Tafel step

Although no actual current is involved in the Tafel step, it is convenient to include F in (27) to express the rate in terms of a current. A similar argument applies to the inclusion of Fin (30).]... 

The highly reactive oxygen atom or hydroxyl species then bonds to the metal of the anode and recombines as in the Tafel step in the cathode to form an oxygen molecule that leaves the anode as a gas. 

A further conclusive support of the Vohner-Tafel mechanism is given by the log-log plot of the current density of HER (i) versus the band intensity shown in Fig. 8.22 B. The slope of 2 indicates iccff, which is the well-known rate equation of the Tafel step at limiting low 0 (i.e., in the framework of the Langmuir isotherm of adsorption) [103-105]. 

Tafel line, which would lead to an overestimate of the exchange c.d., and hence, of Vr. On a Ni hydrogen electrode in alkaline solution, Vr-2 was reported, indicating slow discharge followed by a rapid Tafel step on the mechanism. 

The surface coverage of H(a) will grow to a significant magnitude under certain conditions, so that / may not be approximated as unity. If this were the case, e.g., the forward unidirectional rate /+t of the Tafel step is expressed as, with Hi = Hr = 0,... 

Assuming a two-site activated complex for the Tafel step, we develop Eq. (49) as... 

In the present case, however, 6 = So = const, because of assumed quasiequilibrium in the Tafel step. Hence, W = Q -, or Eqs. (60) and (61) both reduce to simple forms, as seen in the literature. The case where W, Q are not equal to unity will be discussed later (Section 5). 

Results of the experiments have been summarized elsewhere, and only brief comments will be given here (a) results which support the Tafel-Volmer reaction route were obtained on Rh (in acid) and Rh and Ni (in alkaline solution), and partly on Pt, Pd, Ir, Au, and Ag (in acid solutions) (b) elementary step rates are frequently of comparable magnitudes, particularly on Rh and Ni (c) correspondingly, the stoichiometric numbers often deviate from integer values and (d) kinetics of the cathodic component rate of the Tafel step was second order in au, while that of the Volmer step indicated its symmetry factor (1 - /3) to be close to unity. Some results are summarized in Table 1/ ... 

Typical Tafel lines on Ni in alkaline solution are seen also in Figure g (53,163,164) anodic limiting current behavior which is observed would be attributable to the rate-determining Tafel step. In support of this, the rate of anodic ionization of H2 on Ni in 1 Af KOH or 0.1 Af KOH was dependent on the first power of On the other hand, an opposite view was... 

The cathodic Tafel line on Ni is reasonably linear, with a slope of 120 mV. However it is not impossible to account for this slope in terms of a rate-determining Tafel step, by considering interaction among H(a) and that... 

The effect of additives on the rates of the individual step rates is experimentally traceable in the case of Pd hydrogen electrode.As discussed above in Section 5.5, analysis of the overpotential decay transient on Pd, upon interruption of the current, directly gives information on the affinity values shared by the Volmer and the Tafel steps during steady polarization. Typical results are shown in Figure Tetrabutylammonium ion (Figure... 

In contrast with these cases, the m vs. 17 relation is significantly different in the absence or presence of reducible substance for the system butenediol-butanediol or allyl alcohol-propanol (Figure 19). Further, it was possible in these systems to observe the case where Th < 1, hence 172 > 0 at cathodic hydrogen overpotentials. This indicates that H(a), supplied through the Vol-mer step, is consumed by the electroreduction process which now exists as a reaction which competes with the Tafel step. The difference between the curves was the smaller the more negative was the electrode potential, namely. 

This is followed by either the Tafel step (10.2) or the Heyrovsky step (10.3). The former step involves the combination of two adsorbed hydrogen atoms, while the latter envisages the combination of a proton in solution, an electron, and adsorbed hydrogen to give dihydrogen ... 

The Volmer and the Tafel steps are coupled (coupled electroadsorptionchemical combination). 

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