Cathode transfer

The notation of the transfer coefficients differs in many sources. Some texts use p or (1-P) in place of a. Others use a for the anodic transfer coefficient and p for the cathodic transfer coefficient. Transfer coefficients are typically close to... 

From the flow of electric charge it follows that the cathodic transfer of metal ions requires the electrode to accept electrons from an external cell circuit, and that the anodic transfer of metal ions requires the electrode to donate electrons to an external cell circuit. No electron transfer, however, takes place across the electrode interface this is the reason why no electrons are involves in the metal ion transfer reactions in Eqns. 7-3 and 7-4. 

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... 

We again consider a transfer of redox electrons via the conduction band medi-anism as shown in Fig. 8-23. The anodic and cathodic transfer currents of redox electrons have been given in Eqns. 8-56 and 8-57, respectively. In these equations, the state density occupied by electrons in the conduction band is approximated by the concentration of conduction band electrons at the electrode interface, n, = j Dsc(E)A(E-eF(8C))dE and the state density vacant for electrons in the conduction band is approximated by the effective state density of the conduction band, Nc Nc n, j Dsd ) 1-f(e-ef ac ) de. Further, the state density of... 

For redox reactions due to cathodic transfer of electrons via the conduction band, the cathodic current is expected to be maximum when the most probable vacant level eqx of the oxidant particle is in the same level as the conduction band edge e this cathodic current gradually decreases with increasing separation of eox firom ej. as shown in Fig. 8-29. The same conclusion may also be drown fh>m Eqn. 8-61. 

Figure 8-30 shows the normalized cathodic transfer current of redox electrons for several redox reactions as a function of the standard redox potential sbdox on n- semiconductor electrodes of zinc oxide in aqueous solutions. The bell-like curve observed in Fig. 8-30 is in agreement with the forgoing conclusion that the maximum current occurs at the electrode potential at whidi tox equals e. ... 

We now consider a cathodic transfer of electrons from the conduction band of electrode to the vacant redox electron level in a hydrated oxidant particle to form a hydrated reductant particle in solution OX , + ecB- RED . Equation 8-72 expresses this reaction current, due to the direct transfer of electrons from the conduction band to the oxidant particle based on Eqn. 8-61 as follows ... 

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.

Similarly, the cathodic transfer current of metallic ions, i", is given by Eqn. 9-7 ... 

Figure 9—4 shows the polarization curves observed for the transfer reaction of cadmium ions (Cd Cd ) at a metallic cadmium electrode in a sulfuric acid solution. It has been proposed in the literature that the transfer of cadmium ions is a single elemental step involving divalent cadmium ions [Conway-Bockris, 1968]. The Tafel constant, a, obtained from the observed polarization curves in Fig. 9-4 agrees well with that derived for a single transfer step of divalent ions the Tafel constant is = (1- P) 1 in the anodic transfer and is a = z p = 1 in the cathodic transfer. 

If the anodic anion transfer (anionic adsorption, Eqn. 9-13a) to form an adsorbed metallic ion complex is the rate-determining step, the Tafel constant, a = 1 - p, win be obtained from Eqn. 9-14. If the anodic transfer of the adsorbed metallic ion complex (desorption of complexes, Eqn. 9-13b) is the rate-determining step, the Tafel constant, a = 2 - p, will be obtained from Eqns. 9-16 and 9-17. Similarly, if the cathodic anion transfer (anionic desorption, Eqn. 9-13a) is determining the rate, the Tafel constant in the cathodic reaction, a = 1 p, will be obtained from Eqns. 9-15 and 9-16 and if the cathodic transfer of a metallic ion complex (adsorption of complexes, Eqn. 9-13b) is determining the rate, the Tafel constant, a-sp, will be obtained from Eqn. 9-18. In this discussion we have assumed Pi = Ps P then, Eqns. 9-19 and 9-20 follow ... 

Next, we discuss the reductive dissolution of ionic compoxmds shown in Fig. 9-15(b). The reductive dissolution is composed of a cathodic reduction of surface cations and a cathodic transfer of surface anions as expressed in Eqns. 9-55 and 9-66 ... 

Figure 10-30(c) applies to the photoexcited cell, where oxj en evolution proceeds via the anodic transfer of holes at the n-type anode and hydrogen evolution proceeds via the cathodic transfer of electrons at the p-type cathode. In order for the photoelectrolytic decomposition of water to proceed in such a cell, the edge level of the valence band sCy of n-type anode needs to be lower than the Fermi level tr(02ai20) of oxygen redox reaction and the edge level of the conduction band p c of p-type cathode needs to be higher than the Fermi level of... 

On the mixed electrode of metallic iron immersed in acidic solutions, the anodic and cathodic charge transfer reactions (the anodic transfer of iron ions and the cathodic transfer of electrons) proceed across the electrode interface, at which the anodic ciurent (the positive charge current) is exactly balanced with the cathodic current (the negative charge current) producing thereby zero net current. 

The potential of a mixed electrode at which a coupled reaction of charge transfer proceeds is called the mixed electrode potential , this mixed electrode potential is obviously different from the single electrode potential at which a single reaction of charge transfer is at equilibrium. For corroding metal electrodes, as shown in Fig. 11—2, the mixed potential is often called the corrosion potential, E . At this corrosion potential Eemt the anodic transfer current of metallic ions i, which corresponds to the corrosion rate (the corrosion current ), is exactly balanced with the cathodic transfer current of electrons for reduction of oxidants (e.g. hydrogen ions) i as shown in Eqn. 11-4 ... 

Equation 11-6 corresponds to the affinity for the reaction of metallic corrosion. As described in Chaps. 8 and 9, the anodic transfer current i of metal ions and the cathodic transfer current i of electrons across the interface of corroding metallic electrodes are, respectively, given in Eqns. 11-7 and 11-8 ... 

Figure 11-7 shows the polarization curve of an iron electrode in an acidic solution in which the anodic reaction is the anodic transfer of iron ions for metal dissolution (Tafel slope 40 mV/decade) the cathodic reaction is the cathodic transfer of electrons for reduction of hydrogen ions (Tafel slope 120 mV /decade) across the interface of iron electrode. 

In the stationary state of anodic dissolution of metals in the passive and transpassive states, the anodic transfer of metallic ions metal ion dissolution) takes place across the film/solution interface, but the anodic transfer of o Q en ions across the Qm/solution interface is in the equilibrium state. In other words, the rate of film formation (the anodic transfer oS metal ions across the metal lm interface combined with anodic transfer of osygen ions across the film/solution interface) equals the rate of film dissolution (the anodic transfer of metal ions across the film/solution interface combined with cathodic transfer of oitygen ions across the film/solution interface). 

Anodic, cathodic transfer coefficient Dimensionless electrode potential Standard potential of the ion transfer Snrface (excess) concentration of species i, molm initial snrface (excess) concentration, molm Maximal snrface concentration, molm ... 

Figure 5. Measurement and analysis of steady-state i— V characteristics, (a) Following subtraction of ohmic losses (determined from impedance or current-interrupt measurements), the electrode overpotential rj is plotted vs ln(i). For systems governed by classic electrochemical kinetics, the slope at high overpotential yields anodic and cathodic transfer coefficients (Ua and aj while the intercept yields the exchange current density (i o). These parameters can be used in an empirical rate expression for the kinetics (Butler—Volmer equation) or related to more specific parameters associated with individual reaction steps.(b) Example of Mn(IV) reduction to Mn(III) at a Pt electrode in 7.5 M H2SO4 solution at 25 Below limiting current the system obeys Tafel kinetics with Ua 1/4. Data are from ref 363. (Reprinted with permission from ref 362. Copyright 2001 John Wiley Sons.)...

In DMSO solution, the standard rate constant and cathodic transfer coefficient of the Cd(II)/Cd(Hg) system decreased with increasing concentration of TEAP [65]. It was found that a chemical reaction, probably partial desolvation of the reactant, precedes the electron transfer, and Cd(II) is reduced according to the CEE mechanism. The kinetic parameters of this process were determined. 

The cadmium electrodeposition on the solid cadmium electrode from the sulfate medium was investigated [217]. The following kinetic parameters were obtained cathodic transfer coefficient a = 0.65, exchange current density Iq = 3.41 mA cm , and standard rate constant kg = 8.98 X 10 cm s . The electrochemical deposition of cadmium is a complex process due to the coexistence of the adsorption and nucleation process involving Cd(II) species in the adsorbed state. 

The electrochemical reaction ox(sin) + 2e- = red(sin) is first order with respect to the reactant ox. The cathodic transfer coefficient is 0.5. How many times is the excliange current density increased when the concentration of ox is increased ten times (Gokjovic)... 

---