Polarizable metallic electrodes

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. 

The development of microscopic models of the double layer began over 100 years ago with work of Helmholtz [20]. He assumed that the charge on the polarizable metal electrode is exactly compensated by a layer of ionic charge in solution located at a constant distance from the geometrical electrode solution interface. The separation distance was assumed to have molecular dimensions. This simple model which gave rise to the term double layer is the equivalent of a parallel-plate capacitor with a capacitance given by... 

In the case where the ionic species in the aqueous electrolyte are fairly hydrophilic and the organic phase features hydrophobic ions, the liquid]liquid junction behaves similarly to an ideally polarizable metal electrode. Under this condition, the Galvani potential difference can be effectively controlled by a four-electrode potentiostat [4,5]. A schematic representation of a typical electrochemical cell is shown in Fig. 1 [6]. Cyclic voltammo-grams illustrating the potential window for the water] 1,2-dichloroethane (DCE) interface for various electrolytes are also shown in Fig. 1. In the presence of bis(triphenylpho-sphoranylidene)ammonium hexafluorophosphate (BTPPA PFe) the supporting electrolyte in DCE, the potential window is limited to less than 200 mV due to the hydrophilicity of the anion. Wider polarizable potential ranges are obtained on replacing... 

Several recent KMC studies of electrode reactions and electrode/electrolyte interfaces of electrochemical cells have been reported. Peterson et al. combined KMC with MD simulations to model the behavior of polarizable metallic electrodes held at a constant potential and separated by an electrolyte. Marcus theory ° was used to calculate the electron transport rates. This... 

Surface studies are difficult in the case of many metal electrodes since their regions of ideal or perfect polarizability are very narrow that is, the potentials of anodic dissolution (or oxidation) of the metal and of cathodic hydrogen evolution are close... 

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. 

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... 

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. ... 

Now consider a polarizable interface that consists of a metal electrode in contact with a solution of a l l-valent electrolyte (i.e., Z+ = 1 and z = -1). It will be remembered that in order to apply electrocapillaiy thermodynamics to a polarizable interface Mj/S, the interface has to be assembled in a cell along with a nonpolarizable interface. Suppose that the nonpolarizable interface is one at which negative ions interchange charge with the metal surface, i.e., Zj = — 1. Hence, Eq. (6.99) for the polarizable interface becomes... 

What can we do for reversible (non polarizable) electrodes In this case we use the fact that different processes occur at different timescales. Instead of the relatively slow change of the voltage in cyclic voltametry, AC potentials with varying frequencies are applied and the current is detected. The method is called impedance spectroscopy. Using impedance spectroscopy, even semiconducting [100] or insulating materials can be analysed by coating them onto metallic electrodes. 

The ET reaction at the polarizable OAV interface thus described is formally similar to that at a metal electrode surface. This allows us to employ usual electrochemical... 

It has been reported that the dielectric constant of thin BST films decreases with decreasing film thickness when metal electrodes are adopted even without any interfacial non-perovskite material. This is due to the intrinsic interfacial low dielectric layer that originates from the termination of the chemical bonding of the perovskite structure at the interfaced. The large dielectric, polarizability of the perovskite can not penetrate into the metal layer due to the extremely high carrier concentration of the metal. 

The absolute potential of the hydrogen electrode may be estimated by another route using the well defined properties of a polarizable mercury electrode [2]. Since the metal only acts as a source or a sink for electrons for the hydrogen electrode reaction, one could use the half-cell... 

This result shows that the relationship between the PZC and the work function is simple only when the dipole contributions are independent of metal nature. The contribution from the reference electrode is constant, provided the electrolyte solution remains unchanged when the polarizable metal is changed. As will be shown below, the dipole potential terms definitely depend on the nature of the metal. 

The most recent experimental work has involved studies of organic adsorption at the single crystal faces of polarizable solid metal electrodes [57]. These experiments provide details of the role of the metal in organic adsorption. By examining these data within the context of the new molecular descriptions of interfacial adsorption the theory of this important process will be greatly advanced. 

The knowledge of the work function of the electrode metal is essential to obtain VNHE(vac. scale) by using Eq. (20). Physical quantities are best known for the perfectly polarizable Hg electrode and it is possible to write for the potential of zero charge of this metal... 

For (ideally) polarizable metals (i.e. electrodes having a large energy barrier for... 

Within the voltage limits set by the thermodynamic stability range of the electrolyte, foreign metal electrodes may sometimes be regarded as ideally polarizable or blocking. The metal electrodes must not react with the electrolyte, and for the moment adsorption and underpotential deposition will be neglected. From an electrochemical point of view, this is the simplest type of interface and has furnished much of the information we have about the electrified interface. 

Gold and mercury electrodes display ideally polarizable behavior in a broad range of electrode potentials. Under these conditions, the metal solution interface behaves as a capacitor. The capacity of the electrode decreases when organic molecules are present at the interface [28,29]. The measurement of the electrode capacity provides a convenient tool to study the spreading of an insoluble mono-layer onto a metal electrode surface. The capacity may be measured by applying a linear voltage sweep to the electrode and recording the voltammetric current (CV-cyclic... 

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