Current flow, comparison between electrode

The overall rate of an electrochemical reaction is measured by the current flow through the cell. In order to make valid comparisons between different electrode systems, this current is expressed as cunent density,/, the current per unit area of electrode surface. Tire current density that can be achieved in an electrochemical cell is dependent on many factors. The rate constant of the initial electron transfer step depends on the working electrode potential, Tlie concentration of the substrate maintained at the electrode surface depends on the diffusion coefficient, which is temperature dependent, and the thickness of the diffusion layer, which depends on the stirring rate. Under experimental conditions, current density is dependent on substrate concentration, stirring rate, temperature and electrode potential. 

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... 

In LEIS, the full electrochemical impedance spectrum of the sample/electrolyte interface can be obtained at the submillimeter level. The system works by stepping a probe tip across the sample surface (the smallest step size is 0.5 pm) while the sample (connected as the working electrode) is perturbed by an ac voltage waveform (usually about the open-circuit potential with an amplitude typically of 20 mV). The probe tip consists of two separated platinum electrodes, separated by a known distance. Measurement of the potential difference between the two electrodes allows the calculation of the potential gradients above the sample surface, which then give the current density. Comparison of the in-phase and out-of-phase current flow produces the impedance data, as with the regular EIS. The data can be plotted as Bode or Nyquist charts for specific points on the surface, or impedance maps of the sample surface can be obtained. 

Figure 1. Comparison of the interface between an electronically conductive electrode and a solution reduction of Fe3+) (A) and the interface between two immiscible solutions of electrolytes (ITIES) during current flow in a closed electric circuit [transport of picrate (Pi ) from nonaqueous phase (n) to water (w)] (B). (Reproduced from reference 4. Copyright 1990 American Chemical...

FIGURE 13.14 (a) Comparison between estimated proton conductivities on Pt whiskers and measured conductivities of the ionomer in a bulk membrane and in a conventional electrode with ionomer-to-carbon content of 0.97, at 80°C as a function of RH. For comparison, the conductivity of distilled water at 25°C is also shown, (b) ORR current density distributions across the (0.23- om-thick) NSTF cathode at i= with high stoichiometric flows of ECS. 

The two fundamental flow cell designs that were discussed differ essentially in the position of the working electrode relative to the direction of the eluent flow. The thin-layer cell exhibit laminar flow parallel to the electrode surface, while the wall-jet design exhibits a flow directiOTi perpendicular to the electrode surface. Comparative studies between the different cell designs were conducted. In spite of the fact that equations that describe the current as a function of the cell-type and geometries have been derived, comparison between the theoretical and experimental results was not satisfactory. The difference between the calculated and experimental was... 

Satisfactory agreement with experimental data was obtained for Cu-SiC composite deposition in a channel flow. Because of the limited range of experimental data it is not clear if the model is also able to describe important features, like the peak in the particle composite content versus current density curve. In comparison to Valdes model, the particle mass transfer is poorly taken into account by using the Reynolds number. The particle-electrode interaction on the other hand is treated much more adequately by the balance between particle adsorption (Co, Sx and Dm) and particle ejection due to hydrodynamics (Gq). For example, a small value for d is obtained, indicating that, in accordance with experimental data (Section III), electro-osmotic interactions between particles and the cathode (Dm) are negligible. 

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