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Volmer-Tafel

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. [Pg.210]

The hydrogen oxidation reaction (HOR) at the anode proceeds on Pt-based catalysts and is one of the simplest reaction systems. ° Nonetheless, fundamental information of the mechanism and kinetics of HOR is still lacking. The most common mechanisms are the so-called Heyrovsky-Volmer and Tafel-Volmer mechanisms involving the following steps ... [Pg.2511]

The adsorption of hydrogen on metal electrodes such as platinum has been studied extensively in electrochemical systems over the last several decades. The mechanism for the hydrogen oxidation reaction on a Pt electrode in an acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrosky-Volmer, both of which involve the adsorption of molecular hydrogen followed by a... [Pg.413]

Hydrogen under electrochemical conditions was investigated very recently [222, 223]. Santana et al. investigated the electro-oxidation of molecular hydrogen at the Pt(110)-water interface [222]. The Tafel-Volmer mechanism with a homolytic H-H bond cleavage followed by the formation of adsorbed terminal hydrogen atoms and further oxidation of the H atoms was observed by the authors. Furthermore, Santana et al. found the potential dependent activation energies for this process to be in accordance with experimental results. [Pg.145]

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/ ... [Pg.261]

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 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. 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.
In acid media, the rate equations for reactions (4), (5) and (7) can be formulated assuming that adsorption of hydrogen atoms on the eleetrode surface follows a Langmuir isotherm, with insignificant adsorption of molecular H2. It is assumed that such reactions occur on a homogeneous surface. Then, eurrent densities for Tafel, Volmer and Heyrovsky reaetions can be written as [45] ... [Pg.256]

When equilibrimn is attained, the partial anodic current density is equal to the partial cathodic current density of the same reaction. Tafel, Volmer and Heyrovsky exchange current densities are given by equations (12), (13) and (14), respectively ... [Pg.257]

HOR studies on smooth noble metal surfaces in acidic media show that platinum is the most active. The reaction mechanism on bulk polycrystalline platimun electrode is usually assumed to proceed through a Tafel-Volmer mechanism via reactions (4) and (5), the dissociative adsorption of hydrogen being the rate-determining step (r.d.s.) [51]. In alkaline electrolytes, HOR on polycrystalline Pt follows a Tafel-Volmer sequence and the r.d.s. is the dissociative hydrogen adsorption given by reactions (4) and (6) [52]. [Pg.263]

A dual-pathway kinetie equation has been proposed to describe the HOR current behavior on Pt over the whole relevant overpotential region [57]. This equation is based on the Tafel-Volmer-Heyrovsky meehanism and has been applied to analyse the polarisation curves measured with platinum microelectrodes and RDE, as well as, with high surface area catalysts operating under PEFCs eonditions. [Pg.264]

HOR on Pt-Ru/C anodes has also been studied in a symmetrical H2IH2 polymer electrolyte membrane fuel cell, because the polarisations involved in this reaction are small [70], This cell contained a MEA in which a proton exchange membrane is sandwiched between a Pt-Ru catalysed anode and a Pt catalysed cathode. The anode (working electrode) was then fed with H2, as well as the cathode, which was used as counter and reference electrode. The analysis of the anodic polarisation scans together with the dependence of the exchange current density on the partial pressme of hydrogen allowed concluding that the Pt-Ru catalysed anode follows the Tafel-Volmer mechanism. [Pg.268]

HOR on a Pt/C electrode in alkaline solutions has also been studied by us in a three-electrode arrangement [71]. A semi-empirical equation in agreement with the Tafel-Volmer mechanism was proposed to quantitatively explain the current vs. potential curves obtained in a variety of experimental conditions ... [Pg.268]

Sequence of reaction pathway 2 at GC (Tafel-Volmer analogue)... [Pg.85]

In the Tafel-Volmer (TV) pathway, the dissociative adsorption of a hydrogen molecule occurs without electron transfer and is followed by two separate one-electron oxidations of the H adatoms. By contrast, in the Heyrovsky-Volmer (HV) pathway, chemisorption and one-electron oxidation occur simultaneously, followed by another one-electron oxidation of the H adatom. [Pg.1046]

In this section, the reaction kinetics of the anode catalyst layer in the presence of carbon dioxide are established. These kinetics can be used in the simulation of mass transport in a similar manner to the CO kinetics. The reaction kinetics formulation given below is the same as that used in the models developed in Janssen [106] and Minutillo and Perna [107]. The Tafel-Volmer mechanism, as shown in equation (7.92) and equation (7.93), is used to describe the hydrogen oxidation. [Pg.255]

The HOR in acid solution is considered to proceed through either the Tafel-Volmer or Heyrovsky-Volmer mechanisms, depending on the nature of the adsorption step. If it is a purely chemical process, the mechanism is Tafel-Volmer and if it is a combined chemical and electrochemical process, it is Heyrovsky-Vohner [3]. [Pg.33]

The mechanism of HOR is thought to follow a similar Tafel-Volmer or Heyrovsky-Volmer mechanism, as discussed in Section 1.2.3, except with OH -mediated reactions ... [Pg.38]

The other studies of nonbulk metal HOR in alkaline come from Cabot et al. [25,44] who used a Pt-containing gas diffusion electrode (GDE) to closely represent the electrodes of a fuel cell in their RDE experiments. They concluded that at low overpotentials (near the OCV). the Tafel reaction is the rate-determining step in a Tafel-Volmer mechanism, with the diffusion of H2 becoming rate determining at higher overpotentials. These studies also showed that the exchange current density for HOR is lower in alkaline media for GDE. [Pg.39]


See other pages where Volmer-Tafel is mentioned: [Pg.79]    [Pg.531]    [Pg.276]    [Pg.279]    [Pg.353]    [Pg.353]    [Pg.353]    [Pg.2511]    [Pg.250]    [Pg.145]    [Pg.251]    [Pg.267]    [Pg.272]    [Pg.257]    [Pg.257]    [Pg.258]    [Pg.262]    [Pg.265]    [Pg.265]    [Pg.267]    [Pg.267]    [Pg.268]    [Pg.136]    [Pg.143]    [Pg.152]    [Pg.130]   
See also in sourсe #XX -- [ Pg.413 ]




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Hydrogen Volmer-Tafel mechanism

Tafel

Tafel plots from Butler-Volmer equation

Tafel-Volmer route

The Butler—Volmer and Tafel equations

Volmer-Heyrovsky-Tafel mechanism

Volmer-Tafel mechanism

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