Tafel-plot

Figure Bl.28.1. Schematic Tafel plot for the experimental detennination of and a.

Fig. 2. Tafel plot, where and are both chosen to be 0.5 the temperature is 298.15 K. A iadicates the linear region (eq. 23) and B the Tafel (eq. 24).

FIGURE 1-8 Tafel plots for the cathodic and anodic branches of the current-potential curve. 

Tafel equation, 14 Tafel plot, 15 Tetrathiafulvalene, 179 Thallium, 85... 

If we want to use the Tafel slopes to obtain the empirical kinetics of polymerization, we have to use a metallic electrode coated with a previously electrogenerated thin and uniform film of the polymer in a fresh solution of the monomer. In some cases experimental Tafel plots present the two components (Fig. 4) before and after coating. 

Figure 4. Log intensity vs. potential plots (Tafel plots) obtained from the voltammograms of a platinum electrode submitted to a 2 mV s l potential sweep polarized in a 0.1 M LiC104 acetonitrile solution having different thiophene concentrations. (Reprinted from T. F. Otero and J. Rodriguez, Parallel kinetic studies of the electrogeneration of conducting polymers mixed materials, composition, and kinetic control. Electrochim, Acta 39, 245, 1994, Figs. 2, 7. Copyright 1997. Reprinted with permission from Elsevier Science.)...

Tafel plots, during electrode polymerization, 316 Technology of electrochemical polymer formation, 427 Temperature coefficient and the interfacial parameter, 183 and the potential of zero charge, 182 of potential of zero charge as a function of crystal phase, 87... 

The usual procedure for extracting the exchange current Io is then to measure qw (=AUwr) as a function of I and to plot lnl vs T w (Tafel plot). Such plots are shown on Figs. 4.11 and 4.12 for Pt and Ag catalyst electrodes. Throughout the rest of this book we omit the subscript "W" from r w and simply write q,... 

Figure 4.11. Typical Tafel plots for Pt catalyst-YSZ interfaces during C2H4 oxidation on Pt the large difference in I0 values between the two Pt films (labeled R1 and R2) is due to the higher calcination temperature of Pt film R2 vs Pt film Rl.4 Reprinted with permission from Academic Press.

Figure 4.12. Effect of temperature on the Tafel plots and corresponding I0 values of a Ag catalyst-YSZ interface during C2H4 oxidation on Ag.12 Reprinted with permission from Academic Press.

As shown in Fig. 9.27 there is a break in the slope of the Tafel plot at Erhe I-OS V with a change in the transfer coefficient from 0.27 to 0.1. As shown below this change is consistent with a change in the surface coverages of adsorbed species as also manifest in the reaction kinetics. 

Figure 11. Tafel plots for methanol oxidation on (a) an E-Tek Pt-C electrode and (b) an E-Tek PtojRuoj-C electrode (1 M CH3OH in 0.5 M HQO4, 50 C, metal loading 0.1 mg cm" ).

Figure 18. Tafel plots from single cell DMFC data from several research groups. (After Ref. 25 reproduced with permission.)...

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

In the crudest approximation, the effect of the efectrical double layer on electron transfer is taken into account by introduction of the electrostatic energy -e /i of the electron in the acceptor into the free energy of the transition AF [Frumkin correction see Eq. (34.25)], so that corrected Tafel plots are obtained in the coordinates In i vs. e(E - /i). Here /i is the average electric potential at the site of location of the acceptor ion. It depends on the concentration of supporting electrolyte and is small at large concentrations. Such approach implies in fact that the reacting ion represents a probe ion (i.e., it does not disturb the electric held distribution). 

Figure 6.4 Dependence of the apparent rate constants and the apparent intrinsic rate constant on the potential ( Tafel plots ), determined by fitting the experimental transients with (6.5).

Using the colloidal Pt(i t ) + RU c/C catalysts described above, the optimal atomic ratio depends upon methanol concentration, cell temperature, and applied potential, as shown by the Tafel plots recorded with methanol concentrations of 1.0 and 0.1 M at T = 298K (Fig. 11.4) and 318K (Fig. 11.5). Some authors have stated that for potentials between 0.35 and 0.6 V vs. RHE, the slow reaction rate between adsorbed CO and adsorbed OH species must be responsible for the rate of the overall process [Iwasita et al., 2000]. From these results, it can be underlined that, at a given constant potential lower than 0.45-0.5 V vs. RHE, an increase in temperature requires an increase in Ru content to enhance the rate of methanol oxidation, and that, at a given constant potential greater than 0.5 V vs. RHE, an increase in temperature requites a decrease in Ru content to enhance the rate of methanol oxidation. 

Figure 11.4 Tafel plots for methanol oxidation on Pt + Ru/C coUoid catalysts with different atomic compositions at T = 298K (a) 1.0 M MeOH (h) 0.1 M MeOH (0.5 M H2SO4 sweep...

Tafel plots and Tafel slopes Eor carbon monoxide oxidation, 164-166, 175... 

Typical results, shown in Fig. 21(a), demonstrate that the rate constant for the reaction between TCNQ and aqueous Fe(CN)g increases with increasing driving force, promoted by decreasing [CIO4 as evidenced by the steeper Fe(CN)g concentration profiles. Moreover, the Tafel plot obtained for ET between Fe(CN)g and TCNQ is linear with an apparent measured a value of 0.31 0.02. In these studies, the concentration of reactant in the droplet phase was always at least 10 times the concentration of the reactant in the receptor phase, to ensure that depletion (and diffusional) effects within the droplet were negligible. 

Plotting the overpotential against the decadic logarithm of the absolute value of the current density yields the Tafel plot (see Fig. 5.3). Both branches of the resultant curve approach the asymptotes for r RT/F. When this condition is fulfilled, either the first or second exponential term on the right-hand side of Eq. (5.2.28) can be neglected. The electrode reaction then becomes irreversible (cf. page 257) and the polarization curve is given by the Tafel equation... 

Fig. 5.39 Tafel plot of hydrogen evolution at a mercury cathode in 0.15 m HC1, 3.2 m KI electrolyte at 25°C. (According to L. I. Krishtalik)...

It was experimentally found that the polarization curves of the investigated air gas-diffusion electrodes in a semi-logarithmic scale at low current densities (below 10 mA/cm2) are straight lines, which can be treated as Tafel plots. At these low current densities the transport hindrances in the air electrode are negligible so that activation hindrances only are available. 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

Equation (1.37) is of the form n = a + b log / an empirical observation first reported by Tafel. Thus, a Tafel plot of rj vs loge/ giving a straight line at high overpotentials is indicative of quasi-reversible kinetics. The slope gives / and the intercept (obtained via the extrapolation back to n = 0) gives /0 see Figure 1.8. 

Figure 1.8 A schematic representation or a Tafel plot of loge / vs. if. showing linearity at high overpotentials. At values of the overpotential < ryL, the current shows a linear dependence on...

A plot of logekt (obtained from the intercept) vs. the potential at which the measurements were obtained is a form of Tafel plot the plot should be linear, with slope fiF/RT and intercept k°. 

The authors also reported that a Tafel plot of the reduction current gave a slope of 107 mV, indicating wc = 1, in agreement with their proposed mechanism. 

Figure 5.3 Tafel plot for the anodic current density of an outer-sphere reaction.

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