Volmer mechanism potentialities

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. 

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 ... 

In 1992, Kakiuchi et al. showed, also by impedance measurements, that the apparent rate constants for anions were even faster than for cations of relatively equal radius [100] (Table 1.2). This led the authors to conclude that dielectric friction at the interface must be considered. They also showed than the Goldman-type current-potential relationship was more appropriate than the Butler-Volmer mechanism (see Section 1.3.3.1) [101]. 

When at eqnilibrium the bulk concentrations are equal, the potential difference is eqnal to the formal potential for the ion-transfer reaction, and the global activation energy barrier is symmetrical [91]. As the potential is varied, the overall driving force zF (A"(l)-A"( ) ) partially lowers the activation energy barrier and, as in the Butler-Volmer mechanism, the current is then given by... 

The two-step charge transfer [cf. Eqs. (7) and (8)] with formation of a significant amount of monovalent aluminum ion is indicated by experimental evidence. As early as 1857, Wholer and Buff discovered that aluminum dissolves with a current efficiency larger than 100% if calculated on the basis of three electrons per atom.22 The anomalous overall valency (between 1 and 3) is likely to result from some monovalent ions going away from the M/O interface, before they are further oxidized electrochemically, and reacting chemically with water further away in the oxide or at the O/S interface.23,24 If such a mechanism was operative with activation-controlled kinetics,25 the current-potential relationship should be given by the Butler-Volmer equation... 

In this notation, anodic current is positive, while cathodic current is negative. As the later section on oxygen reduction will show, the Tafel slope can change with overpotential. This is because the Butler-Volmer law only applies to outer-sphere reactions. Although it can describe electrode reactions, the equation does not account for repulsive interactions of the adsorbates or changes in the reaction mechanism as potential is changed. 

The electrochemical reaction occurs at the surface of graphite anode [37 39]. At potentials lower than 1.25 V, chlorine is formed by a Volmer/Heyrovsky mechanism with the latter being the rate determining step. Chloride ions are initially discharged on surface sites that are not covered by chlorine atoms (Volmer reaction (14.4a)), followed by the discharge of chloride ions on adsorbed chlorine ions (Heyrovsky reaction (14.4b)) [39] ... 

Our chapter has two broad themes. In the first, we will consider some aspects of quantum states relevant to electrochemical systems. In the second, the theme will be the penetration of the barrier and the relation of the current density (the electrochemical reaction rate) to the electric potential across the interface. This concerns a quantum mechanical interpretation of Talel s experimental work of 1905, which led (1924-1930) to the Butler-Volmer equation. 

The rate of electron transfer and its potential dependence can be described by the Butler-Volmer equation (20) (see Section 2). An electron transfer often initiates a cascade of homogeneous chemical reactions by producing a reactive radical anion/cation. The mechanism can be described mathematically by a rate equation for each species these form part of the electrochemical model. The rate law of the overall sequence is probed by the voltammetric experiment. 

It seems that the modihed BDD surface has now a number of RUO2 sites sufficiently high to sustain the required passage of current (which is a function of the applied potential), and accordingly the mechanism does not need any involvement of the diamond surface. This conclusion is further supported by the dependence on pH the Volmer-Heyrovsky sequence of steps does not involve protons and, in fact, a zero reaction order with respect to H has been found for the electrode material under discussion (HMD) [13,26]. 

As already mentioned above, the derivation of the Butler-Volmer equation, especially the introduction of the transfer factor a, is mostly based on an empirical approach. On the other hand, the model of a transition state (Figs. 7.1 and 7.2) looks similar to the free energy profile derived for adiabatic reactions, i.e. for processes where a strong interaction between electrode and redox species exists (compare with Section 6.3.3). However, it should also be possible to apply the basic Marcus theory (Section 6.1) or the quantum mechanical theory for weak interactions (see Section 6.3.2) to the derivation of a current-potential. According to these models the activation energy is given by (see Eq. 6.10)... 

A generalized Volmer-Heyrovsky-Tafel mechanism explained the problem of the intermediate and product adsorption with a nearly zero coverage at potentials approximately equal to that of the net hydrogen evolution process [51]. The electrochemical reaction rates of the individual processes are... 

---