Studying the coupled electron–ion transfer reaction at a thin film-modified electrode by means of square-wave voltammetry

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Abstract

The coupled electron–ion transfer reaction at a thin film-modified electrode is studied under conditions of square-wave voltammetry (SWV). It is assumed that the electrode is covered with a thin film of an electrochemically inactive organic solvent, immiscible with water, containing a neutral redox probe and an electrolyte. The electrode is immersed in the aqueous electrolyte. The oxidation or reduction of the redox probe at the solid electrode|organic solution interface is accompanied by a simultaneous ion transfer across the liquid|liquid (L|L) interface. The overall reaction is modeled considering the diffusion of the redox probe and of the transferring ion within the limited boundaries of the thin film, and the diffusion of the transferring ion in the aqueous phase under semi-infinite conditions. Both reversible and quasireversible cases are considered. Under conditions of SWV, the quasireversible reaction exhibits unique properties, providing an insight into the kinetics of the overall process, which could be controlled either by the electron transfer or the ion transfer across the L|L interface. The theoretical predictions are confirmed by experiments with decamethylferrocene and a series of lutetium phthalocyanine complexes dissolved in nitrobenzene and deposited as a micro-film on the surface of a pyrolytic graphite electrode. The coupled electron–ion transfer reactions of these compounds are quasireversible and controlled by the kinetics of ion transfer (ClO4-, NO3-, Cl) across the water|nitrobenzene interface. It is demonstrated that SWV in combination with the thin film-modified electrode is a new and promising tool for measuring the kinetics of ion transfer across the L|L interface.

Introduction

Coupled electron–ion transfer reactions are frequently encountered at various surface-modified electrodes [1] as well as in mediated interfacial electron transfer processes [2], [3]. The fundamental knowledge of these processes is of importance for understanding the charge transfer at biological cell membranes where the permeation of ions across the membrane is energetically driven by redox transformations [4], [5].

The coupled electron–ion transfer reaction is a complex electrochemical process in which the electron exchange reaction at the working electrode surface is accompanied by simultaneous ion transfer at a separate interface positioned close to the electrode surface. Examples of this type can be found in the processes occurring at a solid electrode covered with a thin solid film of a redox active material [1] (e.g., redox polymer, or a solid inorganic compound) in which the electron transfer at the electrode surface|solid film interface is accompanied by an ion transfer across the solid film|aqueous electrolyte interface. Further examples are found in a plethora of studies of organic redox liquids [6], [7], [8], for measuring Gibbs energies of ion transfer [9], [10], [11], [12], or the rate of electron transfer across the interface between two immiscible liquids [2], [3], for studying biphasic electrocatalytic and photoelectrochemical processes [13], for measuring the dynamics of mass transfer of electrochemically generated ions across the liquid|liquid (L|L) interface [14], [15], etc. Marken et al. [6], [7], [8] modified the surface of graphite electrodes with randomly distributed micro-droplets of an organic water insoluble liquid and studied the redox properties of the liquid in an aqueous electrolyte using conventional three-electrode cell. Anson et al. [16], [17], [18], [19], [20] formed a stable micro-film of an electrochemically inactive organic solvent, at the surface of a pyrolytic graphite electrode, where the solvent contains a redox probe and a suitable electrolyte. Instead of having a thin film covering the entire electrode surface, the paraffin impregnated graphite electrode can be modified by a single macroscopic droplet of an organic solvent containing only a neutral redox probe [9], [10], [11], [12].

For all these electrode systems the electron exchange reaction at the electrode|organic liquid (E|O) interface is accompanied by an ion transfer across the organic phase|aqueous solution (O|W) interface. Several authors attempted to provide a theoretical basis for these processes considering the configuration of the three-phase electrode [9], [10], [11], [12] or the thin film-modified electrode [16], [17], [18], [19], [20]. In the first theoretical study, Myland and Oldham [21] addressed the three-phase electrode experiment assuming that the reaction occurs exclusively along the three-phase junction line, which was described geometrically as a micro-hemitoroid. Later on, the same authors modeled the coupled electron–ion transfer reaction at a thin film-modified electrode in the absence of an electrolyte in the organic phase [22]. In a subsequent theoretical study of three-phase electrodes the organic droplet was approximated as a conic film [23], whereas the role of diffusion and migration governing the distribution of ions in the organic phase has been studied for the thin film-modified electrode system [24]. The behavior of the three-phase electrode under conditions of square-wave voltammetry (SWV), as well as the effect of the uncompensated resistance has been explained on the basis of an electrode reaction occurring in a limited diffusion space [25]. Aoki et al. [26] modeled the three-phase electrode experiment on the basis of a micro-band electrode. It is finally worth noting that Nakatani and Sekine [14], [15] investigated the mass transfer of an electrochemically generated ion across the O|W interface in a system consisting of a single micro-oil droplet attached to a micro-electrode.

In all theoretical studies dealing with the three-phase or thin film-modified electrodes it has been assumed that the overall coupled electron–ion process is at equilibrium. The quasireversible electrode reaction at the thin film-modified electrode has been considered, assuming that the kinetics is controlled by the electron exchange and excluding the accompanying ion transfer from the theoretical treatment [27]. Only recently, the quasireversible coupled electron–ion transfer reaction was addressed on the basis of a simplified theoretical model in which the concentration of the transferring ion in both liquid phases was assumed to be constant at the L|L interface [28]. Furthermore, in most of the theoretical studies [22], [23], [24], [25], the diffusion of the ions in the aqueous phase was neglected, although, as will be demonstrated in this communication, it can play a significant role in determining the mass transfer regime in the system.

In the present study the coupled electron–ion transfer reaction at the thin film-modified electrode is considered for both reversible and quasireversible cases, considering the mass transfer of all electroactive species. Particular attention is paid to reveal the role of the concentration of all electroactive species in this complex system. The system is considered under conditions of square-wave voltammetry [29]. It is demonstrated that this technique is a powerful tool providing an insight into the kinetics of overall transfer processes occurring at the thin film-modified electrodes.

The theoretical results are compared with experiments performed at a pyrolytic graphite electrode covered with a micro-film of nitrobenzene containing decamethyferrocene (DMFC), lutetium bis(tetra-t-butylphthalocyaninato) (Lu[t-Bu4Pc]2), lutetium bis(phthalocyaninato) (Lu[Pc]2), and lutetium (tetra-t-butylphthalocyaninato hexadecachlorphthalocyaninato) (Lu[t-Bu4Pc][Cl16Pc]), all used as redox probes [30].

Section snippets

Experimental

DMFC was a product of Fluka, whereas all lutetium complexes were synthesized as described elsewhere [31], [32]. All other chemicals were of high purity and used as purchased. Redox compounds were dissolved in water-saturated nitrobenzene (2 mmol/L). Besides the redox compound, nitrobenzene contains 0.1 mol/L tetrabutylammonium (Bu4N) salt, perchlorate or nitrate, or tetrahexylammonium chloride as electolyte.

A pyrolytic graphite disk electrode (0.32 cm2) has been used. The preparation of the

Description of the mathematical model

As depicted in Scheme 1, it is assumed that the electrode is covered with a thin film (thickness L) of an electrochemically inactive organic solvent immiscible with water. The organic solvent contains an electrolyte and a neutral redox probe, Red in the present case. The thin film-modified electrode is immersed in an aqueous electrolyte, the anion of which is the same as in the organic phase. Red is oxidized in a one-electron step to a monovalent stable cation. This electron transfer is for

Reversible case

The properties of reaction (I) under conditions of SWV depend predominantly on the thickness parameter Λ and the concentrations of all species participating in the electrochemical process. The thickness parameter, Λ=LfD, relates the thickness of the film with the diffusivity of the electroactive species and the time window of the voltammetric experiments, representing the influence of the mass transfer within the film. This parameter is equivalent to that of a simple electrode reaction in

Conclusion

A coupled electron–ion transfer occurring at a solid electrode covered with a thin film of an organic solvent containing a hydrophobic redox compound has been studied under conditions of square-wave voltammetry (SWV). The theoretical model was developed considering two simultaneous charge-transfer reactions occurring at separate interfaces, i.e., the electron transfer at the solid electrode|organic solvent (E|O) interface and the ion transfer at the water|organic solvent (W|O) interface. In the

Acknowledgments

The authors thank A. Pondaven for the preparation of the lutetium complexes. V. Mirčeski acknowledges gratefully the financial support of A. v. Humboldt-Stiftung and University in Bretagne Occidentale.

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