Tafel-Volmer route

Figure 7. Theoretical curves showing affinity distribution m = Agr/ v for the Tafel-Volmer route as a function of hydrogen overpotential for various values of mo = fov/ ot and of as indicated. Temkin isotherm with u = 5 and j3 = 0.5 are used. Territories indicate schematically the rate-determining characteristics of the constituent steps (A) linked Tafel-Volmer, (B) rate-determining Volmer with coupled Tafel, (C) rate-determining Tafel with coupled Volmer, (D) Tafel uniquely rate determining, (E) Volmer uniquely rate determining.

Figure 9. Current-overpotential relations calculated for the Tafel-Volmer route, with (A) mo = 10, (B) 1, or (C) 10, with various values of Bq. Temkin isotherm with m = 5 and = 0.5 are assumed.

The subscripts which distinguish the steps honor, respectively, Tafel, Volmer and Heyrovsky. Unlike the MCFC cathodic reaction mechanisms, however, these steps combine pairwise to yield the overall reaction. The reaction mechanism graphs for each of the three reaction mechanisms are shown in Figure 6. Notice that it is not possible to represent the entire mechanism by a single reaction mechanism graph. This is because, unlike in the MCFC case, there are now independent full reaction routes which yield the over all reaction. In both of the MCFC examples, there was only one. Still the three separate graphs do clearly convey the three HER reaction routes. 

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/ ... 

Reaction kinetics with comparable magnitudes of rate constants of the constituent steps, hence not having any unique rds, was often discussed. Hammett was probably the first to examine such cases. He conducted polarization experiments on Pt in 0.1 Af HCl or 0.1 Af KOH and interpreted the data on the model of two steps in the Volmer-Tafel reaction route having similar magnitudes of exchange c.d. s. The value of mo required in interpreting the polarization data on Pt was 7 -- 18 for the repeated series of experiments. 

In conclusion, the break of the cathodic Tafel line in the Heyrovsky-Volmer route is not due to onset of surface saturation with H(a) with the Heyrovsky step rate determining, but to the Volmer step becoming rate determining. ... 

In the case of the Volmer-Tafel reaction route, the equivalent pressure is readily obtained by substituting Eq. (71) into Eq. (109) and employing the relation between m and rj, as calculated (see Figure 7). The resultant log Ph2 vs. 7] relation becomes practically linear, and its slope is determined by mo and other kinetic parameters. ... 

There are 3 steps in scheme 7.270, two intermediates (adsorbed hydrogen and vacant sites), one balance equation which relates these two intermediates, and then respectively two independent routes according to the Horiuti-Temkin rule. Steps 1, 2 and 3 are usually referred as Volmer, Tafel and Heirovsky reactions respectively acknowledging the names of researches who emphasized the importance of these processes. 

More detailed analyses were conducted later, both for the Volmer-Tafel and the Volmer-Heyrovsky reaction routes, with allowance for comparable magnitudes of exchange c.d. s of the constituent steps. It became clear that such analyses of the so-called linked/coupled t mechanism could well provide an interpretation of some experimental data, as will be discussed later. Meanwhile, some comments will be made concerning the definition of the now widely accepted terms rds and other related topics. 

The affinity distribution in the present example of the Volmer-Tafel route of HER may be expressed by m ... 

Typical results of calculations of m for the Volmer-Tafel route as a function of 17 are given in Figure it is seen that m has a tendency... 

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