Inert films on electrodes. Electrochemical properties of carbon fibre electrodes modified by a- and P-naphthol electropolymerization

PANTELIS KARABINAS Laboratory of Physical Chemistry, Aristotelian University of Thessaloniki, 540 06, Thessaloniki, Greece

Received July 13, 1989

PANTELIS KARABINAS. Can. J. Chem. 68, 302 (1990). Modification of carbon fibre electrodes has been achieved by electropolymerization of a- and P-naphthols. The formation

of the film was followed spectrophotometrically by probing the decrease of the concentration of the corresponding monomer in dilute solutions. The electrochemical behaviour of several depolarizers on the modified electrodes showed that the charge transfer takes place at the electrode-film interface. 'The film structure can be satisfactorily described by the "pin-hole" model.

Key words: carbon fibres, inert film, electromodification, a-naphthol, P-naphthol.

PANTELIS KARABINAS. Can. J. Chem. 68, 302 (1990). On a modifit des tlectrodes en fibre de charbon en effectuant une tlectropolymtrisation des a- et P-naphthols. On a suivi la

formation du film par spectrophotomttrie en dtterminant l'abaissement de la concentration du monomkre correspondant dans des solutions diluCes. Le comportement Clectrochimique de plusieurs dtpolarisants sur les tlectrodes modifites montre que le transfert de charge se produit l'interface film-tlectrode. On peut dtcrire la structure du film d'une faqon satisfaisante a l'aide du modtle du atrou d'aiguillem.

Mots clts : fibres de carbon, film inert, Clectromodification, a-naphthol, P-naphthol. [Traduit par la revue]

Introduction There is much current interest focused on the electrochemical

applications of modified electrodes especially in electrocata- lysis or passivation of electrodes to corrosion. Films formed by polmerization of radicals obtained during the oxidation of phenol or naphthol derivatives show catalytic or inhibitory behaviour depending on the presence or absence of active redox groups. Moreover, carbon fibres (c.f.) have nowadays been used successfully in numerous fields of technology, mostly as reinforced composites. The permeability and the roughness of a film on c.f. are the major factors that define the quality of the modified fibres as matrix for the attachment of epoxy resins.

Dubois et al. proposed the protective coating of metallic surfaces with a polyphenylene oxide film obtained by the electrooxidation of disubstituted phenols from concentrated alkaline aqueous-methanolic solutions (1). The polymerization mechanism suggested the propagation of the film by head-to-tail coupling of the phenate radicals. In recent studies 5-hydroxy- 1,4-naphthoquinone and its derivatives have been found to follow the same electropolymerization mechanism on carbon, Pt, and Au electrodes (2-4). The film obtained showed catalytic effect on oxygen reduction due to the presence of the redox quinone group.

Furthermore, the electrosorption of nitro- and nitroso- deriva- tives of a- and P-naphthol has been studied lately (5, 6). It was found that the irreversible oxidation of the -OH group of 2-nitroso-1-naphthol results in the formation of a film, while in the case of reversible oxidation of 1-nitroso-2-naphthol no modification is observed.

The stability of the polymeric film depends on the substituent, pH and solvent composition. Thus, the polynaphthoquinone film is not stable in alkaline solutions and films bearing nitro- or nitroso- groups are stable only up to the reduction potential of the corresponding groups.

In all of the above cases the surface coverage exceeds a monolayer. Submonolayer coverage has been found for the electrosorption of P-naphthol, performed at a negative potential range where no oxidation of the -OH group occurs. In this case

the adsorption increases at less negative potentials and follows the Langmuir adsorption isotherm (7).

Recently, Zinger et al. (8) suggested the electropolymeriza- tion of hydroxyalkyl-methacrylates and aniline for the improve- ment of the interlaminar shears strength of the c.f.

In the present paper the electrooxidation and electropoly- merization of a- and P-naphthol is studied, since phenol -the parent compound of the monocyclic series - follows mainly dimerization reactions during its electrooxidation (9, 10). Our objective is to verify the electrochemical formation, the stability and the properties of films which have no functional groups attached to highly oriented c.f. electrodes. The amount of naphthol electrosorbed was calculated from its bulk concentra- tion decrease. The permeability of the film in relation to the film thickness is investigated on the basis of the electrochemical behaviour of several reactants with various molecular sizes. The structure of the film is interpreted according to the existing theories.

Experimental The PAN-based carbon fibres (Celion GY-70) used in this study

were product of Celanese Corporation U.S.A. The fibres were pretreated with ethanol or acetone in a Soxhlet-type

apparatus for 2 h, washed with water, and vacuum dried to remove the adhesive resins which were deposited during the production process.

Absorbance values of the electrosomtion experiments were measured with a PMQ-3 Zeiss spectrophotometer at i92 nm and 224 nm for a - and P-naphthol, respectively.

Micrographs were taken in the Physics Department using the JEOL-CX electron microscope in the SEM mode.

Cyclic voltammetric measurements were carried out using a conven- tional set up consisting of a voltage scan generator Wenking VSG 72 (G. Bank), a standard potentiostat Wenking ST 72 (G. Bank) and an X-Y recorder Servogor 733 B .B .C. (Goertz).

Current densities are expressed per mg fibres. All potential values are referred to the aqueous calomel electrode (SCE). A Pt sheet was used as counter electrode. Both the counter and the referenceelectrodes were separated from the working electrode compartment by glassy diaphragms.

a - and P-Naphthols puriss. p.a. (Fluka) were used after recrystal-

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KARABINAS 303

E/V vs. sc E

FIG. 1. Cyclic voltammogram of P-naphthol (C = 1 mM, pH = 4) on ( a ) bare electrode and (b) modified electrode with 1.7 x lo-' mole cm-2 (u = 100 mV s-').

lization from 80% ethanol. The study was carried out at 25 & O.l°C in aqueous 5% methanolic Britton-Robinson buffered solutions (pH = 1-12) prepared with triply distilled water.

Results and discussion The electrosorption experiments were carried out on large

bundles of highly oriented carbon fibres and the applied electro- sorption potential covered a wide range of positive values. The concentration decrease, after the adsorption equilibrium, was followed spectrophotometrically. The largest amount of naphthol electrosorbed was observed at the potential where the oxidation of the naphthol at the corresponding pH takes place.

Figure 1 curve a represents the cyclic voltammogram of P-naphthol on carbon fibre electrodes at pH = 4. The i, value varies linearly with the square root of the potential scan rate, as is expected for diffusion-controlled processes. The E, is shifted 0.059 V per pH unit towards more negative values up to pH = 9 i.e. for pH < pK ( P K ~ . ~ = 9.57). This behaviour suggests that a one-electron oxidation takes place, resulting in the formation of the naphthoxy radical. a-Naphthol exhibits exactly the same electrochemical behaviour.

The naphthoxy radical formed on the electrode surface undergoes further reactions, depending on the naphthol concen- tration. At high concentrations the polymerization through head-to-tail coupling is the predominant process (1-4), while at lower concentrations tail-to-tail dimerization (9, 10) as well as addition of water molecule (10, 11) can also take place.

Controlled potential electrolysis at 180-200 mV more positive than the E, of the wave of Fig. 1 and simultaneous determination of the bulk concentration decrease showed that the amount of naphthol electropolymerized levels off to a limiting value with increasing electrolysis time. Each limiting value, which is attained at greater electrolysis time as the concentration of naphthol in the solution is decreased, suggests termination of the polymerization. These limiting values, expressed in mg naphthol per g of c.f., obtained from various naphthol concentrations (5 x 10-~-10-~ M ) reach a maximum as in Fig. 2. It may also be concluded from Fig. 2 that the

FIG. 2. Concentration dependence of the electrosorbed quantity of (a-) 0 and (P-) CB naphthol (pH = 4).

FIG. 3. Cyclic voltammogram of AgN03 (u = 100 mV s-', C = 1 mM, pH = 4) on electrodes modified with P-naphthol charged from the following concentrations: (a) without naphthol, (b) lop5 M, (c) 5 X lop5 M, (d) M, (e) 2 x M, and (f) 5 x M.

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304 CAN. J. CHEM. VOL. 68, 1990

I I , I 0.5 0.0 0.5 0.0

E/V VS. SCE E/V VS. sc E

FIG. 4. Cyclic voltammogram of K4Fe(CN)6 (u = 100 mV s-', FIG. 5. Cyclic voltammogram of p-benzoquinone (u = 100 mV s-', C = 2 mM, pH = 4) on electrodes modified with P-naphthol charged C = 1 mM, pH = 4) on electrodes modified with P-naphthol charged from the following concentrations: (a ) bare electrode, (b) 5 x M, from the following concentrations: (a ) bare electrode, (b) M, (c) (c) 2 X lO-'M, (d) 5 x M, (e) M. 5 x loP5M, (d ) M, (e) 2 X M, and (f) 5 x M.

oxidation of naphthol itself is totally hindered, as curve b in Fig. 1 shows.

maximum amount of naphthol electropolymerized is achieved for concentrations greater than 3 X lop4 M. This behaviour

The film remains strongly attached to the carbon fibre over a wide potential range of about 2.5 V and is removed only after prolonged remaining at potentials where extreme hydrogen evolution occurs. After the film detachment the oxidation and electrosorption of naphthol can be carried out again od the regenerated fibre. The film is very stable in aqueous solutions

suggests that the film formed from relatively low concentrations 1.0 can not exceed a critical thickness.

( i ~ = 1-12) and in benzene. However, it is removed after remaining for 24 h in 1 M NaOH solution and by immersion of the fibre in ethanol, acetone, and acetonitrile.

Cyclic voltammetry has been employed to verify the film

-

structure. It was found that at the whole potential range where

Projected molecular areas for both naphthols of 6.6 x o 10-l5 cm2 and 2.6 X 10-l5 cm2 in the planar and perpendicular orientations (12, 13) yield values for surface coverage of 70 and 25 equivalent monolayers, respectively. These values were obtained taking into account that 1 mg of carbon fibres corres- pond to 1.5 cm2 of effective area (14).

On an electrode bearing the limiting amount of the film the

FIG. 6 . Dependence of Ai,=, for 0 AgN03, @ K4Fe(CN)6, and @ p-Benzoquinone on the bulk equilibrium concentration of P-naphthol employed for modification.

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KARP

FIG. 7. Scanning electron micrograph of ( a ) bare carbon fibre ( x

the film exists, several electrochemical reactions are consider- ably hindered, with the magnitude of inhibition depending on the extent of modification.

Figures 3, 4, and 5 show the gradual i, decrease with increasing film thickness for three different depolarizers Ag', Fe(CN)64-, and benzoquinone. It can be seen that in all cases the redox potential remains constant and the shape of the wave is the same, independent of the film thickness. On the other hand, on electrodes with the same modification a different inhibition is observed depending on the molecular size of the reactant. Thus, for an electrode coated with 1.7 X lops mole P-naphthol cmp2 the Ag' reduction is almost 65% inhibited, whereas the ~ e ~ ' / F e ~ ' and quinone/hydroquinone reactions are inhibited about 80% and 90%, respectively.

The plot of (Ai),, = ip - i,/i,, where i, and i, represent the peak current on pristine and modified fibres respectively, vs. the concentration of p-naphthol employed for modification, results in the curves presented in Fig. 6. It is observed that, for low p-naphthol concentrations, the limiting value of (Ai),, is different for each depolarizer depending on its diffusion through the film.

Three possibilities can be considered for the reduction of the above depolarizers: (a) electron exchange taking place at the film-solution interface considering electron propagation through the film (15-18), (b) diffusion of the species through pores or channels (pin-hole model) (1 9-21) or through the film itself (membrane model) (22) with electron exchange at the electrode-film interface (23), and (c) diffusion of the species and propagation of the electrons with electron exchange in the film (24, 25). It must be noted that the film under consideration is inactive due to the absence of functional groups.

If the reduction of the depolarizer occurred at the film- solution interface (case a ) , the electrons had to diffuse through the film and the electrochemical reaction should occur at a potential different than that for a bare electrode (17). It follows that the magnitude of inhibition should be the same for each depolarizer depending only on the film thickness.

If electron propagation and species diffusion simultaneously occurred in the film (case c) a shift of the redox potential and a change of the shape of the wave should be observed. These effects should be more pronounced as the film thickness is increased (1 6, 24).

In case b, where the species is diffused through the film, the redox potential remains unaffected by the extent of modification

30 000) and (b) partially modified fibre with (3-naphthol ( X 40 000).

0.5 0.0 E/V vs S C E

FIG. 8. Cyclic voltammogram of AgN03 ( u = 100 mV s-', C = 1 mM, pH = 4) on modified electrodes (15 min electrolysis in a 1 mM (3-naphthol solution at 0.75 V for different supporting electrolyte concentrations (a) CH3COOH 1.6 M, CH3COONa 0.4 M, and (b) CH3COOH 0.16 M, CH3COONa 0.04 M.

and the process is kinetically controlled by the diffusion of the species through the film (case "S" in the Andrieux and SavCant notation) (16).

As it has already been pointed out, in the case of films derived from naphthol electropolymerization, the redox potential of the depolarizers remains unaffected by the film thickness and is the same as that on a bare electrode. This indicates that the charge transfer takes place at the electrode-film interface and the height of the wave reflects merely the magnitude of the species

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306 CAN. J. CHEM. VOL. 68. 1990

FIG. 9. Variation of the ratio i,/i, and of E,, vs. u for &Fe(CN),j electrooxidation on a modified electrode bear~ng 1.5 X lop8 mole P-naphthol ~ m - ~ .

penetration through the film. Thus, it may be considered that on a modified electrode there are still active regions on the surface which are exposed to the solution.

The film formation may be described as an island growth of inactive regions on the electrode, so that the surface becomes inhomogeneous and the active species from the solution penetrates the film only through pores or channels. As the film thickness increases the size of the holes and the distribution of active sites decrease significantly and a smaller wave is observed. This also explains why for the same film thickness the inhibition is proportional to the molecular size of the corresponding reactant and even though the surface coverage exceeds several monolayers the electrochemical reaction is still observed. The structure suggested is very similar to that presented in the scanning electron micrographs of Fig. 7 . Figure 7 a shows a top view of pristine c.f. and Fig. 7 b shows an image of the polymer particles deposited on the electrode where pores are clearly observed. The deposit appears distally brightened due to its insulating properties.

Further evidence is provided by the effect of the concentra- tion of the supporting electrolyte on the film compactness. At high concentrations the film is swelled with solvent and electrolyte ions. This results in an increase of holes and/or channels in the film and consequently the peak current of the depolarizer is increased (Fig. 8).

In Fig. 9 the variation of the ratio i,/i, vs. u for the ferrocyanide on an electrode modified with 1.5 x lo-' mole P-naphthol/cm2 is given. This variation is rather in agreement with the theory predicted by Gueshi et al. (21) and further supports that the film structure fits the pin-hole model.

When the modification is performed from a concentrated solution of naphthol ( l o p 2 M in 10% MeOH) the film becomes compact, the naphthol molecule is unable to reach the electrode surface and termination of the electropolymerization occurs. In this case channels and/or pores are blocked and the film exhibits properties of an impermeable inert insulator. These properties are different from those predicted by the membrane model since in the latter the current is governed by the extraction and diffusion coefficients of the species in the two phases (26).

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