Electron transfer current

Introducing Eqn. 8—17 into Eqns. 8—7 and 8-8, we can derive from integration of Eqn. 8-9 the cathodic and anodic electron transfer currents, t and i. ... 

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. 8-8. Diffusion of redox particles before and after electron transfer at metal electrode i (i ) = cathodic (anodic) electron transfer current, t fr (>dfr) = cathodic (anodic) difiusi

Figure 8-11 shows as a function of electron energy e the electron state density Dgdit) in semiconductor electrodes, and the electron state density Z e) in metal electrodes. Both Dsd.t) and AKe) are in the state of electron transfer equilibrium with the state density Z>bei)ox(c) of hydrated redox particles the Fermi level is equilibrated between the redox particles and the electrode. For metal electrodes the electron state density Ai(e) is high at the Fermi level, and most of the electron transfer current occurs at the Fermi level enio. For semiconductor electrodes the Fermi level enao is located in the band gap where no electron level is available for the electron transfer (I>sc(ef(so) = 0) and, hence, no electron transfer current can occur at the Fermi level erso. Electron transfer is allowed to occur only within the conduction and valence bands where the state density of electrons is high (Dsc(e) > 0). 

When the transport current of electrons or holes in semiconductor electrodes more or less influences the interfacial electron transfer current, the overvoltage T) consists of an overvoltage of space charge layer iisc, an overvoltage of compact layer t]h, and a transport overvoltage tit in semiconductors as expressed in Eqn. 8-68 ... 

Figure 8-40 shows the electron transfer current of two redox reactions (outer-sphere electron transfer) observed at constant potential for platinum electrodes covered with a thin oxide film in acidic solutions as a function of the film thickness. As e3q>ected fium Eqns. 8-84 and 8-85, a linear relationship is observed between the logarithm of the reaction ciirrents and the thicknesses of the film. 

Fig. 8-41. Electron transfer reaction of hydrated redox particles in equilibrium on a metal electrode covered with a thick film (F, solid curve) and with a thin film (F, dashed curve) >cs = electron transfer current via the conduction band >scl = tunneling electron current through a depletion layer in the conduction band >vb = hole transfer current via the valence band.

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.

The main thing to be interpreted is the peak itself, the principal characteristic of the potential sweep. The processes corresponding to each peak differ only in finer detail, specific to the reaction encountered. Basically, they can be explained in terms of the effect of potential on the Faradaic (electron transfer) current, iF, and of time on the value of the limiting current, iL. 

If the electron-transfer step in an electrode reaction is preceded by a chemical reaction that involves proton transfer, the polarographic current often will be a complex function of the concentration of the electroactive species, the hy-dronium ion concentration, and the rate constants for proton and electron transfer. Currents controlled by the rate of a chemical reaction are called kinetic currents and often are observed in the reduction of electroactive acids (e.g., pyruvic acid), in which the protonated form of the acid is more easily reduced than the anion. A polarogram of pyruvic acid in unbuffered solution exhibits two waves whose relative wave heights depend on the concentration of pyruvic acid and the solution pH.59... 

Kovacic, P. and Jacintho, J. D. Mechanisms of carcinogenesis focus on oxidative stress and electron transfer. Current Medical Chemistry 8 773-796 2001. 

To investigate the effect of reactant transport on the electron-transfer current density, we can express Eqn (2.28) as Eqn (2.61) ... 

For evaluating the diffusion-limited electron transfer current, the following equations are important ... 

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