Primary current distributions parallel plate electrodes

Fig. 6C Primary current distribution and potential profiles for a parallel-plate configuration, with the Luggin capillary placed close to the working electrode. K = 50 mS c/ r. Top equipotential lines. Bottom current lines. Reprinted with permission from Landau, Weinberg and Gileadi, J. Electrochem. Soc. 135, 396. Copyright 1988, the Electrochemical Society.

Primary Current Distribution in Parallel Plate Electrodes.299... 

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. 

As was shown in Fig. 2.2, the primary current distribution is only uniform when all points on the electrode surface are strictly equivalent and the current density is low. This is possible only with two reactor designs, a parallel-plate reactor having... 

As an example, Fig. 2.9 is a sketch of the various current distributions for a parallel-plate cell with electrodes of length L and of infinite width, and with fully developed laminar flow. The primary current distribution shows the current to be uniform over most of the electrode but with a considerable edge effect at x = 0 and jc = L the current goes to a very high value at these ends. The secondary distribution is similar but is even closer to the ideal while the limiting tertiary distribution shows that in these conditions the current density drops sharply along the electrode. 

Optimization of the electrochemical cell s geometry is the primary factor that determines the uniformity of current distribution. The two major geometrical arrangements are parallel-plate electrodes and concentric cylinders. PaiaUel-plate geometry is common in large-scale production of base metals when the metal concentration in the electrolyte is high. Cylindrical cells are used in the treatment of less concentrated solutions, in the recovery of noble metals, and also in the production of base metals. The current distribution between two parallel electrodes is only uniform when a nonconducting containment of the same cross section surrounds the interelectrode space. 

FIGURE 5.23. Primary and limiting current distribution at parallel plate electrodes in a flow channel with various d/L ratios. 

According to Eq. (5.6), if L/d = 10 (a comparatively small electrode), i/i v — 0.958 at X = L/2. This is not far from the uniform current density (i/i = 1) for very large electrodes. Thus variations in primary current distribution have little significance in practical parallel plate electrode systems. 

Although in theory only parallel plate flow electrolyzers with electrodes whose lengths tend toward infinity, or that fill the channel completely have the ideal geometry that leads to uniform primary current distribution, in practice these effects are not important. It is different, however, for laboratory cells with very small electrodes. 

Consider parallel plate electrodes with the current uniformly distributed along one edge, i.e., unidirectional current flow. For reactors with narrow electrode gaps, primary and secondary current density distributions for linear polarizations are given by ... 

As was shown in Fig. 2.5, the primary current distribution is only uniform when all points on the electrode surface are strictly equivalent and the current density is low. This is possible only with two reactor designs, a parallel-plate reactor having electrodes of equal area and occupying opposite walls and the concentric-cylinder reactor. There will be a variation of potential and current density over the surface for all other electrode arrangements an example is shown in Fig. 2.16(a) where the broken lines join equipotential points and the current densities are inversely proportional to the lengths of the arrowed lines. The highest current density is between the points closest together on the two electrodes and almost no current flows on the reverse side of the anode. 

---