Current oscillation

In the potential range catliodic to one frequently observes so-called metastable pitting. A number of pit growtli events are initiated, but tire pits immediately repassivate (an oxide film is fonned in tire pit) because the conditions witliin tire pit are such that no stable pit growtli can be maintained. This results in a polarization curve witli strong current oscillations iU [Pg.2728]

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

Now commence the voltage sweep using a scan rate of 5 mV per second, or with a manual polarograph, increase the voltage in steps of 0.05 V. The recorder plot will take the form shown in Fig. 16.4 if a manual instrument is used, then since the current oscillates as mercury drops grow and then fall away, the plot will have a saw-tooth appearance, and for measurement purposes a smooth curve must be drawn through the midpoint of the peaks of the plot. 

According to the literature [21], all reported electrochemical oscillations can be classified into four classes depending on the roles of the true electrode potential (or Helmholtz-layer potential, E). Electrochemical oscillations in which E plays no essential role and remains essentially constant are known as strictly potentiostatic (Class I) oscillations, which can be regarded as chemical oscillations containing electrochemical reactions. Electrochemical oscillations in which E is involved as an essential variable but not as the autocatalytic variable are known as S-NDR (Class II) oscillations, which arise from an S-shaped negative differential resistance (S-NDR) in the current density (/) versus E curve. Oscillations in which E is the autocatalytic variable are knovm as N-NDR (Class III) oscillations, which have an N-shaped NDR. Oscillations in which the N-NDR is obscured by a current increase from another process are knovm as hidden N-NDR (HN-NDR Class IV) oscillations. It is known that N-NDR oscillations are purely current oscillations, whereas HN-NDR oscillations occur in both current and potential. The HN-NDR oscillations can be further divided into three or four subcategories, depending on how the NDR is hidden. 

Another example is dendritic crystal growth under diffusion-limited conditions accompanied by potential or current oscillations. Wang et al. reported that electrodeposition of Cu and Zn in ultra-thin electrolyte showed electrochemical oscillation, giving beautiful nanostmctured filaments of the deposits [27,28]. Saliba et al. found a potential oscillation in the electrodeposition of Au at a liquid/air interface, in which the Au electrodeposition proceeds specifically along the liquid/air interface, producing thin films with concentric-circle patterns at the interface [29, 30]. Although only two-dimensional ordered structures are formed in these examples because of the quasi-two-dimensional field for electrodeposition, very recently, we found that... 

Figure 14.1a shows aj vs. U curve in Cu + Sn + H2SO4 with (solid curve) and without (dashed curve) cationic surfactant. The addition of the surfactant causes a drastic change in they vs. U curve. Namely, an NDR appears in a narrow potential region of about 5 mV near —0.42 V, where the Cu-Sn alloy is electrodeposited. Another notable point in the surfactant-added solution is that a current oscillation appears when the Uis kept constant in (and near) the potential region of this NDR, as shown in Figure 14.1b. It was also revealed that both the NDR and current oscillation appeared only in the presence of cationic surfactant and not in the presence of anionic surfactant.

On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the tme electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given hy E=U —JAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, andj is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, is much more... 

Nakanishi, S., Sakai, S., Nagai, T. and Nakato, Y. (2005) Macroscopically uniform nanoperiod alloy multilayers formed by coupling of electrodeposition with current oscillations. /. Phys. Chem. B, 109, 1750-1755. 

Survila, A., Mockus, Z. and Juskenas, R. (1998) Current oscillations observed during codeposition of copper and tin from sulfate solutions containing Laprol 2402C. Electrochim. Acta, 43, 909-917. 

Samjeske G, OsawaM. 2005. Current oscillations during formic acid oxidation on a Pt electrode Insight into the mechanism by time-resolved IR spectroscopy. Angew Chem 44 5694-5698. 

Similarly to the induction or the inhibition of the current oscillation at a biomembrane with a sodium channel, the current oscillation at the liquid membrane (without any channel proteins) caused by the transfer of Na can be induced by acetylcholine ion, Ach, or inhibited by such rather hydrophobic ions as alkylammonium and glutamate ions. 

The current oscillation observed by applying AFwi-wi = —0-48 V was inhibited when one of the rather hydrophobic ions such as TEA, TBA+ or glutamate ion was added to Wl to more than 10 BSA, was added to W2. 

It is noteworthy that the current oscillation was induced or inhibited by the addition of Ach or TEA, respectively, into W1 although the Wl/LM interface is not directly related to the voltammetric maximum essential for the oscillation. 

Since the final descent in the voltammogram at the Wl/LM interface in the presence of 0.01 M TEA in Wl (curve 2 ) lies at much more positive potentials than those in the absence of TEA (curve 2), the maximum in the voltammogram for the transfer of Na through the LM in the presence of TEA (curve 1 ) appears at more positive potentials than that in the absence of TEA (curve 1) for the sake of the relation of Eq. (2). Therefore, the AF i-wi value necessary to observe the current oscillation in the absence of TEA (e-g-, —0.48 V) is too negative to continue the oscillation after the addition of TEA+. 

Weder s experiments were carried out with opposing body forces, and large current oscillations were found as long as the negative thermal densification was smaller than the diffusional densification. [Note that the Grashof numbers in Eq. (41) are based on absolute magnitudes of the density differences.] Local mass-transfer rates oscillated by 50%, and total currents by 4%. When the thermal densification dominated, the stagnation point moved to the other side of the cylinder, while the boundary layer, which separates in purely diffusional free convection, remained attached. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

It must, however, be kept in mind that one cannot eliminate the fraction of the non-compensated solution resistance Rnc, which generates the ohmic drop iRnc. Unfortunately, the positioning of the reference electrode even closer to the working electrode (<2d) would cause current oscillations. 

The other effect considered in this section deals with transients in a single fuel cell. The transient models examine step changes in potential and related phenomena (e.g., gas flow rates, water production, and current density). Hence, they are aimed at examining how a fuel-cell system handles different load requirements, which may occur during automotive operation or start up and shut down. They are not trying to model slow degradation processes that lead to failure or the transients associated with impedance experiments (i.e., potential or current oscillations). These types of models are discussed in section 7. 

In [53], oscillatory wave patterns observed during electrochemical dissolution of a nickel wire in acidic media was reported. It was shown that space-averaged potential or current oscillations are associated with the creation of an inhomogeneous current distribution, and that the selection of a specific spatial current pattern depends on the current control mode of the electrochemical cell. In the almost potentiostatic (fixed potential) mode of operation, a train of traveling pulses prevails, whereas antiphase oscillations occur in the galvanostatic (constant average current) mode. 

The influence of illumination [255, 282] and magnetic field [256] on the passive layer behavior and anodic current oscillations was also studied. 

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