Tafel-plot interfaces

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.

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 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 5.2 Tafel plots of In k versus overpotential for a mixed self-assembled monolayer containing HS(CH2)i600C-ferrocene and HS(CH2)isCH3 in 1.0 M HCIO4 at three different temperatures V, 1 °C O/ 25 °C , 47°C. The solid lines are the predictions of the Marcus theory for a standard heterogeneous electron transfer rate constant of 1.25 s-1 at 25 °C, and a reorganization energy of 0.85 eV (= 54.8 kj moh1). Reprinted with permission from C. E. D Chidsey, Free energy and temperature dependence of electron transfer at the metal-electrolyte interface, Science, 251, 919-922 (1991). Copyright (1991) American Association for the Advancement of Science...

The exchange current and cathodic transfer coefficient for the PEVD system can be extracted from standard Tafel plots (Figure 40) as described in detail previously, and provide a measure of the nonpolarizability of the solid electrolyte/electrode interface. The values of Ig and at various temperatures are shown in Table 3. The activation energy of the exchange current can then be obtained from an Arrhenius... 

Initially, the potential difference across the liquid-liquid interface has appeared to be concentrated in the diffuse double layer (Sec. 2.3). On this basis Koryta [6] concluded that the apparent charge transfer coefficient a is not related to the activation barrier and should have a value close to 0.5, as explained in Sec. 3.1.2. A numerical analysis based on Eq. (36) revealed however that, depending on the value of the parameter p (Eq. (37)), d can vary with the potential difference Aq0, i.e., Tafel plots should not be linear curves [60]. 

Fig. 21. Logarithm of the apparent rate constant k vs the potential E relative to the reversible halfwave potential (Tafel plot) derived from ac impedance measurements of Et4N ion transfer in the absence ( ) and in the presence (V, O) of a DLPE monolayer formed at the interface between an aqueous solution of 0.1 M LiCl and a nitrobenzene solution of 0.1 M Pn4NPh4B-(-50 pM DLPE (O), and at the interface between an aqueous solution of 0.09 M LiCl+O.OI M LiOH and a nitrobenzene solution of 0.1 M Pn4NPh4B-i-20 pM DLPE (V). (After [96]).

As already shown in Fig. 1, a general feature of electrocatalysis is that the current passing through an electrode-electrolyte interface depends exponentially on overpotential, as described by the Butler-Volmer equation discussed in Sect. 2.4.1, so that logi versus r] U — C/rev) gives straight lines, termed Tafel plots (Fig. 1). On this basis, one would expect an exponential-type dependence of current on overpotential in Fig. 12 (curve labeled 7ac). Such a curve would correspond to pure activation control, that is, to infinitely fast mass-transport rates of reactants and products to and from the two electrodes. 

The usual procedure for extracting the exchange current 7o is to measure /jw as a function of 7 and to plot In 7 versus rjw (Tafel plot). Such plots were shown in Fig. 1 for aqueous electrolytes and in Fig. 15 for Pt and Ag catalyst electrodes interfaced with YSZ in solid-state electrochemistry. 

Fuel cell engineers refer to jo/mpt as the mass activity. From an electrochemical perspective. Figure 1.6a represents a Tafel plot. However, the voltage Eceii (jo) does not only account for a single plain electrified interface, but also depends on electron flux, ion flux, and mass transport in all components and across all interfaces of the MEA. 

Tafel plots of the current densities of corrosion (i ) and layer formation (%) of iron in acidic electrolytes as a function of the overvoltage TI23 at the oxide-electrolyte interface. (From Vetter, K.J. and Corn, F., Electwchim. Acta, 18,321,1973.)... 

Work described in Ref. 86 includes a detailed investigation of the temperature dependence of ORR kinetic parameters at the Pt/bulk Nafion interface in the range from 30 to 90 °C. Plots of the log of mass-transport-corrected current density versus the potential of the Pt microelectrode showed an increase of a factor of about five in the rate of ORR at 0.90 V and about three at 0.85 V as the temperature was increased from 30 to 90 °C. The apparent activation energies for the two regions (low and high Tafel slope) were calculated from the dependence of the apparent exchange current density on T according to... 

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