Electrode potential, effect anodic dissolution

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

When such a polyfunctional electrode is polarized, the net current, i, will be given by ii - 4. When the potential is made more negative, the rate of cathodic hydrogen evolution will increase (Fig. 13.2b, point B), and the rate of anodic metal dissolution will decrease (point B ). This effect is known as cathodic protection of the metal. At potentials more negative than the metaTs equilibrium potential, its dissolution ceases completely. When the potential is made more positive, the rate of anodic dissolution will increase (point D). However, at the same time the rate of cathodic hydrogen evolution will decrease (point D ), and the rate of spontaneous metal dissolution (the share of anodic dissolution not associated with the net current but with hydrogen evolution) will also decrease. This phenomenon is known as the difference effect. 

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

Since holes are consumed at the surface during the anodic dissolution, the n-type samples show increasing differences between the measured and the calculated capacities with increasing rate of dissolution, i. e., with increasing anodic polarization. In this case d-c potential curves also show deviations from the initial exponential slope. At higher anodic potentials a saturation current occurs. Illumination compensates for or decreases the influence of the anodic current on the concentration cf holes. Fig. 11 shows schematically the influence of anodic dissolution and illumination. For p-type Ge the same effects occur, when electrons are consumed by the electrode reaction, i. e., in the cathodic region. 

Whether a rotation rate dependence of the current for metal dissolution is observed depends on the relative values of the rate of dissolution and the rate of mass transport, i.e. the first and second terms in Equation (4.41). For instance, the anodic dissolution of Fe in H2SO4 at potentials close to the corrosion potential is slow, and no dependence of the current on rotation rate is observed. In contrast. Fig. 4.14 shows data [8] for a rotating zinc disc electrode in lmoldm NaOH, and a significant mass transport effect is seen. 

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