Current distributions, electrocatalyst

On the other hand, the selectivity of the electrochemical deposition of the metal on the substrate must be 100% of the current efficiency, with no interference from the other metal deposition processes. Therefore, the potential distribution needs to be presented for any serious electrochemical reactor study and the electrocatalyst selection problem. The major problem of current distribution depends on the type of the process that controls the entire reaction rate, such as charge transfer, ohmic contributions, or mass transport to or from the electrode. Many parameters have to be evaluated in the course of an electrochemical process to obtain the desired uniform potential and current distributions. One of the conditions that has to be fulfilled is the continuity equation for the current density vector, j ... 

FIGURE 13.4 Current distribution for the primary approach in an electrochemical reactor of two parallel plate electrode geometries. Black and gray rectangles are the electrocatalyst and insulator surfaces, respectively. 

Again a primary current distribution can be obtained for small values of electrolyte conductivities, x- or better with large values of exchange current densities, j0. The latter is of interest in electrocatalysis, because by simply changing the nature or the composition of the electrocatalyst we can achieve small values of Wa, and then a uniform primary current distribution. For the case of the electrocatalytic agents used in the industry, the expression of the Equation 13.33 is not useful at all. By introducing large polarizations, we have... 

FIGURE 13.12 Quasi-cylindrical pore in an electrocatalyst for the analysis of current distribution in a two-phase system. An infinitesimal cylinder element of variable height dy and constant radius r is used for the calculations. 

For the surface growth and the subsequent variation in the boundary layer, the physical change of the electrocatalyst can be easily evaluated with the help of the current distributions. In general, the primary distribution can be used as Ohm s law ... 

Mass transport processes at the surface and in the pores of real and high surface area electrocatalysts can alter considerably the intrinsic activity and specificity as obtained from microscopic principles. This can lead to poor surface utilization, nonuniform distribution of reactants and currents, and decreased product yield and energy efficiency. Possible surface change and... 

Current and Potential Distributions in Rough and Porous Electrocatalysts.306... 

This is the simplest model of an electrocatalyst system where the single energy dissipation is caused by the ohmic drop of the electrolyte, with no influence of the charge transfer in the electrochemical reaction. Thus, fast electrochemical reactions occur at current densities that are far from the limiting current density. The partial differential equation governing the potential distribution in the solution can be derived from the Laplace Equation 13.5. This equation also governs the conduction of heat in solids, steady-state diffusion, and electrostatic fields. The electric potential immediately adjacent to the electrocatalyst is modeled as a constant potential surface, and the current density is proportional to its gradient ... 

When the primary distribution does not illustrate the current or electric potential distribution well, an additional resistance, that is, the charge transfer electrode resistance, has to be considered. In such cases, we need to account for the electrode kinetics, and the secondary current and potential distributions emerge from the models. For industrial purposes the porous or tortuous electrocatalyst has to be considered as a dynamic system. This means that its porosity shape and density besides the surface roughness and the real geometric area changes all the time. This point makes us think that it... 

Wa gives the ratio between the charge transfer resistance and the electrolyte ohmic resistance. This dimensionless value can be taken as a criteria or measure of the current uniformity at the electrocatalyst. When Wa > 0, the primary current contribution prevails. It is clear that the geometric characteristic distance L can be enlarged enough to simplify the analysis to a primary distribution. However, we should not forget that the variation of T with j contains several parameters. In the simplest case of a small polarization change, we have for an elementary electron transfer... 

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