Electrode surfaces, characterization

The dissolved form of O decays to the final electroinactive product via a volume chemical reaction occnrring in the diffusion layer with the volume rate constant (kv), whereas the adsorbed form participates in the surface chemical reaction confined to the electrode surface, characterized by a surface rate constant (kg). These two chemical reactions proceed with different rates due to significant differences between the chenucal nature of dissolved and adsorbed forms of O. Obviously, the mechanisms (2.172)-(2.174) and (2.177) are only limiting cases of the general mechanism (2.178). 

Preparation As compared to single-crystal Ag surfaces, the preparation of pc-Ag electrode may seem to be a relatively simple task. However, a pc-Ag surface, which ensures reproducibility and stabiKty, also requires a special procedure. Ardizzone et al. [2] have described a method for the preparation of highly controlled pc-Ag electrode surface (characterized by electrochemical techniques and scanning electron microscopy (SEM)). Such electrodes, oriented toward elec-trocatalytic properties, were successfully tested in hahde adsorption experiments, using parallelly, single-crystal and conventional pc-Ag rods as references. 

The combination of ultra-high vacuum (UHV) surface science techniques with electrochemical methods of electrode surface characterization (voltammetry, chrono-coulometry) resulted in a spectacular progress in the investigation and molecular level understanding of some processes occurring at electrode/solution interfaces. Evidently the experimental approach strongly depends on the aims of the investigation and the systems to be studied. 

Though important advances were also made in the 1960 s in the area of electrode surface characterization (2), this account will focus almost exclusively on the methods in the second category. In... 

Under complete mass transport control, the rate of reactant supply or product removal determines a limiting current, /l, and this is the maximum possible current for a given reaction. The limiting current, /l, is determined by the mass transport regime close to the electrode surface, characterized by the mass transfer coefficient, k, and related to the relative velocity, v, between the electrode and the electrolyte ... 

Reyter D, Odziemkowski M, Belanger D, Rou L (2007) Electrochemically activated copper electrodes surface characterization, electrochemical behavior, and properties for the electroreduction of nitrate. J Electrochem Soc 154 K36-K44... 

In recent years, advances in experimental capabilities have fueled a great deal of activity in the study of the electrified solid-liquid interface. This has been the subject of a recent workshop and review article [145] discussing structural characterization, interfacial dynamics and electrode materials. The field of surface chemistry has also received significant attention due to many surface-sensitive means to interrogate the molecular processes occurring at the electrode surface. Reviews by Hubbard [146, 147] and others [148] detail the progress. In this and the following section, we present only a brief summary of selected aspects of this field. 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

The presented examples clearly demonstrate tliat a combination of several different teclmiques is urgently recommended for a complete characterization of tire chemical composition and tire atomic stmcture of electrode surfaces and a reliable interiDretation of tire related results. Stmcture sensitive metliods should be combined witli spectroscopic and electrochemical teclmiques. Besides in situ techniques such as SXS, XAS and STM or AFM, ex situ vacuum teclmiques have proven tlieir significance for tlie investigation of tlie electrode/electrolyte interface. 

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

Infrared spectroelectrochemical methods, particularly those based on Fourier transform infrared (FTIR) spectroscopy can provide structural information that UV-visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been fruitful in the characterization of reactions occurring on electrode surfaces. The technique requires very thin cells to overcome solvent absorption problems. 

Figure 3 and 4 show SEM images for the surface of anode catalyst and the cross section of ESC (NiO-YSZ-CeOa I YSZ I (LaSr)Mn03), respectively. The micro structure of the catalyst electrode was characterized by SEM (Hitachi Co., S-4200). The morphology of particles over Ni0-YSZ-Ce02 was uniformly distributed. 

Gasteiger HA, Markovic NM, Ross PN. 1995. Electrooxidation of CO and H2/CO mixtures on a well-characterized Pt3Sn electrode surface. J Phys Chem 99 8945-8949. 

Relatively little work has been done on ORR catalysis by self-assembled mono-layers (SAMs) of metalloporphyrins. The advantages of this approach include a much better defined morphology, structure, and composition of the catalytic film, and the surface coverage, and the capacity to control the rate at which the electrons ate transferred from the electrode to the catalysts [CoUman et al., 2007b Hutchison et al., 1993]. These attributes are important for deriving the catal5d ic mechatfism. The use of optically transparent electrodes aUows characterization of the chemical... 

Simple Fe porphyrins whose catalytic behavior in the ORR has been smdied fairly extensively are shown in Fig. 18.9. Literature reports disagree substantially in quantitative characterization of the catalytic behavior overpotential, stability of the catalysts, pH dependence, etc.). It seems plausible that in different studies the same Fe porphyrin possesses different axial hgation, which depends on the electrolyte and possibly specific residues on the electrode surface the thicknesses and morphologies of catalytic films may also differ among studies. AU of these factors may contribute to the variabUity of quantitative characteristics. The effect of the supporting surface on... 

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

The electrode reaction occurs in the region where by the influence of the electrode charge an electric field is formed, characterized by the distribution of the electric potential as a function of the distance from the electrode surface (see Section 4.3). This electric field affects the concentrations of the reacting substances and also the activation energy of the electrode reaction, expressed by the quantities wT and wp in Eq. (5.3.13). This effect can be... 

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

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