Half-cells, electrochemical

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

Figure 5.4 Atomistic model of the electrochemical half-cell, showing the electrode/electrolyte interface (xi < x < X2), which is connected to the hulk electrode and electrolyte (reservoirs). The lower panel indicates the electrostatic potential within the electrode and the bulk electrolyte (solid lines), and possible shapes for the potential drop between them (dashed lines).

Here, the last term accounts for the excess ions in the interfacial region, which compensate the excess charge on the electrode surface and keep the overall interface electroneutral. What in electrochemical terms is often described as a polarizable active electrode and an unpolarizable reference electrode ensures that any change of the number of ions in the electrochemical half-cell under consideration, caused by an electrochemical reaction, is just compensated by a corresponding counter-reaction at the reference electrode. 

Equations 7.2 and 7.3 are examples of electrochemical half-cell reactions. Since free electrons are not found in nature, half-cell reactions always occur in pairs such that the electrons generated by one are consumed by the other. The half-cell reaction that releases electrons is referred to as an oxidation reaction. The half-cell reaction that consumes electrons is referred to as a reduction reaction. For the redox reaction shown in Eq. 7.1, the oxidation and reduction half-cell reactions are given by Eqs. 7.2 and 7.3,... 

When two interval scales are used to measure the amount of change in the same property, the proportionality of differences is preserved from one scale to the other. For example. Table 1.4 shows reduction potentials of three electrochemical half-cell reactions measured in volts with reference to the standard hydrogen electrode (SHE, E°) and in millivolts with reference to the standard silver-silver chloride electrode (Ag/AgCl, ). For the SHE potentials the proportion of differences between the intervals +0.54 to +0.80 and +0.34 to +0.80 is... 

Electrochemical half-cell potentials vs. the standard hydrogen electrode (SHE, E°) and vs. the standard silver-silver chloride electrode (Ag/AgCl, E). 

A fuel cell is a device that converts the free energy change of a chemical reaction directly into electrical energy. This conversion occurs by two electrochemical half cell reactions. 

Up to this point, most of the discussions in this chapter have focused on descriptions of electrochemical half-cells of the... 

The Butler-Volmer (BV) approximation is the simplest approach to model and capture the essential features of the empirical Tafel equation. It considers an electrochemical half-cell reaction as an activated process, with the forward and backward reaction rates following an Arrhenius type law according to... 

Quantitative treatment of overpotential and related phenomena goes back to 1905, when Tafel showed empirically that, for an electrochemical half-cell from which a net electrical current I is being drawn, an excess potential AE away from the equilibrium potential will inevitably exist, and AE will be a linear function of the logarithm of the current density i (i = I/area of interface) ... 

Ion-Selective Electrodes [22a] Ion-selective electrodes (ISEs) are usually electrochemical half-cells, consisting of an ion-selective membrane, an internal filling solution, and an internal reference electrode (Eq. 5.37) ... 

When two electrochemical half-cells, each containing the components of a half-reaction, are connected, electrons tend to flow to the half-cell with the higher reduction potential. The strength of this tendency is proportional to the difference between the two reduction potentials (AE) and is a function of the concentrations of oxidized and reduced species. 

A similar situation also is encountered in a miniaturized photoelectrochemical cell, i.e., on a metallized semiconductor powder, Figure 2. Here, the individual particle can be thought of as two electrochemical half cells which have eventually collapsed onto each other as the conductive wire connecting them became shorter and shorter. The oxidizing and reducing sites are thus found in close spatial proximity and the potential for subsequent chemical reaction between the initial oxidation and reduction products is excellent. In fact, so long as the respective rates of the oxidation and reduction half reactions differ appreciably, it may be unnecessary to metallize the semiconductor powder in order to... 

The second meaning of the word circuit is related to electrochemical impedance spectroscopy. A key point in this spectroscopy is the fact that any -> electrochemical cell can be represented by an equivalent electrical circuit that consists of electronic (resistances, capacitances, and inductances) and mathematical components. The equivalent circuit is a model that more or less correctly reflects the reality of the cell examined. At minimum, the equivalent circuit should contain a capacitor of - capacity Ca representing the -> double layer, the - impedance of the faradaic process Zf, and the uncompensated - resistance Ru (see -> IRU potential drop). The electronic components in the equivalent circuit can be arranged in series (series circuit) and parallel (parallel circuit). An equivalent circuit representing an electrochemical - half-cell or an -> electrode and an uncomplicated electrode process (-> Randles circuit) is shown below. Ic and If in the figure are the -> capacitive current and the -+ faradaic current, respectively. 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

Figure 5.2. The GDE technique in an electrochemical half-cell configuration...

This overall reaction can be considered as two electrochemical half-cell reactions each involving the transfer of one electron (Eqs. (5) and (6)). 

J.M. Rheaume, B. Muller, M. Schulze, XPS analysis of carbon-supported platinum electrodes and characterization of CO oxidation on PEM fuel cell anodes by electrochemical half cell methods. J. Power Sources 1998, 76, 60-68. 

No or very little fiquid is used in the analytical process on dry reagent systems such as the Vitros and Reflotron Plus. For color reactions, the Vitros uses a multilayered, 16-mm square slide (Figure 11-3) in which reagents dispersed in emulsions are activated by diffusion of the specimen fluid into the layers. From three to seven layers containing reagents are used for each of the different tests available. The Vitros also uses slides for electrometric assays that incorporate miniature ion-selective electrodes. On these, a reference solution and patient specimen provide fluid that turns the electrodes into electrochemical half-cells (see Chapter 4). Slides of this type that measure sodium, potassium, carbon dioxide, and chloride are currently in use. 

Half-cell potential The potential of an electrochemical half-cell measured with respect to the standard hydrogen electrode. Half-life, fi/i The time interval during which the amount of reactant has decreased by one half. 

Another important experiment used to measure the Volta potential difference between two liquids was described by Kenrick [9] and is illustrated here for the mercury electrolyte solution interface (see fig. 8.12). An air gap between the two liquids is established in a cylindrical tube T. The mercury emerges from a central reservoir in a stream of small droplets which flow down the center of the tube. On the other hand, the solution, namely 0.1 M HCl, flows down its walls. The experiment is designed so that the two liquids flow sufficiently rapidly that no charge can be built up on their surfaces. As a result the Volta potential difference across the air gap is zero. The HCl solution is part of an electrochemical half-cell connected to a calomel electrode. The total cell may be described as... 

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

One type of simple electrochemical half-cell consists of a metal strip dipping into a solution of its ions, e.g. a Cu strip immersed in an aqueous solution of a Cu(II) salt. No chemical reaction occurs in such a half-cell, although an equation describing the half-cell refers (by convention) to the appropriate reduction process (equation 7.6). The reaction is written as an equilibrium. 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

For an electrochemical half-cell reaction, such as the generic one given in Eq. (2), the activation energies for the anodic and cathodic reactions are given by ... 

Nemst equation—an equation describing the potential of an electrochemical half-cell neutron—an uncharged, subatomic particle found in the nucleus of an atom noble gas—Group 18 of the periodic table, characterized by their general inertness due to their complete valence shell of electrons... 

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