Three-electron charge transfer processes

From the derivations in Appendix B, it is evident that the present faradaic rectification formulations for multiple-electron charge transfer not only enable the determination of kinetic parameters for each step of three-electron charge transfer processes but may also be extended to charge transfer processes involving a higher number of electrons. However, the calculations become highly involved and complicated. 

The first step in Pt metal deposition is the reduction to Pt(II), followed by the actual metal deposition. Also shown in Figure 2.84 are Si surface states. Three principal charge transfer processes have been drawn 1, hole injection into the valence band which is a potential-independent process 2, hole injection into occupied surface states and 3, reduction of the Pt complexes by electrons from the conduction band. The latter two processes are potential dependent. At potentials for which the semiconductor is in depletion or in a normal (compared to strong) inversion condition (between OV and flatband), the structure under the peak Cl is attributed to charging and discharging of surface states and at V < -0.48 V (Vfb) reduction occurs by electrons from the conduction band. 

Another quantity that has to be taken into consideration is the density of states at the Fermi level, g E, and any alterations caused to it by the charge transfer process. The importance of g E on the adsorptive and catalytic properties of a metal surface has been stated by some investigators [131-133]. More specifically, the density of states defines the ability of the surface to respond to the presence of an adsorbate [132]. Theoretical calculations for the density of states function, g E), have been reported in the literature for certain metals [134]. The g E) function for the d metals Ru, Rh, and Pd is characterized by the participation of the d electrons. All three metals have a high density of states at the Fermi level (1.13 for Ru, 1.35 for Rh,... 

Figure 11.13 illustrates a basic equivalent circuit to represent a general electrochemical reaction. Rs represents the electric resistance, which consists of the ionic, electronic, and contact resistances. Since the electronic resistance is typically much lower than the ionic resistances for a typical fuel cell MEA, the contribution of the electronic resistance to Rs is often negligible. Cj is the double-layer capacitance associated with the electrode-electrolyte interfaees. Since a fuel cell electrode is three-dimensional, the interfaces include not only Arose between Are surfaces of the electrodes and the membrane but also those between the catalysts and the ionomer within the electrodes. Ret is the resistanee associated with the charge transfer process and is called charge transfer resistanee. Z is called the Warburg impedance it deseribes the resistance arising from the mass transport processes. 

The elucidation of the nature of charge-transfer processes in electrochemically active polymer films may be the most interesting theoretical problem of the field and a question of great practical importance. A polymer film electrode can be defined as an electrochemical system in which at least three phases are contacted successively in such a way that between an electronic conductor (usually a metal) and an ionic conductor (usually an electrol)fie solution) is an electrochemically active polymer layer. The fundamental processes of insertion and transport of charged and noncharged species through this type of electrochemical system are described below and illustrated in Figure 9-1 [8] ... 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

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