Disk electrodes current densities

For nonlimiting diffusion conditions, the disk electrode current density would be ... 

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... 

We have used voltammetric measurements in the absence of the electroactive species to quantitatively evaluate this heat-sealing procedure. The magnitude of the double layer charging current can be obtained from these voltammograms [25,68-70], which allows for a determination of the fractional electrode area (Table 1). This experimental fractional electrode area can then be compared to the fractional pore area calculated from the known pore diameter and density of the membrane (Table 1). In order to use this method, the double layer capacitance of the metal must be known. The double layer capacitance of Au was determined from measurements of charging currents at Au macro-disk electrodes of known area (Fig. 6, curve A). A value of 21 pF cm was obtained. 

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.

As noted earlier, nonlinear diffusion to disks is predicted to produce nonuniform current densities across the electrode surface. Similarly, the current den-... 

FIGURE 10 Current distribution on a disk electrode. The primary current distribution approaches infinity at the junction of the electrode and the coplanar insulator. The secondary current distribution is more uniform. Average current density is /aVg and the electrode radius r0. 

The primary potential distribution is, by definition, uniform adjacent to the electrode surface, but the current distribution is highly nonuniform (Fig. 10). It is a general characteristic of the primary current distribution that the current density is infinite at the intersection of an electrode and a coplanar insulator. This condition obtains at the periphery of the disk electrode, and the current density becomes infinite at that point. Additional resistance due to kinetic limitations invariably reduces the nonuniformity of the current distribution. In this system the current distribution becomes more uniform as the Wagner number increases. Theoretically, the current distribution is totally uniform as the Wagner number approaches infinity. 

Channel flow between plane parallel electrodes is shown in Fig. 11. This geometry is similar to that of the disk in that an electrode and an insulator intersect in the same plane. Because of many geometric similarities, the general characteristics of the primary and secondary current distributions are similar. At the edges the local current density is infinite for the primary current distribution (Fig. 12). Increasing the kinetic limitations tends to even out the current distribution. The significant contrasts appear in a comparison of the tertiary current distributions. In channel flow, the fluid flows across the electrode rather than normal to it. Consequently, the electrode is no... 

Let us consider the potential distribution for one such mixed boundary value problem in more detail. If a disk electrode of radius p0 embedded in an infinite insulating plane and with the counter electrode far away has a uniform double layer potential, DL, then the current distribution at the electrode normalized to the average current density, /avg, is given by [40]... 

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