Electron Transfer at Electrodes

Simple electron transfer at electrodes Proton transfer to cyanocarbon bases... 

The reaction of electron transfer at electrodes in aqueous electrolytes proceeds either with hydrated redox particles at the plane of closest approach of hydrated ions to the electrode interface (OHP, the outer Helmholtz plane) or with dehydrated and adsorbed redox particles at the plane of contact adsorption on the electrode interface (IHP, the inner Helmholtz plane) as shown in Fig. 7-2. 

The basic division between the roots and the more modern papers is arbitraiy (1931-1964 is about one generation). The basic theory of proton and electron transfer at electrodes was formulated by Gurney in 1931 and by Weiss in 1954. 

The Marcus treatment for heterogeneous electron transfer at electrodes is analogous to the homogeneous case of redox reactions in solution discussed in Vol. 2, Chap. 4. The free energy of activation can also be expressed by... 

These quotes were chosen to introduce this chapter on chemically modified electrodes because they are from some of the earliest papers in the field and because they review the concepts and objectives of this research area. We learn that the field of chemically modified electrodes involves attaching specific molecules to the surfaces of conventional inert electrodes. We also discover the two major reasons for wanting to attach molecules to electrode surfaces. As explained by Lane and Hubbard, one objective is to obtain fundamental information about the mechanism of electron transfer at electrode surfaces. The second objective, as expressed by Watkins et al. and Elliott and Murray, is to impart to the electrode surface some chemical specificity not available at the unmodified electrode. For example, the modified electrode might catalyze a specific chemical reaction. Alternatively, the modified electrode might be able to recognize a specific molecule present in a contacting solution phase. 

Table 9.1 shows the numerical values of the standard redox potentials for a few reactions of electronic transfer at electrodes. Electrochemical handbooks provide the standard redox potentials for various other transfer reactions of redox electrons. As mentioned in section 9.3, the redox potential is independent of the electrode materials. 

Electrons transfer at electrodes on the basis of one electron per functional site for each reaction step, i.e. each electron-transfer step is an elementary reaction. From a kinetic view this is a powerful advantage. 

Refs. [i] Born M (1920) Z Phys 1 45 [ii] Marcus Y (1997) Ion properties. Marcel Dekker, New York [iii] Rashin AA, Honig B (1985) ] Phys Chem 89 5588 [iv] Stilly WC, TempczykA, Hawley RC, Hendrickson TA (1990) ] Am Chem Soc 112 6127 [v] Rashin AA (1990) / Phys Chem 94 1725 [vi] Shoichet BK, Leach AR, Kuntz ID (1999) Proteins 34 4 [vii] Marcus RA (1977) Theory and application of electron transfer at electrodes and in solutions. In Rock PA (ed) Special topics in electrochemistry. Elsevier, Amsterdam, pp 61 [viii] Millery C/ (1995) Heterogeneous electron transfer kinetics at metallic electrodes. In Rubinstein I (ed) Physical electrochemistry. Marcel Dekker, New York, pp 46-47... 

The - Marcus theory [vi-vii] gives a unified treatment of both heterogeneous electron transfer at electrodes and homogeneous electron transfer in solutions. 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

The model of Presnov and Trunov [341,345] is based on the transition state theory and modern ideas on electron transfer at electrodes, and allows a quantitative calculation of the rate of oxygen electroreduction based on... 

A final comment pertains to the continuous electronic manifolds in the case of electron transfer at electrodes. Here one may formulate Tg to depend explicitly on the states within the accessible portion of the electrode band structure ]31]. Alternatively, one may employ a mean value (e.g., based on the Fermi level) [25b, 32b, I06[. 

An important subject in this chapter on Electron transfer at electrodes and interfaces is to draw an analogy between electrochemical and interfacial electron transfer between two solid phases. Any theory dealing with electron transfer has a thermodynamic and a kinetic basis. In Section 4.2, it was shown that electrons flow or tunnel in the direction of decreasing electrochemical potential the gradient of the electrochemical potential is the driving force behind a directed flow of electrons,... 

---