Metal-electrolyte interface, double layer

The model more generally accepted for metal/electrolyte interfaces envisages the electrical double layer as split into two parts the inner layer and the diffuse layer, which can be represented by two capacitances in series.1,3-7,10,15,32 Thus, the total differential capacitance C is equal to... 

Non-situ and ex situ studies can provide important information for understanding the properties of metal/electrolyte interfaces. The applicability of these methods for fundamental studies of electrochemistry seems to be firmly established. The main differences between common electrochemical and UHV experiments are the temperature gap (ca. 300 vs. 150 K) and the difference in electrolyte concentration (very high concentrations in UHV experiments). In this respect, experimental research on double-layer properties in frozen electrolytes can be treated as a link between in situ experiments. The measurements of the work functions... 

Figure 5. A small portion of the electrochemical double layer at the tumor cell-extracellular fluid (electrolyte) interface is shown to depict the microscopic structure and the potential drops involved, by analogy with the metal-electrolyte interface taken from Conway.47...

Since the metal can be treated as a nearly perfect conductor, C is high compared with C, and cannot influence the value of the measured doublelayer capacitance. The role of the metal in the double layer structure was discussed by Rice, who suggested that the distribution of electrons inside the metal decides the properties of the double-layer. This concept was later used to describe double-layer properties at the semiconductor/electrolyte interface. As shown later, the electron density on the metal side of the interface can be changed under the influence of charged solution species (dipoles, ions). ... 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

We start this chapter with electrocapillarity because it provides detailed information of the electric double layer. In a classical electrocapillary experiment the change of interfacial tension at a metal-electrolyte interface is determined upon variation of an applied potential (Fig. 5.1). It was known for a long time that the shape of a mercury drop which is in contact with an electrolyte depends on the electric potential. Lippmann1 examined this electrocapillary effect in 1875 for the first time [68], He succeeded in calculating the interfacial tension as a function of applied potential and he measured it with mercury. 

In its most simple form, this means without effects such as adsorption or formation of coatings at the electrode surface36. The resistance, Rc, represents electrical conductivity of the electrolyte and is not a property of the electrode itself. The differential double-layer capacity, Cmetal surface of the metal-electrolyte interface, which is in equilibrium with an equal excess of charge but opposite in sign at the side of the electrolyte. 

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 total potential drop Apotential drop at the metal/oxide interface, the potential drop in the oxide, the potential drop in the Helmholtz layer and the potential drop in the electrolyte (diffuse double layer) ... 

Fig. 5.2 Model for the double layer region at the metal-electrolyte interface IHP, inner Helmholtz plane OHP, outer Helmholtz plane. (After ref. [4])...

EOD is based on the electrically induced flow (namely, electro-osmosis) of water trapped between the clay particles (Fig. 2). Such electrically induced flow is possible because of the presence of the electrochemical double layer at the clay/water interface in this double layer (Fig. 2), the charges on the clay surface are electrically balanced by the opposite charges in the water this water is actually an electrolyte because of the presence of some salts, hydronium or hydroxyl ions, etc. The structure and potential gradients of such a double layer are shown in Fig. 3 by analogy with a metal/electrolyte interface. 

At low overpotentials nucleation of the Ni deposit starts at the elbows of the reconstruction (Fig. 4(b)), followed by anisotropic growth of monolayer islands perpendictilar to the double rows of the reconstruction (Fig. 4(c)). For multilayer coverages a layer-by-layer growth of the Ni thin film was observed up to thicknesses of six layers. Under UHV conditions, this system exhibits a similar nucleation behavior but the subsequent growth proceeds isotropically and in a more three-dimensional fashion [27]. Both in UHV and in file electrochemical environment the nucleation of islands is preceded by the formation of dqiressions at the elbows. This indicates that Au surface atoms at these sites are replaced by Ni atoms, which subsequently act as centers for adlayer island nucleation [28]. Ihis demonstrates r-reaching mechanistic similarities for deposition at the metal-vacuum and the metal-electrolyte interface, even in complex cases. 

Spohr describes in detail the use of computer simulations in modeling the metal/ electrolyte interface, which is currently one of the main routes towards a microscopic understanding of the properties of aqueous solutions near a charged surface. After an extensive discussion of the relevant interaction potentials, results for the metal/water interface and for electrolytes containing non-specifically and specifically adsorbing ions, are presented. Ion density profiles and hydration numbers as a function of distance from the electrode surface reveal amazing details about the double layer structure. In turn, the influence of these phenomena on electrode kinetics is briefly addressed for simple interfacial reactions. 

In the oldest theory of the electrode-electrolyte interface (compact layer theory), the double layer was considered analogous to a parallel plate condenser with a plane of charges on the metal side and a second plane of opposite charges on the solution side (4,5). According to this theory, the capacity of the double layer should be independent of potential which is not observed experimentally. 

A double-layer term A ifUPD A) is included in the formation reaction which takes into account changes of the double layer structure when the substrate/electrolyte interface (A) is substituted by the UPD metal/electrolyte interface. If the chemisorption from the... 

An electrical double layer is formed at a semiconductor-electrolyte interface, similar to a metal-electrolyte interface [1-3]. The difference, however, is in the charge distribution between the two interfaces. At a metal-electrolyte interface, the charge on the metal side is localized just at the metal surface, whereas, at a semiconductor-electrolyte interface, the charge on the semiconductor side is distributed deep in the interior of the semiconductor, forming a wide space charge layer. In concentrated electrolyte solutions ( 0.5 M and higher), the charge on the electrolyte side is localized at the (outer) Helmholtz layer for interfaces with either metals or semiconductors. 

S. Sevastyanov, and A. Popov, J. Electroanal. Chem. Interfacial Electrochem. 145(2), 225 (1983) B. E. Conway, The Solid-Electrolyte Interface, Nato Conf. Ser., Ser. 6(5), 497 (1983) G. A. Martynov and R. R. Salem, Electronic Capacitor at a Metal/Electrolyte Interface, Elektrokhimiya 19, 1060-1070 (1983) and G. A. Martynov and R. R. Salem, Lecture Notes in Chemistry, Vol. 33 Electrical Double Layer at a Metal-Dilute Electrolyte Solution Interface, Springer-Verlag, Berlin (1983) also B. W. Ninham, Surface Forces— The Last 30 Angstrom, Pure Appl. Chem. 53, 2135-2147 (1981). 

Procednres for preparing electrode snrfaces (Section 4.10), the technical aspects of measnring spectra at the metal-electrolyte interface (Section 4.6), and the problems that can arise in interpreting the resnlting spectra have already been considered (Section 3.7). The contribntion of IR SEC stndies to an nnderstanding of the adsorption of CO and NO and small organic molecnles (methanol, ethanol, formic acid, etc.), the rednction of CO2 on ordered noble metals, electrochemical polymerization, and the strnctnre of the electrochemical double layer (DL) have been discussed in varions recent reviews [635, 638, 641]. Below, the information that can be obtained from the IR SEC measurements is listed and two IR SEC studies of the DL structure are considered. An example in which in sitn IRRAS is used to follow peptide oxidation on a Pt electrode is discussed in Section 7.8.1. 

FIGURE 4.4 Schematic representation of the Stem model of the stmcture of the double layer at the metal-electrolyte interface showing the ions and water moleeules. The inner and outer Helmholtz planes are labeled, along with the dilluse double layer. In the figure, the metal has been positively polarized. 

A. Komyshev, W. Schmickler, and M. Vorotyntsev [1982] Nonlocal Electrostatic Approach to the Problem of a Double Layer at a Metal-Electrolyte Interface, Phys. Rev. B25, 5244-5256. 

Figure 343 Electrical double layer at metal-electrolyte interface in the presence of chemisorbed anions.

The double layer at a semiconductor-electrolyte interface differs from that found at metal-electrolyte interfaces in that the charges in the sohd are distributed over a certain thickness, the space charge layer. 

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