Tafel lines slope

Table 7.2 summarises the conclusions for the mechansims which have been considered. While the mechanisms and different rate determining steps lead to several values for the reaction order and Tafel slope, the experimental observations will not be totally diagnostic because different mechanisms can lead to the same values. Moreover, there are many more mechanistic possibilities than have been discussed here. On the other hand, the behaviour described by the data in Table 7.2 is often observed. Platinum is an example of a cathode material where two Tafel regions are observed, see Fig. 7.3. Close to the equilibrium potential, a Tafel line, slope (30 mV)" is observed, while at high overpotentials a further linear region, slope (120 mV) , can be seen. 

Thus, the observed value of the Tafel line slope does not contradict the hypothesis of a slow charge transfer reaction. Nevertheless, the authors mentioned that the same value may also be obtained in the case when the discharge step is not ratedetermining (for example if adsorbed atoms are involved in a charge transfer reaction). 

After 300-s polarization, the pseudo-steady-state Tafel line slope was A /3oo/A log i = 38-57 mV dec where pseudo underlines the uncertainty attached to the identification of any steady state. 

Tafel constant) obtained from the slope b of Tafel line (Eqns. 7—19 and 7-32) and is defined in Eqn. 7-36 ... 

The steady-state Tafel lines for methanol oxidation in acid solution are 55-60 mV/decade over the potential range 0.4-0.5 (SHE). When the potential of the working electrode on this scale is made more positive, the Tafel slope changes and becomes 110 mV/decade. These two numerically stated Tafel slopes can readily be reexpressed in electrode kinetic terms to correspond, respectively, to ... 

Fig. 1. Sketch showing (a) Tafel lines with different slopes and exchange currents for two different electrocatalysts, and (b) the observable two-section Tafel line in case, e.g., of change in r. d. s. for a single electrocatalyst.

While no effects of simple roughness (except surface area effects) has been observed for the electrocatalytic activity of Ni, the situation appears to differ with mild steel for which a decrease in Tafel slope has been observed in the low overpotential range [272] (Fig. 9). An investigation of the effect of temperature and NaOH concentration has suggested [273] that in fact this may be related to the appearance, at low overpotential, of surface oxides, whose reduction leads to the break in Tafel line. Again, these results warn against interpreting certain apparent Tafel slopes only in terms of kinetic mechanisms. 

With the aim to enhance the apparent current density of Ni cathodes, several surface treatments have been proposed to increase the surface area. The simplest procedure, and therefore the one of routine, consists in sandblasting the surface. The Tafel line turns out to be shifted towards higher currents without changes in its slope [137], Sandblasting is a common operation also prior to proceeding to the further activation of cathodes (or anodes) [259]. The aim is to provide a support of larger surface area and to ensure better adhesion of the overlayer. 

Raney Ni evolves hydrogen exhibiting a two-section Tafel line [93, 248, 406, 408]. At low overpotentials b is around 40-60 mV, at high overpotentials it is close to 120 mV. The Tafel slope of the first section is usually observed to decrease with increasing temperature, which suggests the probable occurrence of a change in the surface state. 

Compare the form of these equations with the Tafel equation, eqn (1) the slope, b, of the Tafel line is thus (f T/( 1 - P)F) (anodic) and (RTj(PF) (cathodic). Note also that, when rj = 0, i = i0 and the exchange current density may be found from the intersection of the anodic and cathodic Tafel slopes (at tj = 0). This is one method of determining corrosion rates since,... 

The Tafel slope for this mechanism is 2.3RT/PF, and this is one of the few cases offering good evidence that P = a, namely, that the experimentally measured transfer coefficient is equal to the symmetry factor. A plot of log i versus E for the hydrogen evolution reaction (h.e.r.), obtained on a dropping mercury electrode in a dilute acid solution is shown in Fig. 4F. The accuracy shown here is not common in electrode kinetics measurements, even when a DME is employed. On solid electrodes, one must accept an even lower level of accuracy and reproducibility. The best values of the symmetry factor obtained in this kind of experiment are close to, but not exactly equal to, 0.500. It should be noted, however, that the Tafel line is very straight that is, P is strictly independent of potential over 0.6-0.7 V, corresponding to five to six orders of magnitude of current density. 

Tafel lines with a single slope, which satisfy the equation of Ref. 158 (see above)... 

Tafel line having two portions with different slopes... 

Tafel line with a slope of 60 mV the stirring effect is absent... 

Conway et al also studied the h.e.r. at Ni under high-purity conditions in methanolic HCl over a wider range of temperature. Under these conditions, the Tafel lines appear as two linear segments,t the slopes of each of which depend on T as shown in Fig. 4. Comparative data for the D2 evolution reaction (d.e.r.) from MeOD/DCl are shown in Fig. 5. 

The two linear Tafel regions at Ni in methanolic HCl vary with r in a continuous and complementary way one has a slope that increases with T while the slope of the other simultaneously decreases with T (Fig. 4), so there is a singular temperature at which the Tafel relation is one line over the whole c.d. range. The directions of change of the slopes of the two Tafel lines at each temperature, other than at the singular temperature, correspond apparently to reaction mechanisms that are consecutive or parallel ). However, we believe, based on new data... 

It is evident that the T-dependence of b again does not follow that conventionally assumed [Eq. (5)] and, moreover, for the same reaction at the same metal, Ni, the two Tafel-line segments have slopes that vary in opposite directions with temperature Fig. 4 shows that this is a systematic variation and not the result of some... 

The mechanism determination put forward here has been based upon only two types of measurements the Tafel lines with the two slopes in different potential regions (not shown) and the spectroscopic analysis indicating the adsorbed radicals. It is an example of how spectroscopic measurements have improved mechanism analysis and preclude the determination of the optimal electrocatalyst. ... 

Thus, the true charge-transfer current can be calculated from the ordinate at the origin in the plot between the reciprocal of the measured current density, j"1, as a function of w. The slope (B 1) is the reciprocal value of the Levich constant, 0.620nFCJoj, because it is the only portion that strictly depends on the co value [107], where D, is the coefficient of diffusion of they-particle. With the currents corrected from the mass transport effects, we can depict the Tafel lines, from which the values of j0 and a can be calculated. 

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