Surface electrode potential dependence

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

FIGURE 1.13. The influence of monoenergetic surface states on Mott-Schottky plot with Vn, shifted to a higher or lower electrode potential depending on the charge on the states, after Morrison."... 

As new information, electrode potential dependent nuclear magnetic resonance (NMR) spectra of surface CO on polycrystalline platinum were reported by Slezak and... 

Comparing the results of EC-NMR and IR investigations, we find that the potential dependence of C NMR shift and the vibrational frequency of adsorbed CO are primarily electronic in nature, and originate from changes in the f-LDOS. C NMR results show that CO adsorbed on Pt, either directly from CO gas or from methanol oxidation, have the same electronic properties. That is, the chemisorbed product (surface CO) from CO solutions and from methanol decomposition is the same. The electrode potential dependence of the C NMR spectra of CO adsorbed on Pt and Pd nanoparticles provide direct evidence for electric field induced alterations in the E/ -LDOS. In relation to fuel cell catalysis, EC-NMR investigations of Pt nanoparticles decorated with Ru show that there exist two different kinds of CO populations having markedly different electronic properties. COs... 

Although the electronic conductivity of an interphase that is present on an electrode can be related to various optoelectronic properties that are also measurable with spectroscopic techniques, the direct measurement of surface conductivities is not a spectroelectrochemical method. It is nevertheless a surface sensitive method that provides results closely related to those of other methods discussed in this book. Data on the electrosorption of alcohols on gold electrodes [45] or the electrode potential dependent conductivity of intrinsically conducting polymers [46] have been obtained with in situ surface conductivity measurements. Figure 4.4 shows the electrical resistance of a poly(2-propylaniline) film measured in situ under experimental conditions suppressing any influence of solution phase conduction. The influence of... 

In a typical study of a polycrystalline platinum surface in contact with a solution of 0.5 M H2SO4, an electrode potential dependent intensity as shown in Fig. 5.132 was recorded [770]. 

C) in electrochemistry, and Vco versus electrode potential relationships for CO on Pt hlack. Third, variations in vco can be directly correlated with changes in 27t back-donation the higher the back-donation, the lower the CO stretch frequency. When taken together, all these observations suggest that the electrode potential dependence of both the C NMR shift and the vibrational stretch frequency of adsorbed CO are primarily electronic in nature, and originate from changes in the rLDOS at the metal surface and at the adsorbate, induced by electrode polarization. 

Roques J, Anderson AB (2004) Electrode potential-dependent stages in OHjas formation on the Pt3Cr alloy (111) surface. JElectroehem Soc 151(11) E340-E347... 

PJ. Slezak, A. Wieckowski, Electrode potential dependent NMR. spectra of surface CO on polycrystaUine platinum, J. Electroanal. Chem. 339 (1992) 401-410. 

Figure 10.4 Electrode potential-dependent Raman spectra of hydrogen adsorption at a roughened polycrystalline platinum electrode surface. Reprinted from Ref [27] with permission from Elsevier.

The electrode potential depends on the ion exchange on metal sites of the OH group on the surface which can be expressed as [16]... 

In the 1970s and 1980s the major trend was to study the electrooxidation reaction mechanisms and elucidate the surface adsorbed species acting as intermediates and/or catalytic poisons. This was facilitated by progress in experimental apparatus (e.g., development of methods for electrode potential-dependent analysis of surface and bulk intermediates and reaction products) and introduction of new techniques for easy preparation of clean single crystal electrodes (i.e., the Clavilier method [19]). 

The simplest DFT model for examination of elementary energetics at an electrode surface is the vacuum slab model, illustrated in Figure 3.6(a) and the most basic form of Model 2a in Figure 3.5. DFT studies of catalysis typically begin by considering the adsorption of reacting species. Often, in electrocatalysis, reactants or products may be ionic species, and calculation of their electrode potential dependent adsorption energy is more complicated than for neutral... 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

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