Elsevier

Electrochimica Acta

Volume 45, Issue 20, 23 June 2000, Pages 3413-3421
Electrochimica Acta

Simultaneous electrochemical formation of Al2O3/polypyrrole layers (I): effect of electrolyte anion in formation process

https://doi.org/10.1016/S0013-4686(00)00423-0Get rights and content

Abstract

We studied the capability of various kinds of sulfonate-based electrolytes to form a bilayered Al2O3/polypyrrole (PPy) film on an aluminum substrate in aqueous solution. As electrolytes, we selected sodium p-toluenesulfonate (monovalent sulfonate), sodium naphthalene-trisulfonate (trivalent sulfonate), poly(sodium 4-styrenesulfonate), poly(sodium vinylsulfonate) (sulfonate polymer), sodium n-dodecylbenzenesulfonate (SDBS), sodium butylnaphthalenesulfonate(BNS), and sodium n-dodecylsulfate (SDS, surfactant sulfonate/sulfate). Among the sulfonates we investigated, SDBS, BNS and SDS appeared to form a barrier-type Al2O3 layer with high coulombic efficiency in the absence of pyrrole. In the presence of pyrrole, anodization of these electrolytes resulted in the formation of Al2O3 and PPy layers, simultaneously.

Introduction

Electropolymerization of pyrrole has been attempted on many electrode substrates including noble metals (such as Pt, Au, and Ag) and non-noble metals like aluminum. Aluminum electrodes are easily oxidized to form Al3+ species, giving a non-conducting dielectric (Al2O3) film on the electrode. The Al2O3 film acts as an excellent barrier, inhibiting an electron transfer and thus inhibiting the electrochemical formation of polypyrrole (PPy) on it. The electrodeposition of PPy on aluminum substrates is difficult, but a few authors have reported successful results [1], [2], [3]. Hülser and Beck reported that both a PPy film and a porous-type Al2O3 film are formed on a pretreated aluminum electrode in an aqueous solution containing oxalic acid and pyrrole [1]. In their method, the aluminum electrode was pretreated by either a mechanical polishing or an anodic activation (in a nitric acid solution containing pyrrole). According to their discussions, such pretreatments are essential for a successful formation of both films (the porous Al2O3 film and the insulated PPy film in their pores) at the same time. Oxalic acid is known as a typical electrolyte producing a porous type anodic oxide [4]. Beck et al. also reported the formation of a similar type of Al2O3/PPy composite using some non-aqueous media such as acetonitrile and methanol [2]. Cheung et al. confirmed a PPy film formation on a thin Al2O3 film in a propylene carbonate solution [3]. However, the PPy and the thin Al2O3 films have not been well characterized.

The present authors previously reported a simultaneous formation of Al2O3/PPy films on aluminum electrode using sodium n-dodecylbenzenesulfonate (SDBS) [5]. The obtained bilayer-film consisted of a ‘barrier-type’ Al2O3 and an ‘electronically-conducting’ PPy film. Fig. 1 shows a brief model for the electron and mass transports during the simultaneous electrochemical formation of Al2O3/PPy films on the Al electrode. The formation of Al2O3 proceeds at two interfaces, namely at the aluminum/Al2O3 and at the Al2O3/PPy. As shown in Fig. 1, Al3+ species are continuously generated at metal/oxide interface. Then they migrate through growing Al2O3 layer toward the solution under high electric field (107 V cm−1), while oxide ions (O2−) migrate toward the aluminum electrode [6]. At the interface of Al2O3/PPy, the Al3+ encounters water species to form Al2O3 film. In the above anodization process of Al electrode, the electrolyte anion plays a key role in determining the type of oxide film (porous or barrier) and its current efficiency.

In the case of simultaneous formation, surfactant electrolytes are considered to play three important roles: (1) as supporting electrolytes for forming Al2O3 film; (2) dopants incorporated in the electropolymerized PPy film, and most importantly; (3) micelle assemblies at the electrode interface to enable the PPy formation simultaneously with the formation of the insulating oxide under-layer film. As shown in Scheme 1, the process involves an electrochemical oxidation to generate Al3+ species followed by a chemical reaction to form either Al2O3 or AlXn(OH)3−n. Aluminum ions generated by the electrochemical oxidation of the aluminum electrode undergo one of two types of reaction: a film-formation process resulting in Al2O3, or a dissolution process resulting in soluble species which migrate into the bulk of the solution. The pKa of the electrolyte anions determines which of these reactions occur (role 1).

Regarding role (2) above, sulfonate anions are often used as dopants for PPy to support the polymerization process and the electrical/electrochemical properties of the resulting PPy. Acids with high pKa values such as carboxylate and pentaborate are not suitable as dopants for PPy [7], [8]. Generally, electrolytes composed of acids with low pKa value such as sulfonate, sulfate, perchlorate, and tetrafluoroborate, exhibit good film forming ability with highly current-efficient electrochemical polymerization of pyrrole. Among them aromatic sulfonates give high electric conductivity, thermal and mechanical stability and electrochemical redox capacity. PPy doped with certain kinds of sulfonate-based dopant anion is known to improve voltage-current characteristics of a solid electrolytic capacitor [9], [10]. One can introduce new functions to PPy like a surface activity or an optical activity by using various sulfonates.

In our study, the anodization of aluminum was investigated in various sulfonate-based solutions with and without pyrrole in them. Sulfonates were carefully selected by considering the three roles of electrolytes described above. p-Toluenesulfonate is a typical electrolyte for the electropolymerization of pyrrole [11]. Multivalent sulfonates such as 1,3,6-naphthalenetrisulfonate form a PPy with both anion and cation exchange occurring during the redox reactions. The PPy doped with this multivalent anion showed a high diffusivity of ions. PPy doped with polymer sulfonates, namely poly(4-styrenesulfonate) and poly(vinylsulfonate), are known to show cation exchange property. n-Dodecylbenzenesulfonate is well known and widely utilized as anionic surfactant. The PPy doped with the micelles of the n-dodecylbenzenesulfonate exhibited a columnar structure, and thus a considerably high ion diffusion coefficient [12]. The other anionic surfactants we investigated were butylnaphthalenesulfonate and n-dodecylsulfate, which is not a sulfonate but sulfate with similar structure. We also did a comparative study using some inorganic electrolytes and carboxylate electrolytes commonly used for the anodic formation of Al2O3. Sodium sulfate and sodium perchlorate are commonly used as electrolytes for the electropolymerization of pyrrole. Sodium sulfate is known as the representative electrolyte giving a porous-type Al2O3 layer, while sodium perchlorate is utilized for polishing aluminum electrochemically. Ammonium adipate and ammonium pentaborate octahydrate are well known as the representative electrolytes forming a barrier type Al2O3 layer with high coulombic efficiency.

In the present paper, we studied the formation of Al2O3 and PPy films on aluminum using these electrolytes. We checked the relationship between the types of the electrolytes and the resulting film formation to find out how the chemical nature of the electrolytes affects the simultaneous Al2O3/PPy formation. We also observed the surface morphology of the electrode in the initial stage of electrogrowth using scanning electron microscopy and Auger electron spectroscopy. The results of these observations were reflected in our mechanism for Al2O3/PPy formation. Role (3) of the surfactant electrolyte will be discussed in the follow-up [13].

Section snippets

Materials

Pyrrole (Kanto Chemical) was distilled under reduced pressure. As supporting electrolytes, sodium n-dodecylbenzenesulfonate, sodium n-dodecylsulfate, sodium sulfate, sodium perchlorate, ammonium pentaborate octahydrate (Wako Pure Chemical Industries), sodium butylnaphthalenesulfonate (40% aqueous solution), sodium naphtalene-1,3,6-trisulfonate (Tokyo Chemical Industry), sodium p-toluenesulfonate (Kanto Chemical), poly(sodium 4-styrenesulfonate) (Mw ca. 70 000), poly(sodium vinylsulfonate) (25%

Results and discussion

Anodization curves were obtained in various electrolyte solutions in the absence of pyrrole. Fig. 2 shows the anodization curves (the formation voltage (Vƒ) plotted as a function of the charge density passed (Q) in various electrolyte solutions (0.1 M) at a current density of 1.0 mA cm−2. As shown by curve a, the Vƒ linearly increases up to 100 V, indicating the formation of a barrier-type Al2O3 layer on Al. The anodization curves for the sodium perchlorate solution and for the sodium sulfate

Conclusions

We studied the influence of electrolytes on the formation of Al2O3 and/or polypyrrole (PPy) film on an aluminum electrode. In the electrolyte solutions which do not contain pyrrole, surfactant sulfonates such as sodium n-dodecylbenzenesulfonate (SDBS) tend to form a barrier type Al2O3 layer with high current efficiency by a galvanostatic anodization of aluminum. It is confirmed by atomic adsorption analysis that the surfactant sulfonate solution suppressed the dissolution of aluminum and formed

Acknowledgements

A part of this study was financially supported by a Grant-in-Aid for Scientific Research (A) from the Ministry of Education, Science, Sports, and Culture of Japan, Project No. 10131219 (1998-1999).

References (14)

  • F. Beck et al.

    J. Electroanal. Chem.

    (1990)
  • K.M. Cheung et al.

    Polymer

    (1988)
  • M.M. Lohrengel

    Mater. Sci. Eng.

    (1993)
  • M. Satoh et al.

    Synth. Met.

    (1994)
  • M. Satoh et al.

    Synth. Met.

    (1995)
  • P. Hülser et al.

    J. Appl. Electrochem.

    (1990)
  • F. Keller et al.

    J. Electrochem. Soc.

    (1953)
There are more references available in the full text version of this article.

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