Electric electrochemical interfaces

M. R. Philpott, J. N. Glosli. Molecular dynamics simulation of interfacial electrochemical processes electric double layer screening. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 2, pp. 13-30. 

This chapter is devoted to the behavior of double layers and inclusion-free membranes. Section II treats two simple models, the elastic dimer and the elastic capacitor. They help to demonstrate the origin of electroelastic instabilities. Section III considers electrochemical interfaces. We discuss theoretical predictions of negative capacitance and how they may be related to reality. For this purpose we introduce three sorts of electrical control and show that this anomaly is most likely to arise in models which assume that the charge density on the electrode is uniform and can be controlled. This real applications only the total charge or the applied voltage can be fixed. We then show that predictions of C < 0 under a-control may indicate that in reality the symmetry breaks. Such interfaces undergo a transition to a nonuniform state the initial uniformity assumption is erroneous. Most... 

It is, of course, usual in discussing the electrochemical interface to use a dielectric constant, which is the ratio of the electric displacement to the electric field. By Fourier transforming the dielectric function e(k), one would obtain an effective dielectric constant, which would, however, depend on position. In fact,48 the screening... 

Electrochemical interfaces are sometimes referred to as electrified interfaces, meaning that potential differences, charge densities, dipole moments, and electric currents occur. It is obviously important to have a precise definition of the electrostatic potential of a phase. There are two different concepts. The outer or Volta potential ij)a of the phase a is the work required to bring a unit point charge from infinity to a point just outside the surface of the phase. By just outside we mean a position very close to the surface, but so fax away that the image interaction with the phase can be ignored in practice, that means a distance of about 10 5 — 10 3 cm from the surface. Obviously, the outer potential i/ a U a measurable quantity. 

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

The electrochemical interface is the site where electrode reactions take place. At equilibrium, differences of chemical potential in the electrode and electrolyte bring about differences in electrical potential across the interface. The structure and models of such an electrochemical interface will be discussed in Sect. 2.3. 

In the non-steady state experiment, however, transient currents may be observed which correspond to interfacial processes not arising from chemical changes at the electrode (non-Faradaic processes), but rather from the electrical relaxation of the electrochemical interface. 

The interface between the electrode and the electrolyte or electrochemical interface is the site where heterogeneous electrode reactions occur. The structure and electrical properties of this interfacial region are therefore relevant to electrode kinetics. 

The electrochemical interface has a very large electrical capacity (— 10"s F cm-2) compared with the solid—gas or solid—vacuum interface due to the existence of an ionic space charge localized at a short distance from the electrode. 

The high electrical field across the interface and localized in a small distance as a consequence of the high capacitance of the electrode-solution interface also affects the adsorption phenomena considerably. Mutual interaction of adsorbed ions and dipoles is possible at the electrochemical interface. 

This volume, based on the symposium Photoeffects at Semiconductor-Electrolyte Interfaces, consists of 25 invited and contributed papers. Although the emphasis of the symposium was on the more basic aspects of research in photoelectrochemistry, the covered topics included applied research on photoelectrochemical cells. This is natural since it is clear that the driving force for the intense current interest and activity in photoelectrochemistry is the potential development of photoelectrochemical cells for solar energy conversion. These versatile cells can be designed either to produce electricity (electrochemical photovoltaic cells) or to produce fuels and chemicals (photoelectrosynthetic cells). 

The experimental set-up is shown in Fig. 7-1 an electrochemical interface with low level noise and a transfer function analyzer (TFA) were used for measurements of the EHD impedance. A matched two-channels 24 db/octave low pass filter (F) was used to remove HF noise and the ripple due to electric network supply, this analog filtering allows the TFA to operate with an increased sensitivity. These instruments were controlled by a computer, which recorded the data. 

The electrostatic aspects of electrochemical systems will be introduced first and the electrochemical potential as a key concept is presented (Sects. 1.2-1.4). The electrochemical equilibrium is discussed and Nemst s equation and standard and formal electrode potentials are introduced (Sect. 1.5). The study of electrochemical interfaces under equilibrium ends with the phenomenological and theoretical treatment of the electrical double layer (Sect. 1.6). 

Electrochemistry deals with charged particles that have both electrical and chemical properties. Since electrochemical interfaces are usually referred as electrified interfaces, it is clear that potential differences, charge densities, dipole moments, and electric currents occur at these interfaces. The electrical properties of systems containing charged species are very important for understanding how they behave at interfaces. Therefore, it is important to have a precise definition of the electrostatic potential of a phase [1-6]. Note that what really matters in electrochemical systems is not the value of the potential but its difference at a given interface, although it is illustrative to discuss its main properties. 

The complications and sources of error associated with the polarization resistance method are more readily explained and understood after introducing electrical equivalent circuit parameters to represent and simulate the corroding electrochemical interface (1,16-20). The impedance method is a straightforward approach for analyzing such a circuit. The electrochemical impedance method is conducted in the frequency domain. However, insight is provided into complications with time domain methods given the duality of frequency and time domain phenomena. The simplest form of such a model is shown in Fig. 3a. The three parameters (Rp, Rs, and C d,) that approximate a corroding electrochemical inter-... 

Figure 3 Electrical equivalent circuit model commonly used to represent an electrochemical interface undergoing corrosion. Rp is the polarization resistance, Cd] is the double layer capacitance, Rct is the charge transfer resistance in the absence of mass transport and reaction intermediates, RD is the diffusional resistance, and Rs is the solution resistance, (a) Rp = Rct when there are no mass transport limitations and electrochemical reactions involve no absorbed intermediates and nearly instantaneous charge transfer control prevails, (b) Rp = Rd + Rct in the case of mass transport limitations.

As Schmickler states [3], Electrochemistiy is the study of structures and processes at the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) or at the interface between two electrolytes . The electrode/electrolyte or electrolyte/electrolyte interface is the region whose properties differ from the two adjoining phases, and/or the place where reactant adsorption and electrochemical reactions occur. Commonly, it is recognized as the interface between an electronic conductor (e.g., metals and semiconductors) and an ionic conductor (e.g., electrolyte solutions, molten salts, and solid electrolytes), known as an electrochemical interface. In a narrow region of an electrode/electrolyte interface, an electrical double layer (EDL) exists. The EDL is believed to be extremely thin, and is an important component of the interface. 

Any electrochemical interface (or cell) can be described in terms of an electric circuit, which is a combination of resistances, capacitances, and complex impedances (and inductances, in the case of very high frequencies). If such an electric circuit produces the same response as the electrochemical interface (or cell) does when the same excitation signal is imposed, it is called the equivalent electric circuit of the electrochemical interface (or cell). The equivalent circuit should be as simple as possible to represent the system targeted. 

The Stark effect requires the application of electric fields of the order of 10 Vcm or higher. The electrochemical interface, where molecules and ions are subjected to fields in the order of 10 Vcm S seems to be the ideal place to study this phenomenon. 

Potential-dependent frequencies in spectra of adsorbates in electrochemical interfaces are commonly observed. Thus so-called Stark tuning rates, Qv/QE, of 30cm have been reported for adsorbed CO on platinum [65, 111] (Fig.59) and adsorbed CN on silver [109, 111, 162]. Even higher values were found for sulfate species adsorbed on polycrystalline platinum (100 cm V [36, 38]) or on single crystal Pt(lll) [141, 143] (120cm V ). In some cases, as for adsorbed tetra-cyanoethylene [163] and anthracene [164], vibrational features which are forbidden by the surface selection rule become active under the influence of the electric field at the interface. 

However, the electrochemical interface is in many respects more complicated than simply a very high electric field. Adsorbed species there, can suffer different degrees of charge transfer, interacting not only electrostatically but also chemically with the metal surface, with neighboring species of their own type, with co-adsorbed ions and sdlvent molecules. These interactions can affect the vibrational energies to an extent similar to that of the electric field, making necessary a separation of effects by an appropriate control of the experimental parameters. 

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