Ohmic effects

Ohmic effects render Epc more negative, Epa more positive, AEp and 8Ep larger, and X smaller than the true values. Since experimental approaches to elimination of iRu errors are not foolproof (see Chap. 7), the presence of ohmic distortions should be tested by measurements on a Nernstian couple such as ferrocene/ferrocenium under conditions identical to those used to probe the test compound. In principle, errors in the measured CV parameters for a test compound can be eliminated by referencing its responses to those of the Nernstian standard. Note that this approach is accurate only if the current level of the standard, rather than its concentration, is equal to that of the test compound, since the diffusion coefficients of the two species may appreciably differ. 

Thus the potential in the macropores is directly affected by transport and charging phenomena in the micropores accessible to electrolyte. These properties of active carbon render it a difficult material to use as an electrode. The large electrochemically active surface area leads to considerable double-layer charging currents, which tend to ob.scure faradic current features. The network of micropores in the electrode material might be expected to result in a significant ohmic effect, which would further impair the potential resolution (IR drop on electrode material) obtainable by PACE voltammetry. CV curves recorded with different masses (and sediment layer thicknesses) of powdered samples of selected carbons in various electrolyte solutions are presented in Fig. 8 as an example [194]. Where amounts of material were greater than 20 mg, the CVs recorded were of the same shape. 

Figure 8 shows the effect of the initial seed layer conductance on the plated thickness nonuniformity. It was determined that the nonuniformity depends upon the initial sheet conductance to the -0.48 power and upon the plated film conductance to the -0.70 power ( Ni K C,f 4SG-° 70). The effect of the Wagner number is shown in Figure 9. The higher the Wagner number the better the non uniformity because the ohmic effects become less important at high Wagner numbers. It was determined that the non uniformity is proportional to the Wagner number to the -0.60 power (Nt K G 0 irG 0 70W

These parameters arise from the boundary conditions on Laplace s equation when finite kinetic rates are included. The first is a ratio between the exchange current density and ohmic parameters such as length and conductivity. The second parameter is a dimensionless current level. Values of either or both parameters much greater than unity indicate that ohmic effects dominate and the current distribution resembles the primary case. Low values of both parameters indicate that kinetics limit the process and the current distribution is uniform. Even though a low value of J indicates substantial kinetic effects, the current distribution resembles the primary case at high values of 6 because the linear ohmic dependence on current density dominates the logarithmic overpotential dependence. These... 

The local current density within corrosion pits may be extremely large. If the precipitation of corrosion products does not occur, the metal dissolution is controlled by charge transfer and ohmic effects, and hence the corrosion process is potential dependent. This situation requires a sufficiently acidic solution to avoid the precipitation of insoluble oxides or a still not saturated or supersaturated pit electrolyte with no formation of a salt layer. Pitting at potentials close to the critical value Ep occurs usually with moderately small local current densities / c,p that, however, may increase with potential to extremely... 

At room temperature, and above, there is no sign of nonlinearities in PHT, and the hopping distances must be smaller. In PBT there are non-ohmic effects, even at room temperature. 

Unusual media Tradihonahy, electrochemistry suffered relahve to spectroscopy because electrochemical measuranents could be made only in highly ionic condncting soluhons. This restrichon arose because resistance between the working or sensing electrode and the reference electrode limited the precision with which the applied potenhal could be accurately controlled. The smaU electrolysis currents observed at microelectrodes often completely eliminate these ohmic effects. 

In general, PEM fuel cell contamination effects are classified into three major categories (1) kinetic effect (poisoning of the catalyst sites or decreased catalyst activity) (2) ohmic effect (increases in the membrane and ionomer resistances, caused by alteration of the proton transport path) and (3) mass transfer effect (mass transport problems caused by changes in the structure of CLs and GDLs, and in the ratio between their hydrophilicity and hydro-phobicity). Of these, the kinetic effect of the electrocatalysts on both anode and cathode sides is the most significant. 

A solution can be found by solving for the slope of the polarization curve in the main linear region. First, plot the polarization curve and select the appropriate region. In this case, because it is a high-temperature fuel cell operating on hydrogen, the overall polarization curve is quite linear and dominated by ohmic effects. 

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