High-current polarization curve

The high-current polarization curve of the CCL follows from Ek[. (2.54). Setting X = 0 in this equation we get... 

This relation determines the position of the intersection of the low- and high-current polarization curves (Figure 2.4). Note that for e -C 1 we have coth(l/e) 1 and the intersection is located at ejo = 2 (Figure 2.4, left plot) . 

Figure 2.10 Upper solid analytical high-current polarization curve for uniform loading. Crosses the exact numerical polarization curve for uniform loading. Lower solid polarization curve of the active layer with optimal shape of catalyst loading. Short-dashed curve nonuniform loading third and fourth order derivatives in Eq. (2.92) are taken into accoimt.

Figure 3.2 The high-current polarization curve of the cathode side of a PEM fuel cell, Eq. (3.7). Note the limiting current density at j = jo-...

This result should obey the normalization condition (4.134) calculating the integral we get the high-current polarization curve of a cell with variable catalyst loading ... 

To conclude this section, note that the low- and high-current polarization curves can be derived directly by dividing Equation 4.69 by Equation 4.70. This yields... 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

One of the goals of these above characterizations is the achievement of high and uniform fuel cell performance. These characterizations provide the necessary knowledge about what parameters should and ean be tailored in order to achieve higher performance. Once a good and uniform performance is consistently achieved, some or all of these characterizations can be eliminated. The best way to gauge the performance of an MEA is to collect voltage -current polarization curves. 

If the cell current is high, the polarization curves of CLs are similar, when either feed or ion transport is insufficient. In both the cases, the curve exhibits doubling of the Tafel slope (Equations 4.88 and 4.134). This makes it difficult to distinguish the physical origin of this doubling, by measuring the polarization curve only. 

In tlie polarization curve of figure C2.8.4 (solid line), tlie two regimes, activation control and diffusion control, are schematically shown. The anodic and catliodic plateau regions at high anodic and catliodic voltages, respectively, indicate diffusion control tlie current is independent of tlie applied voltage and7 is reached. 

Figure C2.8.4. The solid line shows a typical semilogaritlimic polarization curve (logy against U) for an active electrode. Different stages of reaction control are shown in tlie anodic and catliodic regimes tlie linear slope according to an exponential law indicates activation control at high anodic and catliodic potentials tlie current becomes independent of applied voltage, indicating diffusion control.

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

Often, it will be found that currents for a given reaction cannot be measured at all metals at the same value of potential. At some metals the currents would be too low for a reliable, sufficiently accurate determination at others they might be too high for a satisfactory experimental realization. A comparison will then be possible only after an extrapolation of data obtained in a different region of potentials, to the value of selected for comparison. This extrapolation may not be sufficiently reliable where the Tafel section of the polarization curve is too short or indistinct. 

In order to prove the S-shaped character of the polarization curve, the system was studied galvanostatically. The model predicts that the sandwiched branch of the polarization curve should be stable, and therefore measurable under galvanostatic conditions. Figure 6.10 shows the results of the experiment depending on the scan rate, an S-shaped curve can be observed in the back scan, i.e., from high to low current. At low... 

Nevertheless, the comparison (Figure 5) of the polarization curves of the same air electrodes show that the polarization at the high current density range of the most active electrode is much higher that that of the electrodes... 

They recorded such a polarization curve for zinc, for copper in the presence of gelatin and for silver in nitrate solution. Under this mechanism, a negative fluctuation in concentration drives the current density up, resulting in further reduction in interfacial concentration. For this instability to be expressed, the surface concentration must be free to respond to variations in current. As a result, the instability is seen only far from the limiting current, where the interfacial concentration is pinned at zero. At high Peclet numbers, the concentration disturbance is propagated downstream by convection, and the striations follow the streamlines. 

The individual polarization curves for the metals are often modified as a result of interactions resulting from codeposition. If the alloy deposition occurs at low polarization, the nobler metal will be deposited preferentially (Cu in Example 11.1). All factors, however, that increase polarization during electrodeposition, such as high current density, low temperature, and quiescent solution—factors that increase concentration polarization—will favor the deposition of the less noble metal (Zn in Example 11.1). 

Equation 1.7 for the reduction of protons at a mercury surface in dilute sulphuric add is followed with a high degree of accuracy over the range -9 Tafel plot i.s shown in Figure 1.5. At large values of the overpotential, one reaction dominates and the polarization curve shows linear behaviour. At low values of the overpotential, both the forward and back reactions are important in determining the overall current density and the polarization curve is no longer linear. 

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