Partially blocked electrodes

Porous electrodes, partially blocked electrodes, microelectrode arrays, electrodes made of composite materials, some modified electrodes and electrodes with adsorbed species are spatially heterogeneous in the electrochemical sense. The simulation of non-Cottrellian electrode responses at such surfaces is challenging both because of the surface variation and... 

Partially Blocked Active Electrode and Active Islands on Inactive Electrode... 

Composite electrodes used in the electrochemical processes are often partially active since they are composed of the active powder material and the inactive binder and conductor. The partially blocked active electrode can be characterized by the contiguous fractal with dy < 2.0. In the case of the electrodes composed of the active islands on an inactive support, they are characterized by the non-contiguous fractal with dy < 2.0.121... 

Inhibition of Electron Transfer at Partially Blocked Electrodes... 

Uniform accessibility and reactivity of the electrode interface are the main hypotheses for developing the EHD impedance theory. However, in many cases a real interface deviates from this ideal picture due for example either to incomplete monolayer adsorption leading to the concept of partial blocking (2D adsorption) or to the formation of layers of finite thickness (3 D phenomena). 

Before presenting some applications of practical interest, the theoretical EHD impedance for partially blocked electrodes and for electrodes coated by a porous layer will be analyzed. 

In fact, the crucial point in the set-up, common to all variants, is the electromechanical part which must deliver at the output (i.e. at the RDE) a noise free modulation of the angular velocity indeed, regarding the theoretical requirements for some applications (e.g. partial blocked electrodes), well performing motors and servosystems are necessary. 

If the electrode were partially blocked, or behaved as an array of UMEs, then the characteristic length for the scaling would be different and would depend upon whether the concentration boundary layers for the different active areas interacted. If the electrode could be represented as consisting of two or more independent active areas, then two or more characteristic relaxations would be observed in the response, and the length scale could be deduced from the dependence of the measured H values on Re. Addi-... 

Caprani et al. [104], defining the cut-off frequency as the intersection of the low and high-frequency asymptotes, as indicated on Fig. 10.18, have given an approximate method to deduce the size of active sites on a partially blocked electrode from the ratio of the two cut-off frequencies ... 

The effect of a surface film which partially blocks an electrode has been considered above (Section 10.5.2.1). This section considers the effect of a film which is permeable to reagents but which covers entirely the electrode... 

Interestingly, the problem just described was first solved for an entirely different physical situation, referred to as a partially blocked electrode. In this case one assumes that the surface of a... 

Parallel-plate model (for adsorption isotherm), 332 Parallel-plate model (of the double layer), 188 Partially blocked electrode, 450 Partial surface coverage, 131 Passivation, 506... 

The scanning electrochemical microscopy (SECM) technique introduced in recent years by Allen Bard is another area where the smallness of the electrode is essential [38]. The principle in SECM is a mobile UME inserted in an electrolyte solution. The UME is normally operated in a potentiostatic manner in an unstirred solution so that the current recorded is controlled solely by the spherical diffusion of the probed substance to the UME. The current can be quantified from Eqs. 48, 49, or 89 as long as the electrode is positioned far from other interfaces. However, if a solid body is present in the electrolyte solution, the diffusion of the substance to the UME is altered. For instance, when the position of the UME is lowered in the z direction, that is, towards the surface of the object, the diffusion will be partially blocked and the current decreases. By monitoring of the current while the electrode is moved in the x-y plane, the topology of the object can be graphed. The spatial resolution is about 0.25 pm. In one investigation carried out by Bard et al, the... 

The main hypotheses for developing the EHD impedance theory are that the electrode interface is uniformly accessible and the electrode surface has uniform reactivity. However, in many cases, real interfaces deviate from this ideal picture due, for example, either to incomplete monolayer adsorption leading to the concept of partial blocking (2-D adsorption) or to the formation of layers of finite thickness (3-D phenomena). These effects do not involve the interfacial kinetics on bare portions of the metal, which, for simplification, will be assumed to be inherently fast. The changes will affect only the local mass transport toward the reaction sites. Before presenting an application of practical interest, the theoretical EHD impedance for partially blocked electrodes and for electrodes coated by a porous layer will be analyzed. 

Remember 15.4 EHD provides a means to reveal and quantify the influence of partially blocked electrodes and electrodes coated by porous layers. 

Figure 15.5 Concentration field over a partially blocked electrode. The active parts of the electrode are displayed in gray and the concentration is dimensionless d = c/c(oo). (Taken from Caprani et al. )...

For a partially blocked electrode, the diagrams show two characteristic frequencies and, whatever the mean rotation speed Ci, fall onto a single curve when plotted versus a dimensionless frequency p = cofCi, where co/ln is the frequency modulation. The high-frequency domain is characteristic of the response of the sum of the active sites, as if they were not coupled by their diffusion layers. 

This result is phenomenologically based on the theoretical analysis given by Deslouis et al. for the frequency response of a small electrode to a flow modulation. The response of a partially blocked electrode surface was obtained from numerical calculations. The main result of this approach is that, at vairiance with the case of a uniform active disk electrode, such a surface shared between active and passive sites displays two characteristic frequencies, one in the low-... 

Remember 15.5 For a partially blocked electrode, all diagrams obtained at different rotation speeds merge on one diagram by using the normalized amplitude and the dimensionless frequency p. This is not observed for a coated electrode. 

The time constant with calcareous deposits was found to be the same as for a bare surface in the low-frequency range with a slight shift cf diagrams to lower reduced frequencies when the deposition time increases. At high frequency (above p — 1), a second time constant appeared. Both these features characterized the presence cf a partially blocked surface with a moderate value of the active fraction cf the electrode. The slight separation of the EHD diagrams observed in the low-frequency region, when the deposition time is... 

Electrohydrodynamic impedance characterization of calcareous deposits showed mainly partially blocked electrode behavior and allowed the estimation of the average size of characteristic sites of the interface. These results have been confirmed by ex-situ SEM images. 

A. Caprani, C. Deslouis, S. Robin, and B. Tribollet, "Transient Mass Transfer at Partially Blocked Electrodes A Way to Characterize Topography," Journal of Electroanalytical Chemistry, 238 (1987) 67-91. 

The kinetics and mechanisms of 3D Me deposition have been intensively studied by electrochemical impedance spectroscopy (EIS) as well as classical electrochemical techniques during the last two decades. Different simplified models of 2D nucleation and growth were theoretically treated in terms of their impedance behavior by Armstrong and Metcalfe [6.72] and Eppelboin et al. [6.73]. Later, a more realistic model of a partially blocked electrode surface was developed and analyzed by Schmidt, Lorenz et al. [6.75-6.78]. 

Silver electrodeposition was studied by different authors [6.75-6.78, 6.87-6.89]. An exact interpretation of EIS data was found to be only possible on the basis of partially blocked electrode surfaces. The high overall exchange current density of the Ag/Ag electrode requires a special high-frequency (HF) EIS technique [6.77, 6.78]. 

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