Diffusion-limited current plateau

In electroanalysis, electrodes of millimeter dimensions are termed millielec-trodes, while the more recently developed very small area electrodes of micron dimensions are termed microelectrodes there are differences in properties beyond simply the change of dimension. Thus in millielectrode-scale experiments the enhancement of the diffusion-limited current plateau has been observed by a number of other workers—for example, in the reduction of methylviologen in aqueous acetonitrile [32], in the oxidation of bis(cyclopentadienyl) molybdenum dichloride in acetonitrile [33], as well as in several other studies on the aqueous ferrocyanide/ferricyanide couple using wire or disc millielectrodes to study diffu-sional phenomena [34—36], Typical values of the diffusion layer thickness of approximately 5 pm are found under ultrasound [35] in contrast to the normal value of approximately 500 pm in silent conditions. 

With a well-defined polarographic wave where the limiting current plateau is parallel to the residual current curve, the measurement of the diffusion current is relatively simple. In the exact procedure, illustrated in Fig. 16.6(a), the actual... 

On both Pt and polyaniline-coated electrodes, diffusion-limited currents are observed at <250 mV for Fe(CN)500 mV for Fe(CN)g- oxidation. Potentiostatic EHD impedance was measured on both diffusion plateaux (50 mV and 550 mV), using Pt electrodes coated with polyaniline films of various thickness 50 and 130nm. As an example, the results obtained on the cathodic plateau are shown in Fig. 6-13 those observed on the anodic plateau were very similar [93]. 

Compared to conventional (macroscopic) electrodes discussed hitherto, microelectrodes are known to possess several unique properties, including reduced IR drop, high mass transport rates and the ability to achieve steady-state conditions. Diamond microelectrodes were first described recently diamond was deposited on a tip of electrochemically etched tungsten wire. The wire is further sealed into glass capillary. The microelectrode has a radius of few pm [150]. Because of a nearly spherical diffusion mode, voltammograms for the microelectrodes in Ru(NHy)63 and Fe(CN)64- solutions are S-shaped, with a limiting current plateau (Fig. 33a), unlike those for macroscopic plane-plate electrodes that exhibit linear diffusion (see e.g. Fig. 18). The electrode function is linear over the micro- and submicromolar concentration ranges (Fig. 33b) [151]. 

Further increase in anode potential gets into limiting current plateau range (El in Fig. 10.6). In this region, the potential is so high that the electrochemical reaction is faster than mass transport that is, Cu ions produced on the anode surface in a unit time are more than those that mass transport can remove from the anode surface into the bulk solution. As a result, a Cu ion concentrated layer is developed inside the Prandtl boundary layer [4]. The concentrated Cu layer is called Nernst layer or diffusion layer, which has a thickness of [13]... 

The current of interest is that at the limiting current plateau, that is, for Cq(0, t) = 0. For a planar electrode, assuming equal diffusion coefficients for all species (Dq = Dy = D)... 

As in the case of sulfate-supporting electrolyte (Fig. 8.1b), diffusion limiting current density (in this case not represented with the plateau) for Co is higher than that for Ni (Fig. 8.6b). At the same time, all polarization curves for Co-Ni alloy powder electrodeposition are characterized with higher values of current densities. 

It was found that electropolishing occurs in systems which, under anodic dissolution, exhibit a limiting current characteristic of diffusion control, i.e., currents are dependent on flow rate of the electrolyte past the anode, as shown in Figure 24. Over a certain range of potentials at the limiting current plateau, at which maximum brilliance is obtained, oscillations of potential occur at a virtually constant current density, with an amplitude of over 0.40 V, and without appreciable damping. Also, a significant photoelectrochemical effect is found. 

The expression for the diffusion-limited current on the plateau of the wave is analogous to that holding for the linear diffusion-limited current... 

Plain steel corrodes in active-passive state in the ionic liquid. Near the corrosion potential the corrosion current increases abruptly by two orders of magnitude. At about 100 mV above the corrosion potential, the anodic current is observed, which only slightly changes with the potential. This indicates a passive layer. At about 400 mV above the corrosion potential, the passive layer is destroyed, and the anodic current abruptly increases again to reach another plateau. The shape of the corrosion curves suggests that a diffusion-limited current is measured, and the fluctuations may be due to formation of surface layer with some protective properties. The protective layer is stable over a 1... 

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