Electric Double Layer at Metal Electrodes

Hg/OH on which the standard electrochemical free enthalpy of anion adsorption is positive AGli 0), only the ph3 ical adsorption of hydrated anions occurs at the OHP whereas, the chemisorption of anions takes place at the IHP on those electrodes whose standard electrochemical free enthalpy of anion adsorption is negative 0). 

The relationship between the activity of adsorbed ions fiQO in Eqn. 5-21 and the adsorption coverage 6i is known as an adsorption isotherm. Equations 5-23 and 5-24 show simple adsorption isotherms  

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Fig. 5-19. Inteifadal charge at an electric double layer on metal electrodes (a) negative charge on the electrode side without contact ion adsoiption, G>) positive charge on the electrode side without contact ion adsorption, (c) positive charge on the electrode side with contact anion adsorption.

Pig. 3. Representation of the electrical double layer at a metal electrode—solution interface for the case where anions occupy the inner Helmholtz plane... 

The electrical double layer at Hg, Tl(Ga), In(Ga), and Ga/aliphatic alcohol (MeOH, EtOH) interfaces has been studied by impedance and streaming electrode methods.360,361 In both solvents the value ofis, was independent of cei (0.01 < cucio4 <0.25 M)and v. The Parsons-Zobel plots were linear, with /pz very close to unity. The differential capacity at metal nature, but at a = 0,C,-rises in the order Tl(Ga) < In(Ga) < Ga. Thus, as for other solvents,120 343 the interaction energy of MeOH and EtOH molecules with the surface increases in the given order of metals. The distance of closest approach of solvent molecules and other fundamental characteristics of Ga, In(Ga), Tl(Ga)/MeOH interfaces have been obtained by Emets etal.m... 

The electrical double layer at pc-Zn/fyO interfaces has been studied in many works,154 190 613-629 but the situation is somewhat ambiguous and complex. The polycrystalline Zn electrode was found to be ideally polarizable for sufficiently wide negative polarizations.622"627 With pc-Zn/H20, the value of Eg was found at -1.15 V (SCE)615 628 (Table 14). The values of nun are in reasonable agreement with the data of Caswell et al.623,624 Practically the same value of Eff was obtained by the scrape method in NaC104 + HjO solution (pH = 7.0).190 Later it was shown154,259,625,628 that the determination of Eo=0 by direct observation of Emin on C,E curves in dilute surface-inactive electrolyte solutions is not possible in the case of Zn because Zn belongs to the group of metals for which E -o is close to the reversible standard potential in aqueous solution. 

In the active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

The formation of an electrical double layer at a metal-solution interface brings about a particular arrangement of atoms, ions and molecules in the region near the electrode surface, and an associated variation in electrical potential with distance from the interface. The double layer structure may significantly affect the rates of electrochemical reactions. 

Recently, the Pt NMR of commercial fuel cell electrode material has been observed 180,181) (Fig. 61). This material consists of platinum supported on carbon black and pressed into graphitized-carbon cloth. (Similar material has been used to study NMR see Section IV.G.) Because of the conducting nature of the carrier, one might expect to see differences with respect to NMR of particles supported on oxides. Furthermore, if an electrolyte is present in the NMR sample, the electric double layer at the metal/electrolyte interface might influence the Pt surface signal. 

The homogeneous models assttme three phases, i.e., metal, polymer film arrd an electrolyte solutiott. Electrorric, tttixed electrorric (electron or polaron) and iotric charge trarrsport processes are cotrsidered in the metal, within the polymer film and in the solutiott, respectively. The polymer phase itself consists of a polymer matrix with incorporated ions arrd solvent molecrrles. A one-dimensional model is used, i.e., the spatial changes of all qttantities (concerrtrations, potential) within the film are described as a function of a single coordirrate x, which is a good approach when an electrode of usual size is used. The metal Ipolymer and the polymer solution interfacial boundaries are taken as planes. Two intetfacial poterttial differences are considered at the two interfaces, and a potential drop inside the film when crrrrerrt flows. The thicknesses of the electric double layers at the irrterfaces are small in... 

The first reaction is an electron transfer across the double layer at the electrode-electrolyte interface between redox species in the electrolyte that exchange electrons with a metal electrode, the second one is an ion-transfer reaction across the double layer since the electron lost by the Cu atom remains at the metal. The third one is an ion-transfer process across the water-organic solvent interface or ion transfer at immiscible electrolyte solutions (ITIES) without the transfer of electrons. In all cases the electrochemical reaction takes place at an electrified interface and therefore the rate of these reactions follow similar exponential dependence on the interfacial electrical potential. 

The incorporation of microcapsules into the plating bath also influences the electrical double layer at the metal-solution interface and the differential capacitance. Usually, a surfactant is added to the bath to induce adsorption onto the electrode, and this reflects a distinct decrement of the differential capacitance on the corresponding curves. 

It can be concluded from the above-mentioned differential capacitance curve and cathodic polarization curve analyses that the mechanistic model of electrolytic codeposition of Hquid microcapsules is associated with the stable chelation of—OH and -O groups of the wall material (e.g., PVA, gelatin) with metal ions (e.g., Ni ", Cu +), and this gives rise to positively charged microcapsules. This in turn helps to accelerate the electrophoretic migration of microcapsules in the plating solution. The liquid microcapsules were also adsorbed onto the electrode due to the presence of a surfactant. Consequently, it is feasible for microcapsules to enter the electrical double layer at the interface and to become embedded in the co-deposited coating. 

On solid metals the situation for ion deposition or dissolution in electrode reactions is much more complicated. The models for crystal growth from the vapor phase or atomic evaporation have to be applied, being modified by ion discharge or ion formation in passing the electrical double layer at the interface. Figure 2.31 represents the main positions of atoms on the surface of a low index face of a metal with one monoatomic step. It is assumed that the edge of the step is not smooth and contains several kink sites. 

The applied electrode potential generates an electrical double layer at the metal/polymer interface, with excess ionic charge accumulating in the polymer phase near the interface (we assume that the polymer is neither oxidized nor reduced in the potential range Ei-Et). According to the concepts governing the distribution of... 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

Mishuk et a/.675,676 have applied the modified amplitude demodulation method to electrochemically polished pc-Bi in aqueous NaF solution. The curves of the real component of the nonlinear impedance Z" as a function of the electrode potential, unlike pc-Cd and pc-Pb, intersect for various cNaF at E - -0.62 V (SCE),674 i.e., at Ea=0 for pc-Bi, as obtained by impedance.666-672 The different behavior of pc-Bi from pc-Cd and pc-Pb at a > 0 has been explained by the semimetallic nature of pc-Bi electrodes. A comparison of inner-layer nonlinear parameter values for Hg, Cd, and Bi electrodes at a < 0 shows that the electrical double-layer structure at negative charges is independent of the metal.675,676... 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

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