Rotating disk electrode current distribution

The constancy of the diffusion layer over the entire surface and thus the uniform current-density distribution are important features of rotating-disk electrodes. Electrodes of this kind are called electrodes with uniformly accessible surface. It is seen from the quantitative solution of the hydrodynamic problem (Levich, 1944) that for RDE to a first approximation... 

Electrode processes are often studied under steady-state conditions, for example at a rotating disk electrode or at a ultramicroelectrode. Polarog-raphy with dropping electrode where average currents during the droptime are often measured shows similar features as steady-state methods. The distribution of the concentrations of the oxidized and reduced forms at the surface of the electrode under steady-state conditions is shown in Fig. 5.12. For the current density we have (cf. Eq. (2.7.13))... 

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

Several cell configurations are common in electrochemical research and in industrial practice. The rotating disk electrode is frequently used in electrode kinetics and in mass-transport studies. A cell with plane parallel electrodes imbedded in insulating walls is a configuration used in research as well as in chemical synthesis. These are two examples of cells for which the current and potential distributions have been calculated over a wide range of operating parameters. Many of the principles governing current distribution are illustrated by these model systems. 

B. Current and Potential Distributions for the Rotating Disk Electrode... 

The rotating disk electrode will have a uniform tertiary current distribution but an extremely nonuniform primary current distribution with the current density at the electrode edge approaching infinity (8-12). For a disk electrode of radius r0, embedded in an infinite insulating plane with the counterelectrode far away, the primary current distribution is given by... 

Figure 7 (a) Rotating disk electrode, flush mounted into an insulating plane, (b) Current distributions for primary, secondary, and tertiary cases as a function of radial position r/ r . (c) Variation of as a function of rlr0 for various values of the reciprocal Wagner... 

The rotating disk electrode, described in Section 11.6, has the advantage that the fluid flow is well defined emd that the system is compact and simple to use. The rotation of the disk imposes a centrifugal flow that in turn causes a radially uniform flow toward the disk. If the reaction on the disk is mass-transfer controlled, the associated current density is imiform, which greatly simplifies the mathematical description. As discussed in sections 5.6.1 and 8.1.3, the current distribution below the mass-transfer-limited current is not uniform. The distribution of current and potential associated with the disk geometry has been demonstrated to cause a frequency dispersion in impedance results. The rotating disk is therefore ideally suited for experiments in which the disk rotation speed is modulated while im-der the mass-transfer limited condition. Such experiments yield another t)q)e of impedance known as the electrohydrodynamic impedance, discussed in Chapter 15. 

M. Durbha, Influence of Current Distributions on the Interpretation of the Impedance Spectra Collected for Rotating Disk Electrode, Ph.D. dissertation. University of Florida, Gainesville, Florida (1998). 

M. Durbha and M. E. Orazem, "Current Distribution on a Rotating Disk Electrode Below the Mass-Transfer Limited Current Correction for Finite Schmidt Number and Determination of Surface Charge Distribution," Journal of The Electrochemical Society, 145 (1998) 1940-1949. 

FIGURE 26.21 Primary, secondary, and mass transfer limited current distributions on a rotating disk electrode of radius r . 

An Experimental Study of Non-uniform Current Distribution at Rotating Disk Electrodes". 

The question of the frequency dependence of the current distribution and its effect on the measured impedance of a solid state electrochanical system has been hardly considered, although it is important in discussing the impedance of, for example, porous gas electrodes on anion conductors, of rough electrodes (discussed below), and also perhaps of polycrystalline materials. In aqueous electrochemical situations the effects has been considered with respect to the rotating disk electrode, where there may be severe current distribution problems. 

Figure 12.52 Lines of current (in dashes) and equipotential lines (solid) in the electrolyte for a rotating-disk electrode under primary current distribution conditions [35].

Bruckenstein S, Miller B (1970) An experimental study of non-uniform current distribution at rotating disk electrodes. J Electrochem Soc 117 1044-1048... 

With its axisymmetric transport and current distribution, the rotating hemispherical electrode complements the rotating disk as a tool for studying electrode processes. Der-Tau Chin provides a valuable overview and summary of the fundamental theory and applications of this interesting device. 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

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