Current - overpotential behavior

Fig. 39 Current-overpotential behavior l- curves) of a stage II PEVD process at various temperatures.

Fig. 40 Tafel plot (ln(/) vs. ti) of the current-overpotential behavior in Figure 39.

Why did we introduce this purely experimental material into a chapter that emphasizes theoretical considerations It is because the ability to replicate Tafel s law is the first requirement of any theory in electrode kinetics. It represents a filter that may be used to discard models of electron transfer which predict current-potential relations that are not observed, i.e., do not predict Tafel s law as the behavior of the current overpotential reaction free of control by transport in solution. 

Activation overpotential may become important with a number of electrocatalysts however, as Debenedetti and Vayenas (15) have discussed, the actual current-voltage behavior of the unit at moderate and high current densities can be well approximated by subtracting the activation overpotential from E For CO... 

ATafel plot for the same data set that was presented in Fig. D.14 is now shown in Fig. D.15 as a log (j)/overpotential plot. It is relatively simple, using such representation, to obtain the exchange current density values and the parameters behind the slopes of the current/voltage behavior, that is, Eq. (D.76). 

Figure 5 Comparison of experimental and simulated current-potential behavior of Ag/Au dealloying (a) experimental current-potential behavior for varying Ag/Au alloy compositions (atom% Au) within 0.1 M HCIO4 -1-0.1 M Ag" " (reference electrode 0.1 M Ag" /Ag) (b) simulated current-potential behavior of Ag/Au alloys (c) comparison of experimental (line) and simulated (triangles) critical potentials the zero of overpotential has been set equal to the onset of dissolution of pure silver hoth in simulation and in experiment. (Reprinted with permission from Ref. 65. Copyright (2001), Macmillan Publishers, Ltd.)...

At higher overpotentials the second-order terms become important, and Eq. (6.9) is no longer valid. At very large overpotentials, when eorj > A, Eq. (6.8) even predicts a decrease of the current with increasing overpotential, i.e., a negative resistance. However, better versions of this theory to be presented in the following section do not show this behavior. 

The complete current-potential relation is shown in Fig. 6.3. For small overpotentials we observe Butler-Volmer behavior, for large overpotentials a limiting current. 

It is difficult to measure kinetic currents at high overpotentials, since then the reaction is fast and usually transport controlled (see Chapter 13). At small overpotentials only Butler-Volmer behavior is observed, and the deviations predicted by theory were doubted for some time. But they have now been observed beyond doubt, and we will review some relevant experimental results in Chapter 8. 

Fig. 9.21 Simulink electrochemical model with R-C model capacitance behavior. The BV Fen block is the quasi-steady Butler-Volmer overpotential equation giving current through the Ret charge transfer resistor as a function of charge transfer overpotential, r).

Figure 12.87 shows the anodic decay current for the remaining H2 from the voids as a function of time. It is seen that bumps occur on such curves, and from an examination of the behavior of this anodic current-time decay curve, it is possible to obtain the calculated pressure in the voids. The result of this indirect approach was 3600 atm for an overpotential of -0.6 V in 1 M NaOH (Minevski and Lin, 1998). 

Note how, even in the region in which there is linear behavior of V with respect to /, the actual value of the potential that the generator could put out depends on the value of the so-called constant, i.e., on the activation overpotential and thus on the exchange current densities and the catalytic power of the electrodes. 

Anodic polarization of Ca electrodes in TC leads to current passage and dissolution of the active metal at high efficiency. As expected for SEI electrodes, a Tafel-like behavior connects the current and the overpotential applied [see Eqs. (5)—(11) in Section V.C.3], It is assumed that upon anodic polarization the anions (Cl-) migrate from the surface film s solution interface to the surface film s metal interface. Two processes can thus occur ... 

Figure 3.3.10 (A) The electrode potential dependence of the Gibbs free energy reaction pathway of the ORR. While the overall reaction has elementary steps that are energetically uphill at +1.23 V (red pathway), all elementary steps become downhill at +0.81 V (yellow pathway) (i.e. at an overpotential of approximately -0.42 V. At this point, the reaction is not limited by kinetics anymore. (B) The experimentally observed current-potential (j-E) relation of the ORR is consistent with the computational conclusions from (A) between +1.23 V and +0.81 V the j-E curve shows an exponential behavior, while at electrode potentials below +0.81 V, the ORR reaction rate becomes oxygen mass-transport limited, which is reflected by a flat ( j-E) profile. Figure adapted with permission from [19]. 

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