Electrode/solution interface Subject

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. 

Nevertheless, the earlier thermodynamic treatment left one significant equation still very much present and effective when a change toward the kinetic approach occurred. This equation (Nemst s law) is used today and probably will be used even when all electrochemical calculations are wrapped up inside various companies software offerings. Nemst s equation,11 which treats the electrode/solution interface at equilibrium in a thermodynamic way, is the subject of the following section. 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

Since the objective of the studies described herein is the characterisation of the solute species formed following redox reaction the very extensive research dealing with characterisation of the electrode/solute interface will not be discussed, excellent overviews of the experimental aspects of this subject are available. While this contribution focuses on applications involving IR, Raman spectroscopy has proved to be invaluable to many SEC studies where surface-enhanced Raman spectroscopy (SERS) and resonance Raman spectroscopy dominate. Reviews and recent studies attest to the value of these approaches. ... 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

Adsorption of CD is an important subject in CD electrochemistry. Since electrode reaction is essentially heterogeneous reaction with electron transfer occuring at electrode-solution interface, adsorption of organic material on electrode surface has sometimes a critical influence on the total reaction. The adsorption phenomena of CD on a mercury electrode were investigated by means of CV in a phosphate... 

During the last decade the field of electrochemistry has witnessed a very fast progress on the modification of electrode surfaces. From the predominant use of random polymeric structures, prevalent in the electrode modification efforts of the late 70s and early 80s, electrochemists have learnt to control the molecular architecture of the electrode-solution interface to a degree that was clearly out of reach a decade ago. Many electrode modification methods developed recently rely on the use of thiolate self-assembled monolayers (SAMs) [1-3]. These systems offer unparalleled ease of preparation and levels of molecular organization close to those that can be reached with Langmuir-Blodgett film methods. Therefore, electrodes derivatized with unfunctionalized or functionalized alkanethiolate monolayers have been the subject of extensive research work during the last few years [4, 5]. 

Structural control of the electrode/solution interface is a complex problem of fundamental importance in electrochemical sciences (7). To achieve some elements of such control, it would be desirable to impart molecular character onto the otherwise "naked" electrode surface so that, as a result, it might acquire desired catalytic properties, gain some elements of molecular selectivity, or exhibit other desirable molecular characteristics. To accomplish this, electrochemists have explored numerous possibilities of coating the electrode surface with thin films (from a single monolayer to micrometers in thickness) of a wide variety of materials (2). This area of electrochemistry, often referred to as the chemical mo fication of electrodes, is the subject of a number of recent reviews (7-5). 

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

With the development of solid-state semiconductor devices (diodes, transistors), semiconductor/solution interfaces [24] became a subject of scientific interest. Since the 1960s, semiconductor electrochemistry and photo-electrochemistry has become established as an independent subdiscipline in electrochemical science. The basic principles and summaries of experimental results can be found in review papers and textbooks [25]. Here, we will introduce the subject by comparing simple electron transfer at a metal with that at a semiconductor electrode. 

In the following, the equilibrium state of an electroactive polymer in an electroinactive electrolyte is considered (Fig. 20. la). Using an inert electrolyte reduces the complexity of the system it is of interest also because the oxidation/reduction of the polymer in inert electrolytes is a subject of great technical potential (batteries, etc.). For characterizing these systems, one has to consider both the ion-partitioning (ion-exchange) equilibrium across the polymer/solution interface fEqs. (23)-(27)] and the electronic equilibrium between the electrode and the polymer phase [Eqs. (19)-(22)]. 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

The precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

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