Electrical double-layer structure electrode reaction rates

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

On die 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 structure of tine electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. 

Electric Double-Layer Structure and Electrode Reaction Rate... 

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 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. 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

The formation of an electrical double layer at a metal-solution interface brings about a particular arrangement of atoms, ions and molecules in the region near the electrode surface, and an associated variation in electrical potential with distance from the interface. The double layer structure may significantly affect the rates of electrochemical reactions. 

When a metal electrode is placed in an electrolyte solution, an equilibrium difference usually becomes established between the metal and solution. Equilibrium is reached when the electrons left in the metal contribute to the formation of a layer of ions whose charge is equal and opposite to that of the cations in solution at the interface. The positive charges of cations in the solution and the negative charges of electrons in the metal electrode form the electrical double layer [4]. The solution side of the double layer is made up of several layers as shown in Fig. 2.7. The inner layer, which is closest to the electrode, consists of solvent and other ions, which are called specifically adsorbed ions. This inner layer is called the compact Helmholtz layer, and the locus of the electrical centers of this inner layer is called the inner Helmholtz plane, which is at a distance of di from the metal electrode surface. The solvated ion can approach the electrode only to a distance d2. The locus of the centers of the nearest solvated ion is called the outer Helmholtz plane. The interaction of the solvated ion with metal electrode only involves electrostatic force and is independent of the chemical properties of the ions. These ions are called non-specifically adsorbed ions. These ions are distributed in the 3D region called diffusion layer whose thickness depends on the ionic concentration in the electrolyte. The structure of the double layer affects the rate of electrode reactions. 

Parsons, The Structure of the Electrical Double Layer and its Influence on the Rates of Electrode Reactions in Advances in Electrochemistry and Electrochemical Engineering, Ed. Delahay and Tobias, Vol. 1, pp. 1-64, Interscience Publishers Inc. and J. Wiley Sons, New York, 1961. 

R. Parsons, The structure of the electrical double layer and its influence on the rate of electrode reactions, in Modern Aspects of Electrochemistry, J. O M. Bockris (ed), Vol. 1, Butterworths, London, 1952, p. 1. 

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 standard electrode potential (q.v.) is often not greatly different in non-aqueous solvents from that in water, although displacements due to differences in the strength of solvation of the ions are to be expected. The same reference electrodes as are used in water are also usually satisfactory. The rates of electrochemical reactions, however, can be radically altered by changes of solvent, since all the factors which govern the ease of transfer of electrons across the electrode surface are likely to be modified. These include the solvation of the electroactive ions, their tendency to ion-pairing and complex formation, the adsorbability of the solvent and of active species at the electrode surface, and the other factors that may affect the structure of the electrical double layer (q.v.). 

The electrode surface participates actively in an electrochemical reaction sequence by providing adsorption sites for at least one reactant and for the reaction intermediates. Thus, the reaction rate and selectivity depend strongly upon the surface properties and its mode of interaction with reactive species and electrolytes. The existence, however, of the structured double layer interface and of the electric field under which electrosorption takes place distinguishes the latter from gas phase adsorption. Electrolyte ions, solvent molecules, and impurities may adsorb and compete with reactants for surface sites or they may poison the surface or contribute to surface changes under reaction. Despite the wealth of experimental information on the potential dependence of surface coverage and on the nature of some adsorbing species, a fundamental understanding of electrosorption mechanisms is still incomplete. 

In this section we describe the equations required to simulate the electrochemical performance of porous electrodes with concentrated electrolytes. Extensions to this basic model are presented in Section 4. The basis of porous electrode theory and concentrated solution theory has been reviewed by Newman and Tiedemann [1]. In porous electrode theory, the exact positions and shapes of aU the particles and pores in the electrode are not specified. Instead, properties are averaged over a volume small with respect to the overall dimensions of the electrode but large with respect to the pore structure. The electrode is viewed as a superposition of active material, filler, and electrolyte, and these phases coexist at every point in the model. Particles of the active material generally can be treated as spheres. The electrode phase is coupled to the electrolyte phase via mass balances and via the reaction rate, which depends on the potential difference between the phases. AU phases are considered to be electrically neutral, which assumes that the volume of the double layer is smaU relative to the pore volume. Where pUcable, we also indicate boundary conditions that would be used if a Uthium foil electrode were used in place of a negative insertion electrode. 

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