Passivity electrode kinetics

The organic conductor properties of tetrathiaflulvalenetetracyanoquino-dimethane (TTF-TCNQ) as a material for constructing electrodes, viz. its catalytic response and resistance to passivation, are of special interest for the determination of biological compounds, which usually have slow electrode kinetics and a low sensitivity, and tend to foul electrode surfaces. The response of a TTF-TCNQ microarray sensor inserted in a flow system for... 

The older literature on the electrochemistry of dioxygen in acidic media attributes the difference between the thermodynamic potential for the four-electron reduction (02/H20, +1.23 V vs. NHE Table 9.3) and the observed value at a freshly activated platinum electrode [+0.67 V vs. NHE, Eq. (9.13)] to overvoltage (or kinetic inhibition). Likewise, the difference between the thermodynamic potential for the two-electron reduction (02/H00H, +0.70 V vs. NHE, Table 9.3) and the observed value at passivated electrodes [+0.05 V vs. NHE, Eq. (6.12)] was believed to be due to the kinetic inhibition of the two-electron process. 

July 13,1916 in Berlin, Germany - Dec. 12,1974 in Berlin, Germany) Study of chemistry and physics in Gottingen and Berlin from 1955 professor of physical chemistry at the Free University of Berlin. Cooperation with K. F. Bonhoeffer, - Gerischer, and J. W. Schultze, almost 100 publications (fundamentals of electrode kinetics, overpotential, redox reactions, ion transfer, passivity and corrosion, especially of iron), most important is his book [i] ... 

A.C. Makrides, Kinetics of Redox Reactions on Passive Electrodes,... 

Anodic protection was developed using the principles of electrode kinetics and is difficult to understand without introducing advanced concepts of electrochemical theory. Briefly, anodic protection is controlled by the formation of protective passive film on metals and alloys using an externally applied potential. Anodic protection is used to a lesser degree because of the limitations on metal-environment systems for which anodic protection is viable. In addition, it is possible to accelerate corrosion if proper controls are not implemented during anodic protection. 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

The position of the electrodes in the reactor can be optimized as a function of hydrodynamic parameters and current density (j). Complementary rules should include the influences of electrode gap (e) and operating conditions on voltage U (and consequently on energy consumption). The measured potential is the sum of three contributions, namely the kinetic overpotential, the mass transfer overpotential and the overpotential caused by solution ohmic resistance. Kinetic and mass transfer overpotentials increase with current density, but mass transfer is mainly related to mixing conditions if mixing is rapid enough, mass transfer overpotential should be negligible. In this case, the model described by Chen et al., (2004) is often recommended for non-passivated electrodes ... 

In this chapter, these thermodynamic and kinetics aspects of passivity are presented after a brief historical survey The following section discusses the electrode kinetics in the passive state. Next the chemical composition and chemical structure of passive films form on pure mefals are reviewed wifh an emphasis on iron. This is followed by a compilation of data for binary alloys. The elecfronic properties of passive layers are fhen discussed, and the last section covers the structural aspects of passivify. 

The thermodynamic properties and the application of electrode kinetics have been described. As corrosion and passivity are in principle determined by electrochemistry at metal surfaces, a good understanding of the equilibria and the kinetics of electrode surfaces is a necessary requirement for any further study with more sophisticated methods. This involves complicated transients studies as well as electrochemical methods like the RRD electrode with or without hydrod5mamical modulation. Here a systematic research is still needed for a better understanding of pure metals and especially alloys. Results for simplified conditions give answers for the often more complicated situation of corroding systems in a real environment. 

Figure 2. Examples of numerical solutions for the cathodic current distribution on a plate electrode immersed in a cell with the counter electrode at the bottom. Three cases are compared (a) (/ column) completely reversible kinetics (primary distribution) (b) center) irttermedrate kinetics (Ub 0.2) (c) (right column) irreversible kinetics (Wa 10). The top row provides a comparison of the current distribution or the deposit profile on the cathode (cross-hatched region). The center row provides the current distribution along the electrode ( stretched ). The bottom row provides the corresponding poterrtial distributions. It is evident that the current distribution uniformity increases as the electrode kinetics become more passivated (Cell-Design software simulations ).

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

Apart from the work toward practical lithium batteries, two new areas of theoretical electrochemistry research were initiated in this context. The first is the mechanism of passivation of highly active metals (such as lithium) in solutions involving organic solvents and strong inorganic oxidizers (such as thionyl chloride). The creation of lithium power sources has only been possible because of the specific character of lithium passivation. The second area is the thermodynamics, mechanism, and kinetics of electrochemical incorporation (intercalation and deintercalation) of various ions into matrix structures of various solid compounds. In most lithium power sources, such processes occur at the positive electrode, but in some of them they occur at the negative electrode as well. 

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