Primary and Secondary Current Distribution

In electrochemistry, spherical and hemispherical electrodes have been commonly used in the laboratory investigations. The spherical geometry has the advantage that in the absence of mass transfer effect, its primary and secondary current distributions are uniform. However, the limiting current distribution on a rotating sphere is not uniform. The limiting current density is highest at the pole, and decreases with... 

There are other cases in practical electrochemical devices in which current distribution is important. Because of the interplay of interfacial and electrolyte resistance effects (primary and secondary current distribution, respectively , the detailed calculation involve much mathematics. Electroplating deep into crevices of the object to be plated is an example of where current distribution considerations often dominate behavior. Throwing power is a term that describes the degree of penetration of the current— hence the plating—into fissures and irregularities in electrodeposition. 

The complexity of a model increases as we proceed from the primary to the tertiary distribution and as the number of spatial dimensions that are considered increases. Essentially all published solutions have been reduced to one or two dimensions, and most include only simulations of the primary and secondary current distributions. For the special case in which only mass transport is limiting, a large number of correlations for the current distribution are available. 

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... 

FIGURE 12 Current distribution on plane parallel electrodes. Primary and secondary current distributions are symmetric about a centerline plane. When the reactant concentration is considered, an unsymmetric current distribution results. 

Primary and secondary current distributions in electrochemical cells are governed by the Laplace equation.[8] Consider a rectangular geometry governed by the following equation [9]... 

In the absence of concentration gradients, that is, for the primary and secondary current distributions, we can use the Laplace equation when there is no charge accumulation (bulk solution) ... 

Fig. 1.17 The primary and secondary current distribution along the cathode of an arbitrary electrochemical system is shown. Due to overvoltages the current density is more uniform. Zero and infinite current densities no longer exist.

With the cell geometry of fig. 4 14 the process of anodic levelling and electrochemical machining is simulated for a primary and secondary current distribution. The choice of this geometry was based on the important current density variations involved. 

When the irregular shaped electrode is the anode, the primary and secondary current distributions look like those presented in fig. 1.17. 

Savinell, R. Chase, G. Analysis of primary and secondary current distributions in a wedge-type aluminum-air cell. J. Appl. Electrochem. 18 4 (1988), pp. 499-503. 

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