Large overpotentials

For iron it is reasonably well established that the reaction goes by way of chemical recombination under most circumstances, although there is some evidence that electrochemical desorption may take over in very alkaline solutions or at large overpotentials. 

In the diffusion-controlled domain (preferable in situations with large overpotentials) a diffusion layer is formed. This layer is found on the solution side of solid-state membranes it is located with in the membrane surface of liquid and glass membranes. 

This is considerably higher than that of an H2-O2 fuel cell (i.e., 83%). However, under normal operating conditions, at a current density j, the electrode potentials deviate from their equilibrium values as a result of large overpotentials, r, at both electrodes (Fig. 5) ... 

The current increases first exponentially and then levels off. The same dependence follows from Eq. (34.27). At not large deviations of the electrode potential from the equilibrium potential (i.e., at not large overpotentials, r = Eq - E), the approximate form of Eq. (34.27) is as follows ... 

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). 

Both the frequency of the well and its depth cancel, so that the free energy of activation is determined by the height of the maximum in the potential of mean force. The height of this maximum varies with the applied overpotential (see Fig. 13). To a first approximation this dependence is linear, and a Butler-Volmer type relation should hold over a limited range of potentials. Explicit model calculation gives transfer coefficients between zero and unity there is no reason why they should be close to 1/2. For large overpotentials the barrier disappears, and the rate will then be determined by ion transport. 

It is often useful to replace this equation by Eq. (5.2.24) modified for an irreversible process (occurring at large overpotentials), which has the following form for cathodic processes ... 

Since noncatalyzed carbon dioxide reduction shows a large overpotential and potentials far more negative than -2.0 V versus SCE... 

The reduction of C02 can be driven electrochemically at metallic cathodes, however, it requires large overpotentials (<—1.5 V) and electrode poisoning often occurs.65 Those problems can be addressed by adding catalysts. Metal complexes are a priori good candidates as electrocatalysts. It is expected that their reduction will be accompanied by the appearance of a vacant coordination site able to bind C02 and thus activate its reduction in the metal coordination sphere.1... 

Simon et al. [92] investigated a biocatalytic anode based on lactate oxidation by lactate dehydrogenase (LDH). The anodic current is generated by the oxidation of NADH (produced by NAD+ and substrate) while LDH catalyzes the electro-oxidation of lactate into pyruvate. As previously mentioned, the oxidation of NADH at bare electrodes requires a large overpotential, so these authors used poly(aniline) films doped with polyanions to catalyze NADH oxidation. Subsequent research by this group focused on targeting mutants of LDH that are amenable to immobilization on the polyaniline surface [93],... 

At higher overpotentials the second-order terms become important, and Eq. (6.9) is no longer valid. At very large overpotentials, when eorj > A, Eq. (6.8) even predicts a decrease of the current with increasing overpotential, i.e., a negative resistance. However, better versions of this theory to be presented in the following section do not show this behavior. 

The complete current-potential relation is shown in Fig. 6.3. For small overpotentials we observe Butler-Volmer behavior, for large overpotentials a limiting current. 

Equation (14.6) predicts that for constant in and large overpotential r/ the current becomes equal to Bvery large, the current is transport controlled, the surface concentration is negligible, and j = ] nn = Btn 1/2, the limiting current density. This remains true for the scheme of Eq. (14.7) as long as the dissociation... 

Tafel s equation (eqn (30)) is accurate at large overpotentials, but fails as q approaches zero. The Tafel plot is obtained by plotting rj vs. log i, with b referred to as the Tafel slope. The Tafel slope is a function of the transfer coefficients and temperature, where... 

Electrochemistry is in many aspects directly comparable to the concepts known in heterogeneous catalysis. In electrochemistry, the main driving force for the electrochemical reaction is the difference between the electrode potential and the standard potential (E — E°), also called the overpotential. Large overpotentials, however, reduce the efficiency of the electrochemical process. Electrode optimization, therefore, aims to maximize the rate constant k, which is determined by the catalytic properties of the electrode surface, to maximize the surface area A, and, by minimization of transport losses, to result in maximum concentration of the reactants. 

Smooth platinum, lead dioxide and graphite are anode materials commonly used in electrooxidation processes. All show large overpotentials for oxygen evolution in aqueous solution. Platinum coated titanium is available as an alternative to sheet platinum metal. Stable surfaces of lead dioxide are prepared by electrolytic oxidation of sheet lead in dilute sulphuric acid and can be used in the presence of sulphuric acid as electrolyte. Lead dioxide may also be electroplated onto titanium anodes from lead(Il) nitrate solution to form a non-porous layer which can then be used in other electrolyte solutions [21],... 

Mercury, lead, cadmium and graphite are commonly used cathode materials showing large overpotentials for hydrogen evolution in aqueous solution. Liquid mercury exhibits a clean surface and is very convenient for small-scale laboratory use. Sheet lead has to be degreased and the surface can be activated in an electrochemical oxidation, reduction cycle [3, 22], Cadmium surfaces are conveniently prepared by plating from aqueous cadmium(ii) solutions on a steel cathode. 

With an n-type Si electrode, the reduction current density increases exponentially with decreasing potential, the apparent Tafel slope was found equal to 140-160 mV/decade. This is much higher than the 60mV/decade required for the processes that are limited by the supply of electrons from the semiconductor whose space charge is under the accumulation regime. In other words, the HER at the Si surface is a slow electron transfer, that is, a relatively large overpotential is required to... 

Fig. 5.4 Current-potential relations for electrode reactions with (a) a large exchange current (small overpotential) and (b) a small exchange current (large overpotential).

On the other hand, if the exchange current is very small and a large overpotential is needed for the current to flow, the electrode process is said to be irreversible. In some cases, the electrode process is reversible (the overpotential is small) for small current values but irreversible (the overpotential is large) for large current values. Such a process is said to be quasi-reversible. 

Table 17-1 showed that there is a large overpotential for reduction of H+ at the Hg surface. Reactions that are thermodynamically less favorable than reduction of H+ can be carried out without competitive reduction of H+. In neutral or basic solutions, even alkali metal (Group 1) cations are reduced more easily than H+. Furthermore, reduction of a metal into a mercury amalgam is more favorable than reduction to the solid state ... 

Two limiting forms of the Butler-Volmer equation of experimental interest are concerned with the current response of the system at both small and large overpotentials. For small overpotentials (rj < 8 mV/n), the exponential terms may be linearized (remember that e x = 1 - x for small x), so that... 

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