Open-Circuit Phenomena

it is interesting to note that high-purity aluminum rests at a potential at which corrosion is at its minimum and is, indeed, relatively very small. It is also largely independent of the anions present in the electrolyte.69 This may be attributed to the coulombic repulsion of anions away from the surface by the negative charge on the metal. The latter seems not to be completely compensated in a thin oxide film, as shown schematically in Fig. 9, so that the solution side of the double layer formed at the O/S interface contains excess cations, anions being repelled. The anions could approach the O/S interface either at thicker films or at potentials more positive than the OCP. 

When the oxide is formed by anodizing in acid solutions and the sample is then left to rest at the OCP, some dissolution can occur. This process has been studied by a numbers of authors,70-75 especially in relation to porous oxides [cf. Section 111(4)]. It was found that pore walls are attacked, so that they are widened and tapered to a trumpet-like shape.70 71 Finally, the pore skeleton collapses and dissolves, at the outer oxide region. The outer regions of the oxide body dissolve at higher rates than the inner ones.9,19 The same is true for dissolution of other anodic oxides of valve metals.76 This thickness dependence is interpreted in terms of a depth-dependent vacancy concentration in the oxide75 or by acid permeation through cell walls by intercrystalline diffusion, disaggregating the microcrystallites of y-alumina.4 

As for the thinning of the barrier film in such a case, it can be understood in terms of the effects discussed earlier [cf. Section III(l(ii))], as the relaxation of anodic polarization increases the rate of proton transfer. Thus, the hydration of the outer regions of the film takes place, resulting in double-layer withdrawal and chemical dissolution at the surface. 

The open-circuit dissolution is very sensitive to temperature (unlike the field-stimulated dissolution). It was found that a temperature rise from 20 to 25°C doubles the rate of dissolution.74 The rate of dissolution is, of course, pH dependent and is virtually nil at higher pH. 

In the presence of oxygen in solution, the cathodic reaction becomes that of oxygen reduction. This can shift the OCP in the positive direction and bring it to, or close to, the potential of the sharp rise of anodic current [ pitting potential cf. Section III(5(i))]. 

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One should not allow the CT secondary to be open circuited when it is energized, for it may induce dangerously high voltages. This phenomenon is explained in Example 15.8. 

When a polymer film is deformed sinusoidally with time with an angular frequency to, an open-circuit voltage or a short-circuit current of the same frequency is observed across the electrodes on both surfaces of the film. This phenomenon is called the piezoelectricity of the film. The deformation is usually the elongational vibration along an axis in the film plane and sometimes the bending vibration. 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

When an n-type semiconductor which is in contact with a metal ion-containing electrolyte is illuminated, then two equal partial currents occur under open-circuit conditions (Fig. 11.25a). The anodic photocurrent is due to O2 formation in H2O, whereas the cathodic partial current corresponds to the reduction of the metallic ions. Since the holes cannot diffuse very far, most of them collect at the illuminated interface. In the case of an n-type semiconductor, sufficient electrons are availabe everywhere, so that metal deposition should occur at illuminated as well as at dark surface sites (Fig. 11.25b), according to which conclusion, selective deposition would be impossible. Experimentally, however, selective metal deposition has been observed, e.g. at CdS at illuminated surfaces [130] and at Ti02 at the dark sites [131]. In the case of CdS, this phenomenon was interpreted as a downward shift of the energy bands at the illuminated surface which is more favorable to an electron transfer there [130]. The result obtained with Ti02 has been explained by strong internal and external recombination... 

A second technique that has been used is the measurement of photovoltage. The basis of this technique is that, on illumination under open-circuit conditions, the potential distribution will be modified so as to eliminate the potential drop within the depletion layer. In fact, as has been demonstrated by Kautek and Gerischer [7], the theory of the photovoltage effect is far from straightforward, especially in the presence of surface states. The effect is a steady-state rather than equilbrium phenomenon the potential distribution will change until the flux of holes to the surface is equal to the flux of electrons and the potential at which this occurs will depend on the recombination kinetics at the surface. Only when these kinetics are slow, i.e. when the surface states are slow and the main surface state equilbrium is with the redox couple in solution, is the technique likely to give results that can be interpreted within a consistent framework. 

In order to address both fundamental and application aspects of electrochemical promotion in this Chapter, the combustion of ethylene over Ir02 or RUO2 catalysts and the reduction of NO with propylene over Rh catalysts —all deposited on yttria-stabilized zirconia (YSZ) solid electrolyte— have been chosen as model catalytic systems. While the literature of electrochemical promotion deals mainly with metal catalysts, our laboratory has a long experience with promotion of metal oxide catalysts, such as Ir02 and RUO2. In fact, ethylene combustion with Ir02/YSZ film catalyst was the first catalytic system found to exhibit the phenomenon of permanent electrochemical promotion, which manifests itself as a shift in the steady-state open-circuit activity due to polarization, and is attributed to a change in... 

The proposed model of two-stage process is well supported by the cyclic voltammetric experiments presented in Section III.4. The fast reversible stage, attributed to formation of an electric double layer at the catalyst/gas interface via backspillover of promoters, has been discussed in detail in Section III.5. To explain the slow irreversible pretreatment. This phenomenon is called permanent electrochemical promotion or permanent NEMCA effect. The similarity between the regions of rate increase and decrease indicates that similar mechanisms are involved during current application and interruption, but the enhancement of the open-circuit rate indicates that the electrochemical promotion of the Ir02 catalyst is not reversible. This behavior of an oxide catalyst is different from that of a metal catalyst for which the electrochemical promotion is usually reversible. ... 

The phenomenon of oscillating reactions is widely recognized in chemistry. In those cases in which a surface-boxmd species or the surface itself is involved in the complex sequence of steps, the EQCM offers the prospect of additional information. An example of this is the role of an oxide layer in the oxidation of formaldehyde at Pt and Rh [174], Similarly, in the oxidation of 2-propanol at Pt electrodes, it was found that the oscillations (in potential and mass, at constant ourent) increased in amphtude until the positive extreme of the potential excursion reached a value consistent with PtOH and/or PtO formation [175]. Oscillations in mass at open-circuit potential were also observed during the dissolution of Cu in sulfate media when the solution concentration of Cu " " was sufficiently high (c > 0.045 mol dm ) [176], although in this case the potential excursions were such that oxide formation/dissolution was ruled out. 

Measuring a voltage can be based on various physical principles. To minimize any disturbance that may be caused to the phenomenon being studied by the measurement process itself, the current generated by that measurement must be kept extremely low. Therefore, when taking electrochemical measurements, notably at open circuit, voltmeters are used with field effect transistors with extremely high input impedance. ... 

On the other hand, the shape of the current-potential curve is different in the oxidation branch because metallic copper is not concerned by any mass transport phenomenon, since copper is always present at the interface. The zone where the current undergoes large variations is close to the open-circuit potential, which is, in this case, equal to the equilibrium potential of the system. The latter, which can be calculated using the Nernst law, is shifted slightly from the standard potential. To give an order of magnitude for a concentration in Cu ions equal to 10" mol L , there is the following ... 

Here therefore we can obtain the result for spontaneous evolution in the system at open circuit. This equates to the balance of both the oxidation and reduction half-reactions which corresponds to the corrosion phenomenon ... 

If a coil such as that imagined above carries an alternating current, and if a second coil is placed alongside, a varying electromagnetic field is produced in the second coil as well as in the first. A current then flows in the second coil if it is not open-circuit. This is the phenomenon of mutued inductance, and it is the basis of the transformer. 

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