Reactions Tafel

The Tafel reaction would require breaking the adsorption bonds to two hydrogen atoms strongly bound to the electrode, while the Heyrovsky reaction requires breaking only one such bond this reaction then determines the rate of the electrode process. 

It is well accepted7 that the electroreduction of proton to dihydrogen (Equation (1)) at metal surfaces proceeds via the Volmer-Heyrowsky-Tafel reaction mechanism depicted in Equations (2) (4). 

Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have ... 

Denoting the forward rate constant for the Tafel reaction by k2 and that for the back reaction by fc 2, we can write the current density in the form ... 

According to the Volmer-Tafel mechanism, the discharge of a proton forms an adsorbed hydrogen atom (Volmer reaction). Two such adsorbed hydrogen atoms then combine to form molecular hydrogen (Tafel reaction)... 

The electrochemical hydrogen permeation technique has proved to be a valuable tool in the study of these reaction mechanisms. This is mainly due to the ability to estimate the amount of an intermediate (Hads) in the reaction scheme. Such studies have been presented, for example, by Devanathan and Stachurski, by Bockris et and by Iyer et The applicability of the Volmer-Tafel reaction scheme can be evaluated by considering the kinetic expressions for reactions (22) and (23), together with equilibrium in the absorption process (25)... 

It can be assumed that the current representing the Volmer discharge reaction (26) equals the current representing the Tafel reaction (27), i.e., only a minor part of the discharge current (iV) results in hydrogen atoms being absorbed into the membrane the reverse Volmer reaction is taken to be negligible so that the observed current, I obs = iv = h It follows from (27) that... 

Figure 27. Plots of the permeation current density vs. the square root of the Tafel reaction current density [Eq. (33)], recorded in ace-tate/acetic acid buffers at pH = 6 and 30°C, using carbon steel membranes of 1.0 and 0.5 mm thickness. ...

Tafel reaction hydrogen Tafel slope - Tafel equation... 

The rate-determining step varies depending on the specific catalysts and the reaction conditions. For a PEM fuel cell with a Pt anode, the HOR process involves only the Tafel and Volmer reactions, with the Tafel reaction being the rate-determining step." The rate of the overall HOR process can be expressed in the Butler-Vol-mer form (equation 12.1) ... 

At the surface, the (fast) so called Volmer reaction + e -> Had takes place. Subsequently, either the homolytic Tafel reaction, 2Had -> H2, or the heterolytic Heyrovsky reaction. Had + H + e H2, may take place. Using the nudget-elastic-band approach (see, e.g., ref. 2) the authors studied the three reactions on the surfaces. They found that the Heyrovsky reaction may dominate, but the similarity of the calculated activation energies for the Heyrovsky and the Tafel reactions suggests that both reactions may take place in parallel. 

In the second step, adsorbed hydrogen is removed from the electrode either through a chemical process (Tafel reaction) ... 

Reaction (9-3) is called the Vohner reaction or proton discharge and reaction (9-4) the Heyrovsky reaction or electrochemical desorption. Both reactions are not the only ones possible. Desorption of the adsorbed hydrogen may also proceed according to the Tafel reaction (Eq. 9-5) ... 

Initially, the hydrogen remains adsorbed in atomic form on the metal snrface. For the formation of molecular hydrogen, there are two possibilities either two adsorbed hydrogen atoms combine chemically to form an adsorbed hydrogen molecule (Tafel reaction) ... 

Oxidation of the adsorbed molecular hydrogen takes place in a two-step process. In a first step the adsorbed molecular hydrogen dissociates into adsorbed atomic hydrogen, the so called Tafel reaction. 

Combination of two adsorbed hydrogen atoms to form molecular hydrogen (Tafel reaction). 

HER measurements showed the onset overpotential for acidic hydrogen evolution was found to be small (0.078 V) and material was also found to be resistant to corrosion in the acidic media. These two results suggest that both the synergistic effects ofthe alloys and the nitride component make this type of material a promising candidate for the HER reaction. The resulting Tafel slope was found to be 35 mV decade". This suggests the Tafel reaction recombination step to be the rate-limiting step in the HER. 

The initial step, reaction (20), in the HER is charge transfer, leading to the formation of an adsorbed intermediate MH containing the metal M. The MH can either recombine to form molecular hydrogOT by reaction (21) or interact with H+ and an electron to form hydrogen by reaction (22). The former step is termed the recombination or Tafel reaction and the latter, 4he electrdchemical desorption or Horiuti-Heyrovsky mechanism. 

---