Diffusion layer rotating disk electrode

Volt mmetiy. Diffusional effects, as embodied in equation 1, can be avoided by simply stirring the solution or rotating the electrode, eg, using the rotating disk electrode (RDE) at high rpm (3,7). The resultant concentration profiles then appear as shown in Figure 5. A time-independent Nernst diffusion layer having a thickness dictated by the laws of hydrodynamics is estabUshed. For the RDE,... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

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... 

Thin catalyst layers on a GC rotating disk electrode (RDE) or a rotating ring-disk electrode (RRDE) serve for studies of ORR kinetics. In order to separate the kinetic current from the measured current j, Schmidt and co-workers [Schmidt et al., 1998b] corrected the latter for the influence of oxygen diffusion in the aqueous electrolyte and in the polymer film using the foUowing equation ... 

The mass transfer boundary layer thickness, d, on a rotating disk electrode can be estimated by d = 1.6/J V a) where D is the substrate diffusion coefficient, v is the solution viscosity, and CO is the disk rotation speed. 

The basic assumption is that the rotating filter creates a laminar flow field that can be completely described mathematically. The thickness of the diffusion boundary layer (5) is calculated as a function of the rotational speed (to), viscosity, density, and diffusion coefficient (D). The thickness is expressed by the Levich equation, originally derived for electrochemical reactions occurring at a rotating disk electrode ... 

Diffusion layer (cont.) polarography. 12-46 thickness, 1335 turbulent flow and, 1234 Diffusion coefficient, and rotating disk electrode. 1141... 

Rotating disk electrode (cont.) diffusion coefficient, 1141 diffusion layer in, 1234 disk current in, 1141 ECE reactions determination by. 1144 electrooxidation of methanol, 1139 kinematic viscosity, 1141,1234 intermediate radicals, determination of. 1139. 

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

The faster a rotating disk electrode spins, the thinner is the diffusion layer in Figure 17-12b and the greater is the diffusion current. A rapidly rotating Pt electrode can measure 20 nM H202 in rainwater.16 H202 is oxidized to 02 at +0.4 V (versus S.C.E.) at the Pt surface and the current is proportional to [H202] in the rainwater. 

For a rotating disk electrode operating at sufficiently great potential, the redox reaction rate is governed by the rate at which analyte diffuses through the diffusion layer to the electrode (Figure 17-12b). The thickness of the diffusion layer is... 

The thickness of the Nernst layer increases with the square root of time until natural - convection sets in, after which it remains constant. In the presence of forced convection (stirring, electrode rotation) (see also Prandtl boundary layer), the Nernst-layer thickness depends on the degree of convection that can be controlled e.g., by controlling the rotation speed of a -> rotating disk electrode. See also - diffusion layer. See also Fick s law. 

Plot, on an impedance plane format, the impedance obtained for a Nernst stagnant diffusion layer and the impedance obtained for a rotating disk electrode under assumption of an infinite Schmidt number. Show that, while the behaviors of the two models at high and low frequencies are in agreement, the two models do not agree at intermediate frequencies. Explain. 

A Nemst stagnant-diffusion-layer model was used to accovmt for the diffusion impedance. This model is often used to account for mass transfer in convective systems, even though it is well known that this model caimot ac-coimt accurately for the convective diffusion associated with a rotating disk electrode. 

The quantitative and qualitative analysis presented in Section 20.2.1 demonstrates that the finite-diffusion-layer model provides an inadequate representation for the impedance response associated with a rotating disk electrode. The presentation in Section 20.2.2 demonstrates that a generic measurement model, while not providing a physical interpretation of the disk system, can provide an adequate representation of the data. Thus, an improved mathematical model can be developed. 

These criticisms and possible action to eliminate or minimize them were discussed when the methodology was applied to study the complexation of Cd, Cu, Ph and Zn (32-34, 53, 104, 108). The results showed that Cu and Pb complexes were kinetically inert, with A ,/ values of between 10 and 10 s, which means that the lifetime of metal complexes, expressed by /ka, is some orders of magnitude higher than the residence time (1-100 ms) of complexes in the diffusion layer when Rotating Disk Electrodes (RDEs) are used. It can therefore be concluded that the reduction process is not appreciably affected by dissociation reaction inside the diffusion layer. Experiments showed instead that Cd complexes present a kinetic lability when Hanging Mercury Drop Electrode (HMDE) or RDE methods are used at low rotation speed (53). The results emphasized that dissociation from the electrode interface determines an underestimation of the conditional stability constant when low rotation speeds are used. To minimize the risk with respect to this problem the RDE method is normally used at the highest rotation speed. 

In many cases the diffusion is not semi-infinite. This case, for example, is observed for polymer electrodes, for a thin mercury layer deposited on surfaces, and for rotating disk electrodes. In such cases, in Eq. (52) parameters B and B are not equal to 0. Two cases may be distinguished for finite-length diffusion, depending on the condition at the boundary located at a distance I from the electrode ... 

Transfer of electroactive species is possible at x = /, and C(/) = 0, but dC l)/dx 0. This is the conducting or transmissive boundary. It is observed, for example, in a rotating disk electrode, where the diffusion layer thickness is determined by the rotation rate. 

The most appropriate experimental arrangement for the quantitative determination of the stationary curves for the reaction is the rotating disk electrode technique, in which the convective transport is controlled mechanically and thus a constant diffusion layer for each species is achieved [101-106]. Modifications of this technique, to primarily collect the intermediates of the reactions, such as ring-disk electrode techniques [93,94] and a hanging meniscus rotating disk electrode... 

The third example is the reflection measurement at a rotating disk electrode (RDE). Scherson and his coworkers have developed near-normal incidence UV-visible reflection-absorption spectroscopy at RDEs [50-52]. Both (AR/R)dc and (AR/R)er have been measured under hydrodynamic conditions. The use of an RDE enables them to quantitatively control the diffusion layer concentration profile of the solution phase species, especially the species generated electro-... 

In hydrodynamic systems Planar diffusion to a uniformly accessible electrode, e.g. for rotating disk electrodes (hypothetical Nernst model with S = diffusion layer thickness)... 

The concentration of metal atoms in mercury electrodes depends on the potential (/face) and the duration (/acc) of accumulation, the bulk concentration of ions (CMe(b)) and the hydrodynamic conditions in the solution [26]. The simplest model considers the reversible electrode reaction on a thin mercury film rotating disk electrode with a fully developed diffusion layer in the solution and uniform distribution of metal atoms in the mercury film ... 

In Eqn (2.82), Cm is normally treated as the solubility of the oxidant in the ionomer membrane. The diffusion layer thickness 5o can be obtained using a rotating disk electrode technique, which will be given in a very detailed discussion in Chapter 5. The equivalent thickness of the ionomer membrane can be calculated according to the amount of ionomer applied in the electrode layer using the following equation ... 

In practical applications, very often diffusion is not semi-infinite. Such finite-length linear diffusion is observed, for example, for internal diffusion into mercury film deposited on a planar electrode, in deposited conducting polymers, for hydrogen diffusion into thin films or membranes of Pd or other hydrogen absorbing materials, or for a rotating disk electrode where the diffusion layer corresponds to the layer thickness. There are two cases of finite-length diffusion displayed Fig. 4.11 ... 

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