Interface electrode-film

Charge propagation within the film is in principle slower than charge injection (or consumption) at the electrode/film interface Whether electrons are transported... 

Figure 2.61 Schematic representation of the electrode/film/electrolyte interfaces.

Subtle effects arise when the surface layer is modulated by the oscillating flow [116]. For example in response to the flow, the layer thickness may change or even deform. In the high-rate dissolution of copper in HC1 [92], a salt film of CuCl is deposited onto the surface. The film is formed at the electrode-film interface, limited by transport of Cl-, and dissolved at the same rate at the film-solution interface, limited by transport of CuClJ. Because the film thickness varies as 1/2, the presence of a film was only... 

The final part of the transmission line circuit is the charge transfer elements at the electrode/film interface and at the film/solution interface. In the former case, at low AC frequency electrons are transferred from the electrode to the trimer centres. We have shown [5] that this process is controlled by the Nernst driving potential, and that the interfacial resistance is given by... 

Fig. 11.1. The transmission line circuit used to model these data. The left hand end of the transmission line is at the electrode/film interface. The right hand end is at film/electrolyte interface. The extended resistances, RP and Rx, correspond to the resistance to motion of electrons between trimer centres and ions through the pores respectively, (a) The potential in the central line of the diagram is the potential within the film, and the connecting capacitors modify this potential to produce the driving potentials to drive current through the resistors. The CR kinetic circuit elements for the interfacial process can be seen at each end of the transmission line, (b) The modified circuit when the capacitance, C in equation (9) is not negligible. The potential at the trimer and in the pores is given by E and E ...

An important example of the system with an ideally permeable external interface is the diffusion of an electroactive species across the boundary layer in solution near the solid electrode surface, described within the framework of the Nernst diffusion layer model. Mathematically, an equivalent problem appears for the diffusion of a solute electroactive species to the electrode surface across a passive membrane layer. The non-stationary distribution of this species inside the layer corresponds to a finite - diffusion problem. Its solution for the film with an ideally permeable external boundary and with the concentration modulation at the electrode film contact in the course of the passage of an alternating current results in one of two expressions for finite-Warburg impedance for the contribution of the layer Ziayer = H(0) tanh(icard)1/2/(iwrd)1/2 containing the characteristic - diffusion time, Td = L2/D (L, layer thickness, D, - diffusion coefficient), and the low-frequency resistance of the layer, R(0) = dE/dl, this derivative corresponding to -> direct current conditions. 

What is the connection between the two main areas in electrochemistry—the science of solutions (ionics) and that of charge transfer across solid-solution interfaces (electrodics) There is indeed a close connection. The interfacial region at electrodes (and all wet surfaces, including the surface of plants undergoing photosynthesis) is surrounded by ions in solution (or in the moisture films on surfaces). Thus it is important that we know all about them. The electrode is the stage the solution is the theater and the audience. It is also the place that supplies the players—ions and solvent—while electrons are clearly supplied from resources in the wings. 

Electropolymerization Based on 4-Vinylpyrldlne and Related Ligands. The third technique for preparing electrode/film interfaces is in many ways the most interesting both in terms of the chemistry involved and the results so far obtained. The strategy is to induce polymerization directly at the electrode surface by oxidation or reduction and our emphasis has been on the reduction of coordinated 4-vinylpyridine and related compounds. It is known that 4-vinylpyridine is susceptible to anionic polymerization (38). 

The emphasis so far has been on the preparation and chemical manipulation of the sites within polymers films on electrodes. Because of the spatial organization within the interface, the film itself must play an important role in any electrochemical redox... 

In the net sense, the bilayers provide a means for directed or unidirectional electron transfer from an external film to the electrode and in that sense they impart a rectifying character to the electrode-film interface. 

The chronoamperometric behavior of the case just considered is very similar to that seen for partitioning of the solution reactant. A, into the film and diffusion in the film with a diffusion coefficient, D, to the electrode surface. This is just the membrane model or Case S considered in Section 14.4.2(b). The concentration profiles for a potential step experiment where the concentration of A at the electrode/film interface, = 0) 0 are shown in Figure 14.5.3. The expression for the current, normalized to that at the bare electrode is (84)... 

Another important feature for lithium graphite intercalation compounds in Li -containing electrolytes is the formation of solid electrolyte interface (SEI) film. During the first-cycle discharge of a lithium/carbon cell, a part of lithium atoms transferred to the carbon electrode electrochemically will react with the nonaque-ous solvent, which contributes to the initial irreversible capacity. The reaction products form a Lb-conducting and electronically insulating layer on the carbon surface. Peled named this film as SEI. Once SEI formed, reversible Lb intercalation into carbon, through SEI film, may take place even if the carbon electrode potential is always lower than the electrolyte decomposition potential, whereas further electrolyte decomposition on the carbon electrode will be prevented. 

An alternative reflection setup makes use of single (or multiple) internal reflection within an OTE (Figure 4). However, at each internal reflection, a small portion of the intensity leaks out (it is correctly known as an evanescent wave) into the thin film electrode layer and beyond into the solution and can be used to detect any absorbing species. The method is also known as attenuated total reflection (ATR). The evanescent wave intensity decays exponentially as exp( — bix) with distance x from the interface. The penetration depth, 5, depends on the wavelength and the optical properties of the substrate, electrode film, and solution d = /l/(4/i Im... 

Beyond an insight into the ease with which compounds can be oxidized or reduced by measuring the formal potential, cyclic voltammetry can be used to determine the rate of electron transfer across the electrode/solution or electrode/film interface. Optimizing the rate of heterogeneous electron transfer is important for technological applications ranging from the analysis of metals in polymers, foods, and cosmetics to the development of biosensors and molecular electronic devices. 

The parameter x is the distance from the electrode/film interface. 

Aoki also considers the stochastic aspects of phase propagation mechanism and relates his analysis to the theory of percolation and the fractal dimension of the system. In this approach the Nemst equation for charge transfer at the substrate/film interface is used to compute the probability of the presence of a conductive seed or nucleus. When the potential is incremented, this seed can then grow in a one-dimensional manner governed by the propagation rate constant kp or the kinetic parameter to form a conductive pillar of a definite length. New nuclei can also form at the support electrode/film interface during the potential... 

Let us now consider potential step chronoamperometry in some detail. We consider a surface-deposited polymer film of uniform thickness L to which a small amplitude potential step is applied. This ensures that only a small change in the polymer oxidation state is effected. Structural changes will be minimal. Initially at time t = 0 before applying the step the redox center concentration in the layer is uniform and has the value c, = where denotes the total redox center concentration in the layer. After applying the small amplitude step, the redox centre concentration at jc = 0 (which defines the support electrode/film interface) is given by Cf (see Fig. 1.46). Let c x, t) denote the concentration of redox centers in the film as a function of distance x and time t. The boundary value problem can be stated as follows... 

The model we describe was developed in a comprehensive manner by Aoki and coworkers, who examined the situation where the redox transformation in the layer obeys the Nemst equation and the heterogeneous electron transfer kinetics at the support electrode/film interface become important. In the following analysis we concentrate on the former case. 

Analysis becomes more complex when the the interfacial electron transfer kinetics at the support electrode/film interface are slow. In such a situation the full Butler-Volmer equation must be used instead of the Nernst equation. In this more complex case, a new variable A characterizing the degree of reversibility of the kinetics at the inner interface is introduced, where A is given by... 

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