Surface confined electrode reaction

Coupling an electrochemical cell to an analytical device requires that hindering technical problems be overcome. In the last years there has been a considerable improvement in the combined use of electrochemical and analytical methods. So, for instance, it is now possible to analyze on-line electrode products during the simultaneous application of different potential or current programs. A great variety of techniques are based on the use of UH V for which the emersion of the electrode from the electrolytic solution is necessary. Other methods allow the in situ analysis of the electrode surface i.e the electrode reaction may take place almost undisturbed during surface examination. In the present contribution we shall confine ourselves to the application of some of those methods which have been shown to be very valuable for the study of organic electrode reactions. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

Clearly, this thiol can be used to attach ferrocene groups (Fc) to the Au electrode surface. If we abbreviate the surface-confined ferrocene group as -Fc, it should be possible to drive the following surface redox reaction ... 

Heat production associated with the electrochemical reactions is also assumed to be confined at the electrode-electrolyte surface, thus the resulting thermal energy produces a discontinuity of the heat flux. The heat generated within this surface, in fact, represents a heat source for the electrode and the electrolyte domains. The sum of the inward heat fluxes is equal to the heat generated as a result of the electrochemical reactions. As explained in Section 3.3.2, the heat is generated by the increase in entropy, associated with the electrochemical reaction (reversible heat), and to the activation irreversibilities. Therefore, the boundary conditions for Equation (3.7) are ... 

Nevertheless, it is important to refer here to the fact that forced convection alters the electrode response only in the case of this being at a timescale that is long in comparison with the electrode process. For short timescales the (fast) perturbation will be confined to a very short distance from the electrode surface the electrode reaction parameters are not affected by the convection, this being simply a way of achieving good reproducibility. 

Surface confined electrode reactions — see - surface redox reactions... 

The reversible systems Au or Pt/((PQ2+ 2Br )n]8urf. were shown to be superior electrodes for cyt c(ox) reduction compared to the naked electrodes.(6) Reduction of cyt c(ox) was found to be mass transport limited when the electrode potential was held sufficiently negative to reduce the [(PQ2+)n]8urf. to ((PQf)n]8urf.. Thus, the results accord well with a mechanism where the reduction of cyt C(ox) occurs in a mass transport limited reaction with surface-confined PQ+ centers. 

Semiconductor electrodes provide an excellent substrate for the study of redox reactions of surface-confined redox reagents. This follows from the fact that the ratio of oxidized to reduced form of a redox couple on a photoelectrode responds to two stimuli, light and potential, rather than to only potential as is... 

Let us now consider the prototypical case in which the electrode reaction O ne R exhibits reversible kinetics and the solution contains O, but not R, in the bulk. The solution has been homogenized and the initial potential E is chosen well positive of, so that the concentration profiles are uniform as the SWV scan begins. The experiment is fast enough to confine behavior to semi-infinite linear diffusion at most electrodes, and we assume its applicability here. These circumstances imply that we can invoke Pick s second law for both O and R, the usual initial and semi-infinite conditions, and the flux balance at the electrode surface, exactly as in (5.4.2)-(5.4.5). The final boundary condition needed to solve the problem comes from the potential waveform, which is linked to the concentration profile through the nemstian balance at the electrode. It is convenient to consider the waveform as consisting of a series of half cycles with index m beginning from the first forward pulse, which has m = 1. Then,... 

Let us consider the ER signal due to the redox reaction of a surface-confined dye molecule. For simplicity, we assume that, for an electrode/adsorption layer incorporating a chromophore/solution interface, the absorption of the oxidized form is negligibly small, i.e. the oxidized form is colorless. We also assume that the electric dipole moment of the reduced form is of a single hnear dipole and that it has a unique director angle f with respect to the surface normal while its azimuthal angle is two-dimensionally isotropic (Fig. 2.15a). The angle

director vector with respect to the surface normal represents the molecular orientation. 

We now use an equivalent circuit representing an electrode/solution interface where the electrode surface is covered by an electroactive monolayer. The simplest circuit is shown in Fig. 2.18. We assume that the molecules in a Langmuir monolayer undergo an n-electron transfer reaction in response to ac and that the ER signal is exclusively due to this faradaic process [69]. The faradaic process of the surface-confined species at the formal potential is represented by a series connection of a constant capacitance associated with the redox reaction of the adsorbed species Q and a charge transfer resistance Ret. where Q is written for a Nernstian process as... 

The width at half height, A hh, of the voltammetric peak shown in Fig. II.l.lO (for both the adsorption and the thin layer case) can be determined as AFhh = 3.53 RT) / nF) = 90.6 mV for a one-electron process at 25°C [54]. For the case of strongly adsorbed systems, any deviation from the ideal or Langmurian case of no interaction between individual redox centres on the electrode surface manifests itself as a change of AEhh. Interpretations based on regular solution theory models have been suggested to account for non-ideal behaviour [55]. Of course, departure from reversibility also leads to changes in wave shape as a function of scan rate [56]. Further mechanistic details for the case of surface-confined reactions have been discussed [57]. 

Adsorption of reactants, intermediates and products at electrode surfaces significantly influence the current-potential relationship for electrode reactions. In particular species confined to electrode surfaces can be oxidized and reduced without the need for the reactants to diffuse towards the electrode surface. The adsorption free energy plays a key role in electrode kinetics and for charged species on the potential dependence. 

B The monomer is absorbed onto the electrode surface and chain lengthening occurs as a heterogeneous reaction between the surface-confined oligomer and monomer cation radical. 

Table 8.2 lists some characteristic electrode reactions. The linear sweep and cyclic voltammetric diagnostic features of these are described below. They have been selected as examples of the chemical systems most likely to be incorporated in chemical sensors. Further discussion of a wider range of mechanisms can be found in (21). Cyclic voltammetry of surface-confined species is discussed in Chapter 5. 

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