodes is not simple, while both the macrocycle catalyst and the polymer matrix are involved in the whole reduction process. The oxygen reduction is a rather complex reaction, first because several electron transfers are involved in the process and secondly because the first step involves the adsorption of the dioxygen molecule. In previous work [7], we found two different Tafel plots for the oxygen reduction at cobalt tetrasulphonated phthalo-

j / m A cm -2

a

-1.4

b

-1.(~

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(I

0,:"

0.'4

0.(;

0.B

E/V (RHE) Fig. 18. V o l t a m m o g r a m s of P P y - C o T s P c films as grown at a gold e l e c t r o d e r e c o r d e d in a 0.5 M H 2 S O 4 d e o x y g e n a t e d solution (curve al and in a 0.5 M H 2 S O 4 + 10 -3 M H 2 0 2 d e o x y g e n a t e d solution (curve b): c = 2 m V s - 1 /2 = 251710 rev min I T = 20°C.

181

cyanine. The first, at higher potentials, was characterized by a slope value close to -RT/F, i.e. a n = 1 (where ce is the transfer coefficient and n the number of electrons exchanged), and the second, at lower potentials, was characterized by a slope value close to -2RT/F, i.e. a n = 0 . 5 . From these results, we deduced that the adsorption conditions of oxygen at cobalt phthalocyanine depend on the electrode potential, and that the phenomenon was controlled by the Temkin adsorption conditions in the range of low reduction current densities, i.e. the range of higher potentials, and by the Langmuir adsorption conditions in the range of high current densities, i.e. in the range of lower potentials. The difficulty was to determine under which form the oxygen adsorption occurred. At a surface such as a platinum electrode, Damjanovic and Brusic [22] proposed two kinds of oxygen adsorption before the whole reduction process, i.e. the adsorption of oxygen at an electrode surface can involve the activation of the molecule or not: S + 0 2,

' S---O 2

adsorption without electron transfer, and S q- O 2 .

" S---O

2

adsorption with electron transfer leading to the activation of the oxygen molecule. In the case of the adsorption of oxygen at cobalt phthalocyanine, the spectroelectrochemical results that we obtained show that the adsorption of the oxygen molecule involves the activation of this molecule by the transfer of one electron from the central metal of the phthalocyanine towards the oxygen molecule. This proof of the first reaction intermediate thus confirms the proposed mechanism which postulated the formation of the C o I n T s P c - O 2 species before the first electrochemical step of the oxygen reduction reaction. The ESR results allowed us to demonstrate the implication of the polypyrrole matrix in the whole process of the oxygen reduction. It appeared that interactions between the polymer film and hydrogen peroxide were effective. It is interesting to remark that the potential at which the maximum value of the intensity of the ESR signal obtained for the polypyrrole matrix in an oxygenated solution or in a solution containing 10 -3 M H 2 0 2 (Fig. 16, curves b and c respectively) is very close to the potential at which the modified electrode is active for the reduction of hydrogen peroxide (Fig. 18). Therefore, it seems that the polypyrrole matrix plays an important role in the four-electron oxygen reduction process, by reducing into water the hydrogen peroxide first formed. This property of the polypyrrole film has already been found by Jakobs et al. [23,24] and Barendrecht and Vork [19] who reported that, on the basis of the rotating ring disk electrode, the polypyrrole film was active for the oxygen reduction into water

182

c. Coutanceau et al. /Journal of Electroanalytical Chemistry 386 (1995) 173-182

(i.e. t h e f o u r - e l e c t r o n r e d u c t i o n ) via t h e p e r o x i d e i n t e r mediate, and thus that polypyrrole was active for the hydrogen peroxide reduction.

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[9] P. He, J. Lu, C. Cha, P. Crouigneau, J.M. IAger and C. Lamy, J. Electroanal. Chem., 290 (1990) 203. [10] P. He, P. Crouigneau, B. Beden and C. Lamy, J. Electroanal. Chem., 290 (1990) 215. [11] J. Zagal, P. Bindra and E. Yeager, J. Electrochem. Soc., 127 (1980) 1506. [12] J.D.E. Mclntyre, in B.O. Seraphin (ed.), Optical Properties of Solids--Recent Developments, Amsterdam, 1975, p. 555. [13] A. Rakotondrainibe, A. Spinelli, B. Beden and C. Lamy, Spectrosc. Eur., 5-6 (1993) 20. [14] J.H. Weber and D.H. Bush, Inorg. Chem., 4 (1965) 469. [15] L.C. Gruen and R.J. Blagrove, Aust. J. Chem., 26 (1973) 319. [16] E.M. Geni~s and J.M. Pernault, J. Electroanal. Chem., 191 (1985) 111. [17] W.A. Nevin, W. Liu, M. Melnik and A.B.P. Lever, J. Electroanal. Chem., 213 (1986) 217. [18] C.P. Poole, Electron Spin Resonance, Interscience, New York, 1967. [19] F.T.A. Vork and E. Barendrecht, Electrochim. Acta, 35 (1990) 135. [20] J.M. Assour, J. Am. Chem. Soc., 87 (1965) 4701. [21] L.D. Rollmann and S.I. Chan, Inorg. Chem., 10 (1971) 1978. [22] A. Damjanovic and V. Brusic, Electrochim. Acta, 12 (1967) 615. [23] R.C.M. Jakobs, L.J.J. Janssen and E. Barendrecht, Electrochim. Acta, 30 (1985) 1085. [24] R.C.M. Jakobs, L.J.J. Janssen and E. Barendrecht, Electrochim. Acta, 30 (1985) 1433.