Electron transfer anodic polarization

A linear correlation of the oxidation potentials Bp of sulfides with Taft s a values of fluoromethy] groups is obtained as shown in Fig. 1 [42]. This dearly indicates that the polar effect of the fluoroalkyl group plays a significant role in the electron-transfer step from the sulfides to anode. Namely, the oxidation potential increases linearly as the number of fluorine atoms of the fluoroalkyl group increases. However, interestingly the oxidation potential was not appreciably affected by the length of the perfluoroalkyl group (Table 4). 

In electrochemistry, the electrode at which no transfer of electrons and ions occurs is called the polarizable electrode, and the electrode at which the transfer of electrons and/or ions takes place is called the nonpolarizable electrode as shown in Fig. 4-4. The term of polarization in electrochemistry, different from dipole polarization in physics, indicates the deviation in the electrode potential from a specific potential this specific potential is usually the potential at which no electric current flows across the electrode interface. To polarize" means to shift the electrode potential from a specific potential in the anodic (anodic polarization) or in the cathodic (cathodic polarization) direction. 

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

Figures 8-5 and 8-6 are energy diagrams, as functions of electron energy e imder anodic and cathodic polarization, respectively, for the electron state density Dyf.t) in the metal electrode the electron state density AtEDox(c) in the redox particles and the differential reaction current ((e). From these figures it is revealed that most of the reaction current of redox electron transfer occurs in a narrow range of energy centered at the Fermi level of metal electrode even in the state of polarization. Further, polarization of the electrode potential causes the ratio to change between the occupied electron state density Dazc/itnu md the imoccupied...

Fig. 8-5. Electron state density in a metal electrode and in hydrated redox particles, and anodic and cathodic currents of redox electron transfer under cathodic polarization n s cathodic overvoltage (negative) i = anodic current i = cathodic current. [From (lerischer, I960.]...

It is characteristic of metal electrodes that the reaction current of redox electron transfer, under the anodic and cathodic polarization conditions, occurs mostly at the Fermi level of metal electrodes rather than at the Fermi level of redox particles. In contrast to metal electrodes, as is discussed in Sec. 8.2, semiconductor electrodes exhibit no electron transfer current at the Fermi level of the electrodes. 

Fig. 8-7. Cathodic and anodic polarization curves observed for a transfer reaction of redox electrons of hydrated Ti /Ti particles at a mercury electrode in 1 M H28O4 solution containing 0.17 M and 0.03 M Ti 4 at 25°C electrode surface area = 0.15 cm. [From Vetter, 1967.]...

Fig. 8-8. Energy levels for redox electron transfer reaction at a metal electrode (a) in equilibrium, (b) in anodic polarization with reao tion rate determined by interfadal electron transfer, (c) anodic polarization with reaction rate determined by both interfadal electron transfer and diffusion of hydrated partides. EF0)Eooxj.a= Fenni level of redox electrons at an interface.

Fig. 8-16. Electron state density for a redox electron transfer reaction of h3rdrated redox particles at semiconductor electrodes (a) in the state of band edge level pinning and (b) in the state of Fermi level pinning dashed curve = band edge levels in reaction equilibrium solid curve = band edge levels in anodic polarization e p,sq = Fermi level of electrode in anodic polarization e v and c c = band edge levels in anodic polarization.

Next, we consider the anodic reaction current of redox electron transfer via the conduction band, of which the exchange reaction current has been shown in Fig. 8-16. Application of a slight anodic polarization to the electrode lowers the Fermi level of electrode fix>m the equilibrium level (Ep(sc)( n = 0) = eiiOTSDca)) to a polarized level (ep(8C)( n) = ep(REDox)- n)withoutchanging at the electrode interface the electron level relative to the redox electron level (the band edge level pinning) as shown in Fig. 8-20. As a result of anodic polarization, the concentration of interfacial electrons, n, in the conduction band decreases, and the concentration of interfadal holes, Pm, in the valence band increases. Thus, the cathodic transfer current of redox electrons, in, via the conduction band decreases (with the anodic electron im ection current, ii, being constant), and the anodic transfer current of redox holes, (p, via the valence band increases (with the cathodic hole injection... 

A further increase in anodic polarization lowers still further the Fermi level ersc)(ti) which gradually approaches the valence band edge Cy at the electrode interface as shown in Fig. 8-21. As the anodic polarization increases, the concentration of interfacial holes in the valence band increases, thus causing the anodic electron transfer to change from the conduction band mechanism to the valence band mechanism. 

Fig. 8-21. State density of redox electrons and reaction current ibr a redox electron transfer at a semiconductor electrode further polarized beyond that in Fig. 8-20 in anodic direction T =T gc.

As the polarization (the overvoltage t) ) increases of a redox reaction that requires the transport of minority charge carriers towards the electrode interface (anodic hole transfer at n-type and cathodic electron transfer at p-type electrodes), the transport overvoltage, t)t, increases from zero at low reaction currents to infinity at high reaction current at this condition the reaction current is controlled by the limiting diffusion current (iu.)tm or ip.um) of minority charge carriers as shown in Fig. 8-25. 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Fig. 8-42. Anodic and cathodic polarization curves observed for electron transfer of hydrated redox particles at an electrode of metallic niobium covered with a thick niobium oxide NbsOs film (12 nm thick) in acidic solution reaction is an electron transfer of hydrated redox particles, 0.25MFe(CN)6 /0.25M Fe(CN)g, in 0.1 M acetic add buffer solution of pH 4.6 at 25 C. =...

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

Fig. 8-43. Anodic and cathodic polarization curves observed for a redox electron transfer at metallic tin electrodes covered with an anodic oxide Sn02 film of various thicknesses d in a basic solution reaction is a redox electron transfer of 0.25 M Fe(CN)6 A).25 M Fe(CN)6 in 0.2 M borate buffer solution of pH 9.1 at 25°C. d = film thickness dj = 2 nm ...

Charge transfer reactions on semiconductor electrodes proceed under the condition of anodic and cathodic polarization in which the Fermi level epfsc) is different either from the Fermi level Eputicox) of redox electron transfer reactions or from the equivalent Fermi level ep,ioN) of ion transfer reactions. For redox electron transfer reactions, thermodynamic requirement for the anodic and cathodic reactions to proceed is given by the following inequalities ... 

Figure 10-16 shows polarization curves observed for the anodic ox en reaction (anodic hole transfer) and for the cathodic hydrogen reaction (cathodic electron transfer) on an n-type semiconductor electrode of titanium oxide. The data in Fig. 10-16 show that the anodic current due to the transfer of holes (minority... 

In another paper, they confirmed that the absorbance of solutions of NaB(CaHs)4 and KB(C6He)4 and of NaAl(C2H5)4 was strictly proportional to the number of electrons transfer at the electrode. And from the observation that mixing of the anode and cathode solutions resulted in a destruction of the living ends, they reversed the polarity of the electrodes and found a stoichiometric destruction of the living ends of a-methystyrene tetramer and a decrease in absorbance directly dependent on the charge transferred 13). Exactly same quantity of charge was... 

Diamond surfaces after anodic oxidation treatment involve oxygen-containing surface functional groups. The electron-transfer kinetics for ions and polar molecules are expected to be quite different. Fe(CN)l /4 was highly sensitive to the surface termination of diamond. For an anionic reactant, there was an inhibition of the electron transfer for the oxygen-terminated diamond electrodes compared with the hydrogen-terminated diamond electrodes, and there was also an acceleration of the electron transfer for oxygen-terminated diamond for some cationic reactants such as Ru(NH3) +/3+ and Fe2+/3+. These results can be explained by electrostatic effects, which interact between the ionic... 

Because the flow of electric current always involves the transport of matter in solution and chemical transformations at the solution-electrode interface, local behavior can only be approached. It can be approximated, however, by a reference electrode whose potential is controlled by a well-defined electron-transfer process in which the essential solid phases are present in an adequate amount and the solution constituents are present at sufficiently high concentrations. The electron transfer is a dynamic process, occurring even when no net current flows and the larger the anodic and cathodic components of this exchange current, the more nearly reversible and nonpolarizable the reference electrode will be. A large exchange current increases the slope of the current-potential curve so that the potential of the electrode is more nearly independent of the current. The current-potential curves (polarization curves) are frequently used to characterize the reversibility of reference electrodes. 

A second related issue is the asymmetry in the E-i response near Ecelectron transfer reaction that is different from the metal oxidation reaction. Therefore there is no fundamental reason why pa and pc should be equal, and they should be expected to differ. The extent of their difference defines the degree of asymmetry. Asymmetry matters because the extent of the region where Eq. (2) is a good approximation of Eq. (1) then differs for anodic and cathodic polarization (29). The errors in assuming 10 mV linearity using both the tangent to the E-i data at Econ and for +10 or -10 mV potentiostatic polarizations have been defined for different Tafel slopes (30). 

The electrochemical insertion process includes - charge transfer (red-ox reaction) by electrons and ions and -> diffusion of ions in the host to theirs sites. The inserted species can be cations (upon cathodic polarization), anions (upon anodic polarization), atoms, and molecules. Insertion of ions can be accompanied by co-insertion of solvent molecules from the -> solvation shell of the ions, which are dragged to the host together with the ions as the electrical field is applied. Insertion electrodes are highly important for the filed of - Li batteries, as most of the electrodes used in Li/Li-ion batteries are in fact insertion electrodes. Thereby, this entry concentrates mainly on Li ion insertion electrodes. 

The two types of electron transfer in a redox reaction at semiconductors can be distinguished by a number of experimental methods (12,13,14). The mechanisms of some redox reactions at germanium electrodes are summarized In Table I. It is seen that the mechanism of redox reactions with positive normal potentials is associated with the valence band, whereas the mechanism of redox reactions with more negative normal potentials is associated with the conduction band if there is any reaction at all. The situation remains the same when the electrode is moderately polarized in the anodic or cathodic direction. An example is shown in Fig. 11 using a redox system with properties equivalent to those assumed In Fig. 10. 

It is important to emphasize the anodic, chemical and actinic activations of electron-transfer oxidation to be complementary methods that all commonly involve the reactive intermediates like those presented in equations (la) and (2). As such, cognizance must always be taken of the subtle differences of concentration, temperature, solvent polarity, etc. that affect the behavior of the transient radicals and ion radi-... 

Tris(4-bromophenyl)ammoniumyl hexachloro antimonate is commercially available (e.g., Fluka product, 5g cost 70). It is commonly used as an oxidizing reagent by means of electron transfer and is elegantly applied to induce cycloadditions and cyclodimerization ([2 -I- 2] reactions) by Bauld [115]. However, aromatic amine radical cations as the oxidizing reagent can be easily obtained anodically [116] and their redox potentials (between -1-1 V and -1-2 V vs. NHE) modulated as a function of different substituents for utilization if indirect oxidation reactions are to be conducted. Therefore, such a redox catalysis process appears to be a cheap and elegant method to selectively achieved in situ oxidation, provided that polar solvents, electrolytes, and room temperatures are acceptable experimental conditions to perform a given reaction. 

To bridge the gap between ideal and practical catalysts, optical spectroscopies, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and Mossbauer spectroscopy can be used. All have been reviewed recently (373, 396), and some examples have been cited earlier (107, 108). Electron spin resonance has been used in several studies of electroorganic reactions (357,371). It can detect short-lived radicals resulting from electron transfer. Recent application of Mossbauer spectroscopy in situ in electrochemical cells deserves mentioning, although it addressed only the anodic polarization and film stability of Co- and Sn-coated electrodes (397,398). Extension to electrocatalytic studies involving Mossbauer nuclides seems feasible. 

The same type of arguments prove that the anodic decomposition reaction occurs via the valence band. Here we see that the corresponding anodic current at p-GaAs increases steeply with increasing anodic polarization whereas a very small anodic current is found with n-type electrodes. The latter could be increased by light excitation. Accordingly, holes from the valence band are required for the anodic decomposition of the semiconductor. It should be emphasized here that not holes but electrons are actually transferred across the interface, but an injection of electrons into the valence band is only possible if holes are present at the semiconductor surface. 

---