Heterogeneous electrode reaction

The interface between the electrode and the electrolyte or electrochemical interface is the site where heterogeneous electrode reactions occur. The structure and electrical properties of this interfacial region are therefore relevant to electrode kinetics. 

If the electrode potential, , is 118 mV smaller than for the preparative reduction of O, then X is 99 % or, expressed dilferently, only 1 % of O has not been transformed at the end of the electrolysis. Equation 98 is valid not only for fast (nernstian) but also for slow heterogeneous electrode reactions, since an equilibrium situation is always attained at the end of the electrolysis where the current is essentially zero. For an irreversible electrode reduction, the rate of electrolysis as expressed by the electrolysis current is, however, very low if E — Eq is — 118 mV. The employment of much higher overpotentials or mediated electrolysis (see Section 2.7) may then be required for achieving a reasonable rate of transformation. 

The importance of Marcus theoretical work on electron transfer reactions was recognized with a Nobel Prize in Chemistry in 1992, and its historical development is outlined in his Nobel Lecture.3 The aspects of his theoretical work most widely used by experimentalists concern outer-sphere electron transfer reactions. These are characterized by weak electronic interactions between electron donors and acceptors along the reaction coordinate and are distinct from inner-sphere electron transfer processes that proceed through the formation of chemical bonds between reacting species. Marcus theoretical work includes intermolecular (often bimolecular) reactions, intramolecular electron transfer, and heterogeneous (electrode) reactions. The background and models presented here are intended to serve as an introduction to bimolecular processes. 

In the last case, a homogeneous reaction step permits the combination of the intermediates formed by the heterogeneous electrode reactions. 

Heterogeneous electrode reactions can be compared with homogeneous kinetics in solution, with regard to mass transport. The second-order rate coefficient for a fast homogeneous reactions in solution, k(hom), which would be observed if diffusion were infinitely fast, can be related to the measured rate coefficient, kob3(hom) by application of Eick s first law in a spherical continuum diffusion field around the reacting molecule. At a collision distance Tab. this corresponds to the average... 

Besides Ri = and Cj = C, one often finds parallel Ri, Ci response associated with a heterogeneous electrode reaction. For such a case we would set R = Rg and Cl = Cr, where Rr is a reaction resistance and C is the diffuse double-layer capacitance of the polarization region near the electrode in simplest cases. The circuit of Figure 1.3.1Z combines the above possibilities when R2 = Rr and C2 = Cr. The results shown in Figure 1.3.le and/are appropriate for the well-separated time constants, It is also possible that a parallel RC combination can arise... 

The quantitative analysis requires knowledge of the rate(s) of the heterogeneous electrode reaction(s), reagent diffusion coefficients and the transfer coefficient. If the electrode reaction is reversible, most of these parameters can be determined from the CV experiments. The formal reduction potential, differs from the standard potential, °, because the latter is obtained by extrapolation to infinite dilution, while the former refers to the actual experimental conditions of ionic strength and temperature. For a fast, reversible process, E° => j,2 10 mV if the diffusion coefficients of the oxidized and reduced forms are within a factor of two. Potentials are reported relative to some standard electrode, such as ferrocene/ferrocinium ion, saturated calomel, SCE, or Ag/AgCl, and this must be taken into account in comparing results from different sources. 

Bard, A. J. Inner-sphere heterogeneous electrode reactions. Electrocatalysis and photocatalysis The challenge. J Am Chem Soc 2010,132, 7559-7567. 

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