Tafel plots electrode

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 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" ).

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. 

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... 

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. 

The latter discussion confirms the results of the potential dependence of the current in that the activation barrier for the hydrogen evolution reaction is, at least on copper and silver, not affected by the electrode potential. This behavior is, on the other hand, connected with the observation of straight lines in a Tafel plot. It would be premature to come up with a comprehensive model that would explain this behavior more experimental work is necessary to substantiate and quantify the effects for a larger variety of systems and reactions. A few aspects, however, should be pointed out. 

Fig. 5.3 Effect of Ru content on Tafel plot of fresh Ru/Ti oxide electrodes in 5M NaCI (pH of approximately 3.5) at room temperature (with IR compensation).

Fig. 5.4 Time dependence of HE profiles during chlorine evolution at a 40 at.% Ru electrode in 5 M NaCI + 0.1 M HCI at room temperature (without IR compensation), subjected to square-wave potential cycling (from 1.35 to -0.32 V versus SCE at 60s cycle-1). The numbers in the figure refer to the time of electrolysis in hours. Each Tafel plot is shifted to the right by 20 mV to avoid overlapping. 

Let us now consider the charge state of the electrode. The emitter is positively biased. A p-type silicon electrode is therefore under forward conditions. If the logarithm of the current for a forward biased Schottky diode is plotted against the applied potential (Tafel plot) a linear dependency with 59 meV per current decade is observed for moderately doped Si. The same dependency of 1EB on VEB is observed at a silicon electrode in HF for current densities between OCP and the first current peak at JPS, as shown in Fig. 3.3 [Gal, Otl]. Note that the slope in Fig. 3.3 becomes less steep for highly doped substrates, which is also observed for highly doped Schottky diodes. This, and the fact that no electrons are detected at the collector, indicates that the emitter-base interface is under depletion. This interpretation is sup-... 

Figure 8-7 shows the anodic and cathodic polarization curves observed for a redox couple of hydrated titanium ions Ti /Ti on an electrode of mercury in a sulfuric add solution the Tafel relationship is evident in both anodic and cathodic reactions. FYom the slope of the Tafel plot, we obtain the symmetry factor P nearly equal to 0.5 (p 0.5). 

Worked Example 7.5. The one-electron reduction of the heptyl viologen dication (HV +) (to form the radical cation HV+ ) occurs at a platinum electrode with an area of 1.50 cm. Calculate the value of kd, given that the intercept on a Tafel plot of log / (as y ) against tj (as x ) is —2.2. 

Figure 11. Tafel plot of flooded porous-electrode simulation results for the cathode at three different values of xp = 2.2nFIfQ 2 02, z=dbK. The z coordinate ranges from 0 (catalyst layer/membrane interface) to L (catalyst layer/diffusion medium interface), the dimensionless overpotential is defined as // = —o FIRT r]oRR, - ), and the ORR rate constant is defined as A = hFFq 2 (Reproduced with permission from ref 36. Copyright 1998 The Electrochemical Society, Inc.)...

Figure 28. Svensson s macrohomogeneous model for the i— 1/characteristics of a porous mixed-conducting electrode, (a) The reduction mechanism assuming that both surface and bulk diffusion are active and that direct exchange of oxygen vacancies between the mixed conductor and the electrolyte may occur, (b) Tafel plot of the predicted steady-state i— V characteristics as a function of the bulk oxygen vacancy diffusion coefficient. (Reprinted with permission from ref 186. Copyright 1998 Electrochemical Society, Inc.)...

Figure 8. Tafel plots of oxygen reduction on Cu electrodes in 0.1 M HCIO, in air, w - 15 Hz, 25 C. bare Cu, o BTA-coated Cu, APVI-T-coated Cu, UDI-coated Cu. log i was used since no mass transport limited behavior was observed.

Figure 9. Tafel plots of oxygen reduction Cu electrodes in phosphate buffer, pH = 5.6, w = 15 Hz, 25 °C. bare Cu, ...

In alkaline solutions, electrochemistry of oxygen is not as greatly affected by purification of the solutions as in acidic media [45]. Since the earlier RRDE experiments, it has been shown that H2O2 is the main product of the reduction in the activation domain but is not detected anymore at lower potentials, when the electrode reaction is limited by mass transport [59, 64, 65]. The existence of two potential regions for ORR in alkaline media is also illustrated by the Tafel plots, as those reported in Fig. 6 [66]. The existence of two... 

Equation 3.13 predicts a linear dependence of In / on E whose slope depends on the coefficient ana, while the ordinate at the origin depends on the electrochemical rate constant and the net amount of depolarizer deposited on the electrode. Accordingly, both the slope and the ordinate at the origin of Tafel plots become phase-dependent [133, 183]. Since the quantity of depolarizer varies from one... 

Fig. 3.11 Tafel plots for verdigris (A), atacamite (B), paratacamite (C), and cuprite (D) from linear scan voltammograms at sample-modified, paraffin-impregnated graphite electrodes immersed in 0.50 M potassium phosphate buffer (pH 7.0). Potential scan rate 50 mV/s...

Figure 15.4 illustrates the Tafel plot for the hydrogen electrode, operating in both the conventional forward direction (current density if, b negative)... 

Figure 15.4 Tafel plot for the two half-reactions of the reversible hydrogen electrode.

Fig. 7.80. Tafel plots for Oz reduction on Au single-crystal electrodes in 0.1 M NaOH. (Reprinted from R. Adzic and N. M. Markovic, J. Elec-troanal. Chem. 138 189, copyright 1982 with permission from Elsevier Science.)...

Non-linear Tafel plots are predicted when A02 changes appreciably with electrode potential, that is at low ionic concentrations and close to the pzc (Fig. 3). 

Tafel plots have been used successfully for evaluation of slow electrochemical reactions at metal electrodes. Their application to electrochemical sensors is somewhat limited because of the mass transport boundary condition imposed by the nature of the Buttler-Volmer equation. Nevertheless, because it is simple and inexpensive, it should be always tried as the first approach, but bearing in mind its limitations. 

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