Ideal polarizable metal-solution

Kinetics of Molecular Processes at Ideal Polarizable Metal-Solution Interfaces... 

Based on the discussion above, it seems evident that a detailed understanding of kinetic processes occurring at semiconductor electrodes requires the determination of the interfacial energetics. Electrostatic models are available that allow calculation of the spatial distributions of potential and charged species from interfacial capacitance vs. applied potential data (23.24). Like metal electrodes, these models can only be applied at ideal polarizable semiconductor-solution interfaces (25)- In accordance with the behavior of the mercury-solution interface, a set of criteria for ideal interfaces is f. The electrode surface is clean or can be readily renewed within the timescale of... 

Another progress in our understanding of the ideally polarizable electrode came from theoretical works showing that the metal side of the interface cannot be considered just as an ideal charged plane. A simple quantum-mechanical approach shows that the distribution of the electron gas depends both on the charge of the electrode and on the metal-solution coupling [12,13]. 

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. 

Work in this area has been conducted in many laboratories since the early 1980s. The electrodes to be used in such a double-layer capacitor should be ideally polarizable (i.e., all charges supplied should be expended), exclusively for the change of charge density in the double layer [not for any electrochemical (faradaic) reactions]. Ideal polarizability can be found in certain metal electrodes in contact with elelctrolyte solutions free of substances that could become involved in electrochemical reactions, and extends over a certain interval of electrode potentials. Beyond these limits ideal polarizability is lost, owing to the onset of reactions involving the solvent or other solution components. 

When we discussed the double-layer properties of metal electrodes in contact with an electrolyte solution, we introduced the notion of an ideally polarizable interface, which is marked by the absence of charge-... 

The interface is in contact with two bulk phases, the metal electrode (index m ) and the solution (index s). Formally, we consider the metal to be composed of metal atoms M, metal ions Mz+, and electrons e " these particles are present both in the electrode and the interface, but not in the solution. On the other hand, certain cations and anions and neutral species occur both in the solution and the interface. Since the electrode is ideally polarizable, no charged species can pass through the interface. 

Ideal polarizable interfaces are critical for the interpretation of electrochemical kinetic data. Ideality has been approached for certain metal electrode-solution interfaces, such as mercury-water, allowing for the collection of data that can be subjected to rigorous theoretical analysis. 

Herein, criteria are developed for ideal polarizable semiconductor electrode-solution interfaces. A variety of experimental studies involving metal dichalcogenide-solution interfaces are discussed within the context of these criteria. These interfaces approach ideality in most respects and are well suited for fundamental studies involving electron transfer to solution species or adsorbed dyes. 

Most earlier papers dealt with the mercury electrode because of its unique and convenient features, such as surface cleanness, smoothness, isotropic surface properties, and wide range of ideal polarizability. These properties are gener y uncharacteristic of solid metal electrodes, so the results of the sohd met electrolyte interface studies are not as explicit as they are for mercury and are often more controversial. This has been shown by Bockris and Jeng, who studied adsorption of 19 different organic compounds on polycrystaUine platinum electrodes in 0.0 IM HCl solution using a radiotracer method, eUipsometry, and Fourier Transform Infrared Spectroscopy. The authors have determined and discussed adsorption isotherms and the kinetics of adsorption of the studied compounds. Their results were later critically reviewed by Wieckowski. ... 

Mercaptohexadecanol, adsorption, 979 Mercury in electrode kinetics, 1093, 1195 Mercury solution interface, ideal polarizable interface, 848 Metal capacity, 888 determination. 890 -water interactions, 896, 897... 

Consider mercury as the liquid metal under study. One of the advantages of this metal is that the mercuiy/solution interface approaches closest to the ideal polarizable interface (see Section 6.3.3) over a range of 2 V. What this means is that this interface responds exactly to all the changes in the potential difference of an external source when it is coupled to a nonpolarizable interface, and there are no complications of charges leaking through the double layer (charge-transfer reactions). 

The liquid metal mercury-solution interface presents the advantage that it approaches closest to an ideal polarizable interface and, therefore, it adopts the potential difference applied between it and a non-polarizable interface. For this reason, the mercury-solution interface has been extensively selected to carry out measurements of the surface tension dependence on the applied potential. In the case of other metal-solution interfaces, the thermodynamic study is much more complex since the changes in the interfacial area are determined by the increase of the number of surface atoms (plastic deformation) or by the increase of the interatomic lattice spacing (elastic deformation) [2, 4]. 

In the mechanisms to be described in this section, one of the idealizations of electrochemistry is being portrayed. Thus, in perfectly polarizable metal electrodes, it is accepted that no charge passes when the potential is changed. However, in reality, a small current does pass across a perfectly polarizable electrode/solution interphase. In the same way, here the statement free from surface states (which has been assumed in the account given above) means in reality that the concentration of surface states in certain semiconductors is relatively small, say, less than 10 states cm. So when one refers to the low surface state case, as here, one means that the surface of the semiconductor, particularly in respect to sites energetically in the energy gap, is covered with less than the stated number per unit area. A surface absolutely free of electronic states in the surface is an idealization. (If 1012 sounds like a large number, it is in fact only about one surface site in a thousand.) A consequence of this is the location of the potential difference at the interphase of a semiconductor with a solution. As shown in Fig. 10.1(a), the potential difference is inside the semiconductor, and outside in the solution there is almost no potential difference at all. 

There is no thermodynamic equilibrium between the ideally polarizable electrode (more exactly the metal phase) and the solution phase because there is no common component capable of changing its charge and being transferred between the phases, conditions necessary for equilibrium. The state of an ideally polarizable electrode is well defined only if an external source is used to maintain a constant polarization potential, i.e., the double-layer capacitor charged with a definite charge. The polarization potential is an independent parameter of the system. 

For ideally polarizable electrodes - since as a whole, the double layer is electrically neutral - the absolute value of the -> surface charge on the metal (opposite charge accumulated at the solution phase near the metal (surface charge density and for the ideally polarizable electrode it is equal to the surface charge density (Q), i.e., electrocapillary measurements. When oM = os = 0, i.e., at the -> potential of zero charge (pzc, Ea = Eq = 0) the - Galvani potential difference between the two phases is due to the orientation of dipoles (e.g., water molecules) [i.v]. 

The double-layer structure at the electrochemically polished and chemically treated Cd(OOOl), Cd(lOiO), Cd(1120), Cd(lOil). and Cd(1121) surface electrodes was studied using cyclic voltammetry, impedance spectroscopy, and chronocoulometry [9, 10]. The limits of ideal polarizability, pzc, and capacity of the inner layer were established in the aqueous surface inactive solutions. The values of Ep c decrease, and the capacity of the inner layer increases, if the superficial density of atoms decreases. The capacity of metal was established using various theoretical approximations. The effective thickness of the thin metal layer increases in the sequence of planes Cd(1120) < Cd(lOiO) < Cd(OOOl). It was also found that the surface activity of C1O4 was higher than that of F anions [10]. 

In examining the properties of the metal solution interfaee, two limiting types of behavior are found, namely, the ideal polarizable interfaee and the ideally non-polaiizable interface. In the former case, the interface behaves as a capacitor so that charge can be placed on the metal using an external voltage source. This leads to the establishment of an equal and opposite charge on the solution side. The... 

The investigation of the effect made by the applied potential difference on the interfacial tension can be most conveniently carried out on the ideally polarizable surface of liquid metal (most commonly mercury) in aqueous electrolyte solution. It is important that in these experiments one be able to simultaneously measure the potential difference between phases (with respect to some standard electrolyte) and the interfacial tension. The latter is usually... 

An electrode at which no charge transfer can occur across the metal-solution interface, regardless of the potential imposed by an outside source of voltage, is called an ideal polarized (or ideal polarizable) electrode (IPE). While no real electrode can behave as an IPE over the whole potential range available in a solution, some electrode-solution systems can approach ideal polarizability over limited potential ranges,. For example, a mercury electrode in contact with a deaerated potassium chloride solution approaches the behavior of an IPE over a potential range about 2 V wide. At sufficiently positive potentials, the mercury can oxidize in a charge-transfer reaction ... 

In practice the main requirement of a reference electrode is that it has a stable potential and that this is not substantially changed during the experiment. This is the case with the hypothetical, completely non-polarizable electrode, the potential of which is unaffected when electric current flows across the metal-solution interface. For practical conditions this means that the exchange current must be large compared with any net current that it is required to pass in use. Ideally "no" current flows through the reference electrode (in a three electrode system) if a high imp ance (>10Mfl) voltmeter is used. 

Most of the events in electrochemistry take place at an interface, and that is why interfacial electrochemistry constitutes the major part of electrochemical science. Relevant interfaces here are the metal-liquid electrolyte (LE), metal-solid electrolyte (SE), semiconductor-electrolyte, and the interface between two immiscible electrolyte solutions (ITIES). These interfaces are chargeable, that is, when the external potential is applied, charge separation of positive and negative charges on the two sides of the contact occurs. Such an interface can accumulate energy and be characterized by electric capacitance, within the range of ideal polarizability beyond which Faraday processes turn on. 

The principal object of electrochemical interest is given by another type of electrified interface, contacts of an electronic (liquid or solid metal, semiconductor) and an ionic (liquid solution, SEs, membranes, etc) conductor. For numerous contacts of this kind, one can ensure such ionic composition of the latter that there is practically no dc current across the interface within a certain interval of the externally apphed potential. Within this potential interval the system is close to the model of an ideally polarizable interface, the change of the potential is accompanied by the relaxation current across the external circuit and the bulk media that vanishes after a certain period. For sufficiently small potential changes, d , the ratio of the integrated relaxation current, dQ, to dE is independent of the amplitude and it determines the principal electrochemical characteristics of the interface, its differential capacitance per unit surface area, C ... 

Despite of this charge transfer many of these systems may be treated as ideally polarizable electrodes, if the adsorbed species are not transformed into a different component present inside the bulk phase. The latter condition is violated, for example, in the hydrogen adsorption at metals of the platinum group in which the adsorbed hydrogen atoms can be in equilibrium with protons in solution and hydrogen molecules in gas phase or hydrogen dissolved inside the metal. The latter system corresponds to perfectly polarizable electrodes, see Ref. [13] for further discussion. 

---