Rotating-disc electrode current density

Beside laminar flow created by e.g. a rotating disc electrode mrbulent flow provides a means of artificially enhanced transport. A consistent mathematical description and analytical treatment of this mode of transportation is not possible. Various approximations have been proposed and tested for correctness [84Barl], an experimental setup has been described [78Ber, 83Her, 831wa]. From comparisons of measured and calculated current density vs. electrode potential relationships exchange current densities are available. (Data obtained with this method are labelled TPF.)... 

Nature of the relation between current density and hydrogen peroxide concentration observed for different rotating-disc electrode materials with a rotation speed of 16.67 rotations per second. 

Miller and Bruckenstein [27,28] introduced the hydrodynamically modulated rotating disc electrode (HMRDE) in 1974. The steady-state current density at a rotating disc electrode is well-defined, given by the Levich equation (equation (10.15)) ... 

The anodic limiting current in lithium salt solutions is determined by the diffusion of the solvated electrons to the electrode. This was quantitatively established by the measurements taken on rotating disc electrodes and also by galvanostatic measurements In fact, as seen from Fig. 8, the limiting current density is proportional to the square root of the disc electrode rotation rate. This, in accordance with the rotat-... 

Inserting Eq. (5.37) into Eq. (5.5) gives the equation for the current density on a rotating-disc electrode... 

An example is shown in Figure 6.16. The reciprocal current is plotted versus the reciprocal value of the rotation frequency of a rotating disc electrode. The currents taken from the extrapolation to 1/Vf = 0 (rotation frequency/ = 00) are represented versus the potential in Figure 6.17. The current-potential plot shows a current-potential curve in the sub-Tafel region. An approximate current-potential line is shown in Figure 6.17. An approximate value of the charge transfer resistance and of the exchange current density... 

The number of particles adsorbed on the metal surface depended on hydrodynamic shear forces. These forces were simulated by a rotating disc electrode and then compared with experiments studying polystyrene particle co-deposition with copper. One result was a calculation of the adhesion force of the particles on the copper surface as a function of the current density showing a maximum between 1 and 2 A dm (Figurel2.10). 

It has already been noted that the flux of material to the rotating disc electrode is uniform over the whole surface, and it is therefore possible to discuss the mass transport processes in a single direction, that perpendicular to the surface (i.e. the z direction). Furthermore, it has been noted that the velocity of movement of the solution towards the surface, is zero at the surface and, close to the surface, proportional to Hence, even in the real situation it is apparent that the importance of convection drops rapidly as the surface is approached. In the Nernst diffusion layer model this trend is exaggerated, and one assumes a boundary layer, thickness 6, wherein the solution is totally stagnant and transport is only by diffusion. On the other hand, outside this layer convection is strong enough for the concentration of all species to be held at their bulk value. This effective concentration profile must, however, lead to the same diffusional flux to the surface (and hence current density) as it found in the real system. 

The wall-jet disc electrode is clearly not uniformly accessible (current density oc r s/4 ). Another important point is that iL depends on the three-quarter power of the flow rate it is more sensitive in this sense than rotating or tube/channel electrodes. 

The rotating ring-disc electrode is perhaps the most useful extension of the idea of the rotating disc. We have already seen (cf. Eq. 32D) that the ring current has the same form as the disc current, namely that it is also proportional to the bulk concentration of the electroactive species C° and to the square root of the rotation rate. A numerical comparison of Eqs. 32D and 25D shows that the current density at the ring is higher than at the disc, indicating that some increase in analytical sensitivity may be attained. This, however, is not the... 

Fig. 9. Dependence of the anodic current density on the potential of a rotating platinum disc electrode (960 rpm) in hexamethylphosphotriamide solution of solvated electrons against a background of 0.3 M NaClO at electrons concentration corresponding to an equilibrium potential of —3.00 V (vs. aqueous SCE). Temperature 5.5 °C (166)...

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