Current Nonuniform distributions

The following section provides a qualitative insight into the essentials of the corrosion process. Important factors such as current distributions, nonuniform metal and environment compositions, and finite resistance of the metal are considered later in the text. 

Electrolytically evolved gas bubbles affect three components of the cell voltage and change the macro- and microscopic current distributions in electrolyzers. Dispersed in the bulk electrolyte, they increase ohmic losses in the cell and, if nonuniformly distributed in the direction parallel to the electrode, they deflect current from regions where they are more concentrated to regions of lower void fraction. Bubbles attached to or located very near the electrodes likewise present ohmic resistance, and also, by making the microscopic current distribution nonuniform, increase the effective current density on the electrode, which adds to the electrode kinetic polarization. Evolution of gas bubbles stirs the electrolyte and thus reduces the supersaturation of product gas at the electrode, thereby lowering the concentration polarization of the electrode. Thus electrolytically evolved gas bubbles affect the electrolyte conductivity, electrode current distribution, and concentration overpotential and the effects depend on the location of the bubbles in the cell. Discussed in this section are the conductivity of bulk dispersions and the electrical effects of bubbles attached to or very near the electrode. Readers interested in the effect of bubbles dispersed in the bulk on the macroscopic current distribution in electrolyzers should see a recent review of Vogt.31... 

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

Tertiay Current Distribution. The current distribution is again impacted when the overpotential influence is that of concentration. As the limiting current density takes effect, this impact occurs. The result is that the higher current density is distorted toward the entrance of the cell. Because of the nonuniform electrolyte resistance, secondary and tertiary current distribution are further compHcated when there is gas evolution along the cell track. Examples of iavestigations ia this area are available (50—52). 

The anode mountings are welded to lap joints in the yard, and the anodes are installed at a minimum distance of 30 cm from the structure to achieve the most uniform current distribution [1-3]. Nonuniform potential distribution occurs even with this distance. 

FIGURE 18.4 Concerning the derivation of equations for nonuniform current distribution (a) in a flat electrode (b) in a cylindrical pore. 

In this example the current density distribution is nonuniform in the vertical, since at all heights x the sums of ohmic potential drops and polarization of the two electrodes must be identical. In the top parts of the electrodes, where the ohmic losses are minor, the current density will be highest, and it decreases toward the bottom. The current distribution will be more uniform the higher the polarization. 

One of the main reasons for a lower specific activity resides in the fact that electrodes with disperse catalysts have a porous structure. In the electrolyte filling the pores, ohmic potential gradients develop and because of slow difiusion, concentration gradients of the reachng species also develop. In the disperse catalysts, additional ohmic losses will occur at the points of contact between the individual crystallites and at their points of contact with the substrate. These effects produce a nonuniform current distribution over the inner surface area of the electrode and a lower overall reaction rate. 

In many limiting-current measurements the expected current distribution is only moderately nonuniform, and a single unsegmented electrode will yield well-defined limiting-current plateaus. The various techniques by which the limiting current at a single electrode can be generated are discussed in the next section. 

Equation (23) implies that the current density is uniformly distributed at all times. In reality, when the entire electrode has reached the limiting condition, the distribution of current is not uniform this distribution will be determined by the relative thickness of the developing concentration boundary layer along the electrode. To apply the superposition theorem to mass transfer at electrodes with a nonuniform limiting-current distribution, the local current density throughout the approach to the limiting current should be known. 

Figure 8. Primary current distribution on the front surface of the electrodes based on Kirkhof s law calculation for three different cell constructions (A) Both connections to the cell are at the top. The higher resistance path at the bottom sections of the electrode reduces the current flow and results in a nonuniform current distribution. (B) All paths have equal resistance, and a uniform current distribution results. (C) The bipolar construction has equal resistance from one end to the other.

Although the matrix may have a well-defined planar surface, there is a complex reaction surface extending throughout the volume of the porous electrode, and the effective active surface may be many times the geometric surface area. Ideally, when a battery produces current, the sites of current production extend uniformly throughout the electrode structure. A nonuniform current distribution introduces an inefficiency and lowers the expected performance from a battery system. In some cases the negative electrode is a metallic element, such as zinc or lithium metal, of sufficient conductivity to require only minimal supporting conductive structures. 

To expand on the last remark, the simulation results from Fuller and Newman are shown in Figure 17. The curves clearly show a nonuniform current distribution that is mainly due to the change in the gas concentrations and the membrane hydration. In the simulation, the initial decrease in the current density is due to the change in the oxygen concentration. However, once enough water is generated to hydrate the membrane, the increased conductivity yields higher local current densities. What... 

The two major causes of uneven current distribution are diffusion and ohmic resistance. Nonuniformity due to diffusion originates from variations in the effective thickness of the diffusion layer 8 over the electrode surface as shown in Figure 10.13. It is seen that 8 is larger at recesses than at peaks. Thus, if the mass-transport process controls the rate of deposition, the current density at peaks ip is larger than that at recesses since the rate of mass transport by convective diffusion is given by... 

Figure 10.15. True leveling on a V-groove produced by nonuniform current distribution, ir > ip, in the presence of a leveling agent (a) nonuniform current density and deposit thickness, hs and hg, after time t = l (b) evolution of a groove profile during deposition activation control.

---