Low Ohmic Resistances

Apart from the above requirement of a low ohmic resistance, it is nevertheless recommended to use a three-electrode system in view of the more precise establishment of the dme potential. 

AGC has developed the low ohmic resistance membrane (F-8934) for high current density operation up to values of approximately 6 kA m-2. The new arrangement of the sub-structure of the membrane has contributed to wider distribution of the current passing through the membrane. This configuration decreases the actual current density localised over the membrane. Thus, the F-8934 shows 25% lower ohmic resistance than that for the F-893, as is shown in Fig. 19.10, even though the former comprises almost the same materials as the F-893. 

Mass-transport deposition control occurs when the exchange current density P is high and the limiting current density is low. Ohmic resistance can be a cause of nonuniformity if there is an appreciable difference in solution resistance from the bulk of the solution to peaks or to recesses. Distribution of the current density will be such that ip > i. and peaks will receive a larger amount of deposit than will recesses. Distribution of deposit in the triangular groove under conditions of mass transport and ohmic control nonuniform deposition, with ip > is shown in Figure 10.14. 

The bipolar plates are usually fabricated with non-porous machined graphite or corrosion-resistant metal plates. Distribution channels are engraved in these plates. Metallic foams can also be used for distributing the reactants. One key point is to ensure a low ohmic resistance inside the bipolar plate and at the contact with the M EA. Another point is to use materials with high corrosion resistance in the oxidative environment of the oxygen cathode. 

All chelates discussed here are bad conductors of electricity. In order to study their electrocatalytic properties, it is therefore essential to maintain a very low ohmic resistance between the reaction site and the circuit connection. This is generally done by mixing the catalyst with an inert conductor such as powdered charcoal or powdered gold. Differences in the quantity and nature of the substance mixed with the catalyst further complicate comparison of the activity values obtained in different laboratories. The various groups have hitherto used different processes for testing chelates. 

Very thick metallic electrodes (up to 30 pm Au have been used) provide low ohmic resistance and lower rc constants. 

A cell system has been designed for the potentiostatic transient investigation of fast electrode reactions.9 The main novel feature of the cell is the elimination of the classical Luggin capillary, as shown in Figure 6.4. The design provides a low-ohmic-resistance reference electrode with low stray capaitances. 

The choice of the anion is most important in anodic reactions. Perchlorates have been found very useful as they are difficult to oxidize and are often soluble both in water and nonaqueous solvents. In anodic (Section VI, F) or cathodic (Section IV, A) substitution reactions the nucleophilicity of the anion is of interest. High concentrations of tetraalkylammonium p-toluenesulfonates in water make the solubility of organic compounds higher than in pure water, and such solutions combine a low ohmic resistance with good dissolving power. 

A low ohmic resistance of the cell is desired from several points of view this may be achieved by having a small distance between anode and cathode, a large electrode surface, a compact cell, and a low-resistance diaphragm. A distance between anode and cathode as small as 0.1-0.2 mm has been reported [9], although usually larger distances are employed. A large electrode area may involve the use of many electrodes a further development in this direction is the fluidized bed electrode (Chapter 31). 

If we draw an analogy between the structure of battery plates and the structure of a tree with a large crown, the plate grid would play the role of the tree trunk. The trunk spUts into several thick branches, which develop further into a system of thinner branches from which the leaves and buds grow. In the case of a battery plate, the AMCL plays the role of the thick branches from which the tree crown grows. That is why the skeleton of the AMCL should be built of thick branches so as to have low ohmic resistance. Otherwise, the AMCL of the plates will get strongly polarized when the high formation current is switched on. 

The operation temperature of portable SOFC may lie weU below 600 °C and can be reduced to 350 °C. These low temperatures can be achieved by extremely thin electrolyte layers being produced with MEMS techniques. Thin electrolyte layers have a low diffusion resistivity for oxygen anions and thus a sufficiently low ohmic resistivity at low temperature. 

Membrane selectivity is probably the most important parameter to compare potential polymeric membranes for DAFC because low ohmic resistance and low alcohol crossover are desired. Kim and Pivovar [4] have noted that both of these quantities are independent of membrane thickness, which affect resistance and alcohol crossover in opposite ways. The thinner is a membrane in a DAFC, the lower the resistive losses, but the higher the crossover losses. Thus, a minimum conductivity is required in membranes for DAFC, regardless of how high its... 

Minimal Ohmic resistance/excellent conductivity Quite surprisingly, even intimately coated graphite particles in the CP(E) configurations exhibit a very low ohmic resistance often, in tens of ohms [5,39,40, 50-52, 55] and, in special cases, even below 10 Q [5,16]. 

Current collector at low Ohmic resistance for supplying with electrons... 

Use of FIFE as the backbone provides high structural strength and enables the polymer electrolyte to be made into a thinner membrane and hence results in low ohmic resistance. The lower limit is set by the structural strength of the membrane. 

W, specific dissipating power 0.4 W/cm temperature coefficient 5.10" in low-ohmic resistances, temperature coefficient one order less)... 

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