Electrode reactions electrical double layer

This volume has been organized to include eleven well-defined areas Solubilities of Electrolytes EMF and Potentiometric Titrations Vapor Pressures Cryoscopy Heats of Solution Calorimetry Polarography Ligand Exchange Rates and Electrode Reactions Electrical Double Layer Spectroscopy and Structure of Electrolytes Organic Electrolyte Battery Systems and Additional References and Data Sources. 

The solvent index is arranged according to single solvents, nonaqueous-aqueous mixtures and nonaqueous-nonaqueous mixtm-es. Individual solvent systems are indexed accordii to solubilities, pKsp, emf, potentiometric titrations, vapor pressure, cryoscopy, heats of solution, polarography, ligand exchange rates, electrode reactions, electrical double layer, spectroscopy, and organic electrolyte batteries. 

The treatment may be made more detailed by supposing that the rate-determining step is actually from species O in the OHP (at potential relative to the solution) to species R similarly located. The effect is to make fi dependent on the value of 2 and hence on any changes in the electrical double layer. This type of analysis has permitted some detailed interpretations to be made of kinetic schemes for electrode reactions and also connects that subject to the general one of this chapter. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

Activation Processes. To be useful ia battery appHcations reactions must occur at a reasonable rate. The rate or abiUty of battery electrodes to produce current is determiaed by the kinetic processes of electrode operations, not by thermodynamics, which describes the characteristics of reactions at equihbrium when the forward and reverse reaction rates are equal. Electrochemical reaction kinetics (31—35) foUow the same general considerations as those of bulk chemical reactions. Two differences are a potential drop that exists between the electrode and the solution because of the electrical double layer at the electrode iaterface and the reaction that occurs at iaterfaces that are two-dimensional rather than ia the three-dimensional bulk. 

The potential difference across the electric double layer A. This cannot be determined in absolute terms but must be defined with reference to another charged interface, i.e. a reference electrode. In the case of a corroding metal the potential is the corrosion potential which arises from the mutual polarisation of the anodic and cathodic reactions constituting the overall corrosion reaction see Section 1.4). 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution into the present and future of the industrial world. It illustrates the transition of electrochemical sciences from a solid chapter of physical electrochemistry (covering mainly electron transfer reactions, concepts of electrode potentials and stmcture of the electrical double layer) to the field in which electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is supported by high-level theory, connects to other areas of science, and includes focus on electrode surface structure, reaction environment, and interfacial spectroscopy. 

Parsons, R., Equilibrium properties of electrified interfaces, MAE, 1, 103 (1954). Parsons, R., The structure of electrical double layer and its influence on the rates of electrode reactions, AE, 1, 1 (1961). 

The electric current connected with chemical conversion is termed the faradaic current in contrast to the non-faradaic current required to charge the electrical double layer. The equation for the electrode reaction is formulated similarly to that for the half-cell reaction (Section 3.1.4) for the cathodic reaction, the electrons are placed on the left-hand side of the equation and, for the anodic reaction, on the right-hand side. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The effect of the electrical double-layer structure on the rate of the electrode reaction... 

The Frumkin theory of the effect of the electrical double layer on the rate of the electrode reaction is a gross simplification. For example, the electrode reaction does not occur only at the outer Helmholtz plane but also at a somewhat greater distance from the electrode surface. More detailed considerations indicate, however, that Eq. (5.3.20) can still be used to describe the effect of the electrical double layer as a good approximation. 

Various approaches to the theory of electrode reactions with participation of adsorbed reactants are beyond the scope of the present book. The effect of of the electrical double layer is negligible for chemisorption (see Section 4.3.3 and 5.7.1). 

In the closely related coulostatic method based on injection of a charge from a small condenser into an electrode in equilibrium with a redox system. The resulting time dependence of the electrode potential originates from the discharging of the electrical double layer by electrode reactions... 

In an analysis of an electrode process, it is useful to obtain the impedance spectrum —the dependence of the impedance on the frequency in the complex plane, or the dependence of Z" on Z, and to analyse it by using suitable equivalent circuits for the given electrode system and electrode process. Figure 5.21 depicts four basic types of impedance spectra and the corresponding equivalent circuits for the capacity of the electrical double layer alone (A), for the capacity of the electrical double layer when the electrolytic cell has an ohmic resistance RB (B), for an electrode with a double-layer capacity CD and simultaneous electrode reaction with polarization resistance Rp(C) and for the same case as C where the ohmic resistance of the cell RB is also included (D). It is obvious from the diagram that the impedance for case A is... 

The second component of the overpotential, rjs, is associated with the passage of reacting species and electrons across the electric double layer, discharge of the reacting species, and changes in the electrode surface structure. Following Newman (N8a), this component is called the surface overpotential. It depends on the reaction rate, the species concentrations in the double layer, and the kinetic characteristics of the electrode reaction at the surface in question. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

In the equivalent electric scheme of the entire electrochemical cell (Figure 1.5b), we note, starting from the working electrode, the presence of a capacitance, Cd, in parallel with an impedance, Zf, which represents the Faradaic reaction. The presence of the supporting electrolyte in excess indeed induces the formation of an electrical double layer, as sketched in... 

Electrochemistry at Electrodes is concerned with the structure of electrical double layers and the characteristic of charge transfer reactions across the electrode/electrolyte interface. The purpose of this text is to integrate modem electrochemistry with semiconductor physics this approach provides a quantitative basis for understanding electrochemistiy at metal and semiconductor electrodes. 

Electroanalytical chemists and others are concerned not only with the application of new and classical techniques to analytical problems, but also with the fundamental theoretical principles upon which these techniques are based. Electroanalytical techniques are proving useful in such diverse fields as electro-organic synthesis, fuel cell studies, and radical ion formation, as well as with such problems as the kinetics and mechanisms of electrode reactions, and the effects of electrode surface phenomena, adsorption, and the electrical double layer on electrode reactions. 

Figure 2. Representation of (A, top) an electrochemical capacitor (supercapacitor), illustrating the energy storage in the electric double layers at the electrode—electrolyte interfaces, and (B, bottom) a fuel cell showing the continuous supply of reactants (hydrogen at the anode and oxygen at the cathode) and redox reactions in the cell.

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