Electrosorption electrode

Researchers have described the electrosorption of organics, ions, and bacteria onto carbon [61-65] and polypyrrole composite [66]. Electrosorption electrodes are usually porous since their sorption capacity relates to their surface area the electrodes do not normally fulfill any separation function. Bladergroen and Linkov described the tubular electrosorption... 

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

Claviher J, Orts JM, Gomez R, Pehn JM, Aldaz A. 1996. Comparison of electrosorption at activated polycrystaUine and Pt(531) kinked platinum electrodes. Surface voltammetry and charge displacement on potentiostatic CO adsorption. J Electroanal Chem 404 281-289. 

Beden B, Lamy C, Bewick A, Kunimatsu K. 1981. Electrosorption of methanol on a platinum electrode. IR spectroscopic evidence for adsorbed carbonyl species. J Electroanal Chem 121 343-347. 

Beden B, Bewick A, Lamy C. 1983. A study by electrochemically modulated infrared reflectance spectroscopy of the electrosorption of formic acid at a platinum electrode. J Electroanal Chem 148 147-160. 

For example, the investigations of the current-generating mechanism for the polyaniline (PANI) electrode have shown that at least within the main range of potential AEn the "capacitor" model of ion electrosorption/ desorption in well conducting emeraldine salt phase is more preferable. Nevertheless, the possibilities of redox processes at the limits and beyond this range of potentials AEn should be taken into account. At the same time, these processes can lead to the fast formation of thin insulation passive layers of new poorly conducting phases (leucoemeraldine salt, leucoemeraldine base, etc.) near the current collector (Figure 7). The formation of such phases even in small amounts rapidly inhibits and discontinues the electrochemical process. 

There is a formal similarity between adsorption and reactions such as metal deposition which gives rise to the concept of electrosorption valence. Consider the deposition of a metal ion of charge number on an electrode of the same material. If the electrode potential 4> is kept constant, the current density j is ... 

The electrosorption valence can be related to the dipole moment of an adsorbed species introduced in Chapter 4. For this purpose consider an electrode surface that is initially at the pzc and free of adsorbate. When a small excess charge density o is placed on the metal, its potential changes by an amount A given by ... 

Electrochemical Surface Characterization of Platinum Electrodes Using Elementary Electrosorption Processes at Basal and Stepped Surfaces... 

The same type of voltammogram has been obtained with a Pt (111) electrode after its ordered surface was subjected to argon ion bombardment, introducing structural defects like randomly distributed steps (14). The similar effects of oxygen electrosorption and ion bombardment show clearly that the former perturbs the surface order,... 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

The influence of the metal on electrosorption was also considered in the adsorption of aromatic compounds. Extensive studies of adsorption of different aliphatic and aromatic compounds on a polycrystalline Pt electrode in O.OIMHCI solution were done by Bockris etal. Using different techniques (radiotracer, FTIR, and ellipsometry), they were found that aromatic molecules are adsorbed parallely to the electrode surface and that the potential dependence of adsorption is symmetrical around the pzc. It was concluded that the dependence of adsorption on the potential is mostly determined by the interaction of water with the Pt surface. 

In any study of electrosorption of neutral molecules on metallic electrodes, the ions of supporting electrolytes should not be specifically adsorbed. Nevertheless, the interaction of the electrolyte ions with the electrode surface may depend on the interaction of the ions with the solvent. Usually, the stronger the ion-solvent interaction, the weaker the adsorption of the ion. Since the ions are more weakly solvated in nonaqueous solvents than in water, the ions that are not adsorbed from aqueous solutions may still be adsorbed from organic solvents. However, even in the absence of... 

The same approach may also apply to the adsorption of redox particles other than the adsorption of proton-hydrogen atom on metal electrodes. To understand electrosorption phenomena, various concepts have been proposed such as the charge transfer coefficient and the adsorption valence [Vetter-Schultze, 1972]. The concept of the redox electron level in adsorbed particles introduced in this textbook is usefiil in dealing with the adsorption of partially ionized particles at electrodes. 

Most frequently, this electrode mechanism has been analyzed in connection with a mercury electrode hence the following reaction schemes are pertinent to this electrode. Note, that the electrosorption mechanism can serve as a theoretical basis for these processes as well [135, 139, 140], The simplest case of an accumula-tion/stripping equilibrium is given by the following equation ... 

E. Ayranci, B.E. Conway, Adsorption and electrosorption of ethyl xanthate and thiocyanate anions at high-area carbon-cloth electrodes studied by in situ UV spectroscopy development of procednres for wastewater pnrification. Anal. Chem., 73, 1181-1189 (2001). 

This method involves electrochemical deposition of the MPc onto CNT-modified electrode surface by repetitive cycling in a concentrated MPc solution (1 mM) within a specific potential window. The first cyclic voltammetric scan is usually similar to subsequent scans, indicating the formation of monomeric species only. Ozoemena et al [11] found that on certain occasions, as reported recently [11] during the electro-deposition of CoTAPc onto a basal plane pyrolytic graphite electrode (BPPGE) pre-modified with SWCNT, both cathodic and anodic waves may decrease continually and then stabilizes at a certain scan (a process known as electrochemical adsorption or simply called electrosorption ). 

The EQCM method is used to evaluate the processes that occur in/on the palladium electrode in acid and basic solutions. It was concluded that hydrogen electrosorption in palladium is accompanied by an additional frequency shift of... 

The study of hydrogen and deuterium electrosorption in palladium limited volume electrodes (LVE) was carried out by the same group in both acidic and basic solutions [124,130,134]. It was found that the hydrogen capacity, H (D)/Pd, measured electrochemically, depends significantly on sweep rate in cyclic voltammetric experiments and also on the thickness of the LVE. Two different mechanisms of hydrogen desorption, that is, the electrochemical oxidation and the nonelectrochemical recombination step, which take place in parallel within the Pd—LVE, have been postulated. 

A physical model and a theory have been proposed [72], which might be helpful in comparative studies on electrocompres-sive behavior of electrodeposited chloride, bromide, and iodide monolayers on the Au(lll) electrode. The theoretical results were in good agreement with the experimental data, which evidence that the adatom-adatom interactions (especially repulsive ones) and electrosorption valency of halide anions determine the compressibility within halide adlayers. Also, Lipkowski et al. have discussed various aspects of adsorption of halide anions on Au(lll) in a review paper [36]. From this paper, we have taken quantitative data concerning adsorption of halide anions on Au(lll) (cf Fig. 3). 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

Sander and Henze [50] have performed ac investigations of the adsorption potential of metal complexes at Hg electrode. Later, Sander etal. [51] have studied electrosorption of chromium - diethylenetriaminepentaacetic acid (DTPA) on mercury in 0.1 M acetate buffer at pH 6.2 using a drop-time method. The changes in the interfacial activity of the Cr(III)-DTPA complex with the bulk concentration obeyed the Frumkin adsorption isotherm. 

The electrosorption parameters of unsaturated cations, at the electrode-solvent interface were determined by phase selective a.c. polarography181 on dropping and... 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

The electrosorption of reactive intermediates and of organic molecules at this interface is generally weak, due to physical adsorption. Nonetheless, in particular if the reactive intermediates are so reactive that they do not survive for much longer than 10-9 sec and therefore cannot escape from the electrode surface, the chemical composition of the adsorbate layer being different from that of the bulk electrolyte composition influences the course of consecutive reactions and their yields and selectivities decisively. 

Partial charge transfer during adsorption is difficult to evaluate because separation of the charge transferred to the electrode and that part of the charge transferred across the double layer to give specifically adsorbed ions cannot be done through measurements of the total charge in the external circuit. Vetter and Schultze [102] defined the electrosorption valence as... 

The electrosorption valency usually increases as the underpotential decreases to approach the ionic charge (total discharge of the cation) close to the Nernst potential, for instance in the case of lead and thallium upd on silver [114]. However, the co-adsorption of anions may contribute to the observed apparent electrosorption valence, as rotating ring disc electrode (RDE) experiments have shown [113]. 

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