Rotating disk electrode current densities

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... 

Figure 6.10 Galvanostatic scans of a Pt(l 10) rotating disk electrode in a CO-satuiated 0.1 M HCIO4 solution at two different current scan rates (disk rotation rate 400rev/min). The insert shows the potential fluctuations observed at an apphed current density of 0.74 mA/cm (disk rotation rate 900 rev/min).

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))... 

Figure 3a is an illustration of the effect of surface overpotential on the limiting-current plateau, in the case of copper deposition from an acidified solution at a rotating-disk electrode. The solid curves are calculated limiting currents for various values of the exchange current density, expressed as ratios to the limiting-current density. Here the surface overpotential is related to the current density by the Erdey Gruz-Volmer-Butler equation (V4) ... 

Fig. 9. Logarithmic plot of apparent limiting-current density as a function of current increase rate at a rotating-disk electrode i — apparent limiting current density i, = true steady-state limiting current density di/dt = current increase rate (A cm-2 sec-1) (u = rotation rate (rad sec-1). [From Selman and Tobias (S10).]...

It was not until 1987, before a second model on electrocodeposition was published by Buelens [37, 58], From experimental observations on the codeposition of particles on a rotating disk electrode (RDE) as a function of current density, rotation speed and bath composition, that could not be explained by Guglielmi, she suggested that a particle will only be incorporated into the deposit if a certain amount of the adsorbed ions on the particle surface is reduced. This is one possible way to account for the field-assisted adsorption, held responsible for the transition between loosely and strongly adsorbed particles in the model of Guglielmi. This proposition yields the probability P(k/K,i) for the incorporation of a particle based on the reduction of k out of K ions, bound to its surface, at current density i... 

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

Using the rotating disk electrode, Seliv-anov et al. [214] have investigated the zinc electrodeposition from zincate electrolyte containing polyethylene polyamine. The limiting current density of [Zn(OH)4] ion diffusion through a film of zinc oxides and hydroxides is shown to be responsible for the formation of dark zinc deposits in the potential range from —1.33 to —1.47 V. 

Influence of Rotating Disk Electrode Condition (Stationary or Rotating) on the Diffusion-Layer Thickness and the Limiting Current Density for the Reaction... 

Electrochemical oxidation of hydroquinone was investigated on a rotating disk electrode in a solution containing 0.01 M quinone and hydroquinone in 0.5 M H2so4 at 298 K. The following values of current density at different electrode potential values and RDE rotation rates were obtained ... 

Fig. 8.3. ATafel line. No special effort is made to reduce the limiting current density in case 1 and the Tafel line has a relatively small range, e.g., only 102 times in current density. In case 2, a rotating disk electrode is used to cause an increased limiting current density, and hence a larger range (say, 104 times) of current density.

Figure 7 (a) Cathodic polarization data for a low carbon steel rotating disk electrode in 0.6 M NaCl with ambient aeration. Oxygen reduction limiting current densities are shown for the indicated rotation rates, (b) Plot of experimental limiting current density versus square root of the rotation rate, showing the experimental and predicted linear behavior. 

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... 

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

The current densities in KIO3 are lower than in K3pe(CN)6 solutions. The pH of minimum potential is also shifted to somewhat higher pH in KIO3 solution. The difference in their behavior may be attributed to ionic radii of IO3 and Fe(CN)6 ions. IO3 ions have much larger size and are thus may provide a hindrance to the anodic dissolution reactions. The value of 5ln(Iss)/8pH is about 1.45. This is lower than the value of 2.303 obtained by Macdonald et al. This study was carried out in static environment, unlike the rotating disk electrode set-up used by MacDonald et al. The differences in the value of this slope may probably be attributed to the differences in mass transfer properties in solution. 

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. 

Figure 3 Plots of the total number of electrons Ut (a), of the Tafel slope b (b), of the limiting current density IjA,-, (c) and of the exchange current density I/A (d), versus the platinum loading for the reduction of oxygen on a platinum-modified polyaniline-glassy carbon rotating disk electrode (O2 saturated 0.5 M H2SO4 2mVs 25 °C A, is the true surface...

FIGURE 26.26 Plating current efficiency for NiFe film deposition on a rotating disk electrode vs. the total plating current density, for different disk rotation speeds. Plating bath composition was the same as that in Figure 26.25. Data from [106]. (Reproduced by permission of ECS—The Electrochemical Society.)... 

---