Oxidation overpotential curve

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

Deposition of mercury at boron-doped diamond (BDD) and platinum electrodes has also been studied [33]. Deposition and oxidation of mercury was performed by cyclic voltammetry from the solution of 1 mM Hg2 ( 104)2 in 1 M Na l04. In order to learn more about this deposition, it was carried out also under chronoamperometric conditions. The results obtained are shown in Fig. 2 in the form of dimensionless current-time transients. Experimental curves obtained at two different overpotentials were compared with the theoretical curves calculated for instantaneous and progressive nucleation. A good agreement of experimental plots with the instantaneous nucleation mechanism was... 

In practice, one often has cases where the course of the cathodic and anodic current densities across the electrode with change of overpotential is symmetrical (except for the signs for the same numerical value of T)). It will be seen in the next section that this is so if the symmetry factor, P, is exactly 0.50. More often, the anodic and cathodic curves are nearly symmetrical. However, sometimes they are importantly and even dramatically different. For example, the anodic current is oxidizing and could provoke... 

Consider an electrolyte that contains an electroactive species E, which is either oxidized or reduced, and a species P with a potential-dependent adsorption isotherm. In the absence of species P the I/DL curve be N-shaped (Fig. 18a, dashed line). Furthermore, the isotherm of P be such that its coverage decreases with increasing overpotential for the conversion of E somewhere in the N-NDR range of k ( DL) (Fig. 18a, solid line). In addition, adsorbed P inhibits the electrochemical conversion of E (Fig. 18b). The following set of equations represents the simplest formulation of the mechanism ... 

For irreversible reactions, k0 kd. Kinetics has an important role, especially for potentials close to Eeq. It is necessary to apply a higher potential than for a reversible reaction in order to overcome the activation barrier and allow reaction to occur—this extra potential is called the overpotential, rj. Because of the overpotential only reduction or only oxidation occurs and the voltammogram, or voltammetric curve, is divided into two parts. At the same time it should be stressed that the retarding effect of the kinetics causes a lower slope in the voltammograms than for the reversible case. Figure 6.4 shows, schematically, the curve obtained, which is explained in greater detail below. 

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

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