Kinetic effects, polarization curves

Based on cathodic polarization curves, Dexter and Gao concluded that the increase of E for 316 stainless steel exposed to natural seawater was due to an increased rate of the cathodic reduction of oxygen at a given potential. It is not possible from Ecorr or polarization curves to decide whetlier the increase in Ecorr is due to thermodynamic effects, kinetic... 

The kinetic and polarization equations described in Sections 6.1 and 6.2 have been derived under the assumption that the component concentrations do not change during the reaction. Therefore, the current density appearing in these equations is the kinetic current density 4. Similarly, the current density appearing in the equations of Section 6.3 is the diffusion current density When the two types of polarization are effective simultaneously, the real current density i (Fig. 6.6, curve 3) will be smaller than current densities and ij (Fig. 6.6, curves 1 and 2) for a given value of polarization. 

The exchange CD determined by different methods will coincide only in the case of quasi-one-step reactions mentioned above. Thus, when the value of if is so much higher than if that the extreme anodic section cannot be measured and there is no break in the polarization curve, all three methods of determination lead to the same value of if. This implies that step 1 has no effect at all on the kinetics of the overall reaction and that its (high) exchange CD cannot be determined. The same conclusion holds in the opposite case of if [Pg.227]

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

This chapter is based on the thermodynamic theory of membrane potentials and kinetic effects will be considered only in relation to diffusion potentials in the membrane. The ISE membrane in the presence of an interferent can be thought of as analogous to a corroding electrode [46a] at which chemically different charge transfer reactions proceed [15, 16]. Then the characteristics of the ISE potentials can be obtained using polarization curves for electrolysis at the boundary between two immiscible electrolyte solutions [44[Pg.35]

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

A kinetic analysis is not complete without determination of the temperature effects and activation energies. Figure 6 summarizes some of the polarization curves for the ORR recorded at 333 K and 298 K for details, see [41]. Clearly, results obtained at 333 K are qualitatively similar to the curves recorded at room temperature, and the order of activity remains the same as at room temperature, i.e., Pt(lll)elevated temperatures in both the mixed diffusion-kinetic potential region and the hydrogen adsorption potential region. These higher currents reflect the temperature dependence of the chemical rate constant, which is approximately proportional to jRT where is the apparent enthalpy of activation at the reversible... 

Polarization curves over a wide range of overpotentials, both anodic and cathodic, for the hydrogen reaction at 1 atm on rotating Pt(hkl) disk electrodes at 298 K are shown in Figure 12. At low positive overpotentials, the order of activity for the HOR increased in the sequence (111) (100) <<(110). These differences in activity with crystal face can be attributed to the different state of adsorbed hydrogen and to different effects of these states on the mechanism of the hydrogen reaction [52,53]. The HOR on Pt(lll) and (100) in alkaline solution is purely kinetically controlled... 

The kinetics of several well-known electrochemical reactions have been studied in the presence of an ultrasonic field by Altukhov et al. [142], The anodic polarization curves of Ag, Cu, Fe, Cd, and Zn in various solutions of HC1 and H2S04 and their salts were measured in an ultrasonic field at various intensities. The effect of the ultrasonic field on the reaction kinetics was found to be dependent on the mechanism of metal anodic dissolution, especially on the effect of this field on the rate-determining step of the reaction. The results showed that the limiting factor of the anodic dissolving of Cu and Ag is the diffusion of reaction products, while in the case of Fe it is the desorption of anions of solution from the anode surface, and at Cd the limiting factor is the rate of destruction of the crystal lattice. Similar results were obtained by Elliot et al. [ 143] who studied reaction geometry in the oxidation and reduction of an alkaline silver electrode. 

More recent findings hy Waskaas using iron electrodes in ferric chloride solutions tend to indicate that anode and cathode polarization curves depend on the magnetic field strength, but this effect is not purely kinetic, since it involves bulk-to-drffusion layer transfer of ferric ions. 

Reference has been made to the observation that both anionic and cationic species in the environment can influence the anodic polarization of active-passive types of metals and alloys. Specific examples have related to the effect of pH as it influences the stability and potential range of formation of oxide and related corrosion product films. The effect of pH, however, cannot be treated, even with single chemical species, independent of the accompanying anions. For example, chloride, sulfate, phosphate, and nitrate ions accompanying acids based on these ionic species will influence both the kinetics and thermodynamics of metal dissolution in addition to the effect of pH. Major effects may result if the anion either enhances or prevents formation of protective corrosion product films, or if an anion, both thermodynamically and kinetically, is an effective oxidizing species (easily reduced), then large changes in the measured anodic polarization curve will be observed. 

Omran et al. have proposed a 3D, single phase steady-state model of a liquid feed DMFC [181]. Their model is implemented into the commercial computational fluid dynamics (CFD) software package FLUENT . The continuity, momentum, and species conservation equations are coupled with mathematical descriptions of the electrochemical kinetics in the anode and cathode channel and MEA. For electrochemical kinetics, the Tafel equation is used at both the anode and cathode sides. Results are validated against DMFC experimental data with reasonable agreement and used to explore the effects of cell temperature, channel depth, and channel width on polarization curve, power density and crossover rate. The results show that the power density peak and crossover increase as the operational temperature increases. It is also shown that the increasing of the channel width improves the cell performance at a methanol concentration below 1 M. 

We will consider only the influence of activation overpotential or overvoltage on secondary current distribution. It is useful to regard the slope of the polarization curve dE /di (if any effect of concentration overpotential can be ignored) as a polarization resistance R. This represents the slowness of charge transfer across the interface and is based on the electrode kinetics of the reaction. If acts in series with R, the resistance of the electrolyte, we can distinguish between two situations. If R R, then the kinetics of charge transfer and not electrolyte resistance determine the current distribution, i.e., secondary current distribution dominates. Conversely, if R R, primary current distribution dominates. Secondary current distributions tend to smooth out the severe nonlinear variations of current associated with primary distributions and they eliminate infinite currents associated with electrode edges. 

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