Batteries electrochemical kinetics

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

J. S. Newman, Electrochemical Systems, Prentice-Hall, Englewood Cliffs, NJ, 1991 A. J. Bard and L. R. Faulkner, Electrochemical Methods. Fundamentals and Applications, John Wiley and Sons, New York, 1980 J. O M. Bockris and S. Srinivasan, Fuel Cells Their Electrochemistry, McGraw-Hill Book Company, New York, 1969 J. O M. Bockris and A. K. V. Reddy, Modern Electrochemistry, Plenum Press, New York, 1970 C. Julien, G. A. Nazri, Solid State Batteries, Kluwer Academic Publishers, Norwell, 1994 M. Winter, J. 0. Besenhard, M. E. Spahr, and P. Novak, Adv. Mater. 10 (1998) 725 F. von Sturm, Elektrochemische Stromerzeugung, VCH, Weinheim, 1969 K. J. Vetter, Electrochemical Kinetics, Academic Press, New York, 1967. 

Another consideration in the use of hydride materials in Ni/MH batteries is related to the electrochemical kinetics and transport processes. The power output of the battery depends critically on these processes. During discharge, hydrogen stored in the bulk metal must be brought to the electrode surface by diffusion. The hydrogen then must react with hydroxyl ions at the metal electrolyte interface. As a consequence, surface properties such as oxide thickness, electrical conductivity, surface area, porosity and the degree of catalytic activity... 

The basic thermodynamic and electrochemical kinetic concepts involved in batteries and the parameters used to evaluate their performance are summarized in Section 2.2. The most widespread primary and rechargeable systems are described by highlighting the most recent advances in Section 2.3. Supercapacitors and fuel cells, whose importance in the field of energy conversion is growing, are also briefly treated in this section. The lithium-based rechargeable systems, the most advanced batteries with the highest performance, are discussed in detail in Section 2.4, with particular emphasis on the new materials on which these batteries are based. 

Since it is impossible to measure the individual electric potential differences at the phase boundaries, we shall hereinafter speak only in terms of the difference in electric potential across the two terminals connected to the electrodes of the battery. When in a battery the current is not flowing or tends to zero, the measurable potential difference across the two terminals is called the open-circuit voltage (OCV), fJc, and it represents the battery s equilibrium potential (or voltage). Since it is related to the free energy of the cell reaction, the OCV is a measure of the tendency of the cell reaction to take place. Indeed, while the conversion of chemical into electric energy is regulated by thermodynamics, the behavior of a battery under current flow (the current is a measure of the electrochemical reaction rate) comes under electrochemical kinetics. 

A.R. Jha, author of 10 books on alternative energy and other topics, outlines rechargeable battery requirements for electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). He identifies the unique materials for electrolytes, cathodes, and anodes that are most cost-effective with significant improvements in weight, size, efficiency, reliability, safety, and longevity. Since electrode kinetics play a key role in the efficient operation of fuel cells, the book also provides you with a foundation in the basic laws of electrochemical kinetics. 

In 1997, a model was developed by Gu et al. to predict transient behaviors of EV LABs during discharge and charge processes [66]. The model not only accounts for the coupled processes of electrochemical kinetics and mass transport occurring in a battery cell, but also considers free convection resulting from density variations due to acid stratification. For the positive electrode, the material balance (Equation 9.169) becomes... 

Electrochemical kinetics and safety of 2-volt class li-ion battery system using lithium titanium oxide anode. 

Takami, N., Inagaki, H., Kishi, T., Harada, Y, Fujita, Y, Hoshina, K. 2009. Electrochemical kinetics and safety of 2-volt class Li-ion battery system using lithium titanium Oxide anode. T. Electrodiem. Soc. 156 A128-A132. 

T. Osaka, K. Naoi, S. Ogano, S. Nakamura, Dependence of film thickness on electrochemical kinetics of polypyrrole and on properties of lithium/polypyrrole battery, J. Electrochem. Soc., 1987,134, pp. 2096-2102. 

The alkaline hydrogen electrode (-0.83 V vs. SHE), a dose equivalent of metal hydride electrode (-0.8 vs. SHE) but with extremely high cell capacity and favourable reversible electrochemical kinetics, can be coupled with V V as a full-flow pH differential battery. The electrochemistry of this vanadium-hydrogen full-flow battery is shown below. 

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. 

An excellent review covers the charge and discharge processes in detail (30) and ongoing research on lead—acid batteries may be found in two symposia proceedings (32,33). Detailed studies of the kinetics and mechanisms of lead —acid battery reactions are pubUshed continually (34). Although many questions concerning the exact nature of the reactions remain unanswered, the experimental data on the lead—acid cell are more complete than for most other electrochemical systems. 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

Electrochemical systems are found in a number of industrial processes. In addition to the subsequent discussions of electrosynthesis, electrochemical techniques are used to measure transport and kinetic properties of systems (see Electroanalyticaltechniques) to provide energy (see Batteries Euel cells) and to produce materials (see Electroplating). Electrochemistry can also play a destmctive role (see Corrosion and corrosion control). The fundamentals necessary to analyze most electrochemical systems have been presented. More details of the fundamentals of electrochemistry are contained in the general references. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

As noted earlier, the kinetics of electrochemical processes are inflnenced by the microstractnre of the electrolyte in the electrode boundary layer. This zone is populated by a large number of species, including the solvent, reactants, intermediates, ions, inhibitors, promoters, and imparities. The way in which these species interact with each other is poorly understood. Major improvements in the performance of batteries, electrodeposition systems, and electroorganic synthesis cells, as well as other electrochemical processes, conld be achieved through a detailed understanding of boundaiy layer stracture. 

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. 

At present, intercalation compounds are used widely in various electrochemical devices (batteries, fuel cells, electrochromic devices, etc.). At the same time, many fundamental problems in this field do not yet have an explanation (e.g., the influence of ion solvation, the influence of defects in the host structure and/or in the host stoichiometry on the kinetic and thermodynamic properties of intercalation compounds). Optimization of the host stoichiometry of high-voltage intercalation compounds into oxide host materials is of prime importance for their practical application. Intercalation processes into organic polymer host materials are discussed in Chapter 26. 

The electrochemical intercalation/insertion is not a special property of graphite. It is apparent also with many other host/guest pairs, provided that the host lattice is a thermodynamically or kinetically stable system of interconnected vacant lattice sites for transport and location of guest species. Particularly useful are host lattices of inorganic oxides and sulphides with layer or chain-type structures. Figure 5.30 presents an example of the cathodic insertion of Li+ into the TiS2 host lattice, which is practically important in lithium batteries. 

Although the diffusion of the counterion is faster in polypyrrole than in polyacetylene, its value is still low enough to influence the rate of the electrochemical charge and discharge processes of lithium/polymer batteries. Indeed the current output of these batteries is generally confined to a few mA cm . Possibly, improvements in the electrode kinetics, and thus in the battery rates, may be obtained by the replacement of standard ... 

Both batteries and fuei cells utilize controlled chemical reactions in which the desired process occurs electrochemically and all other reactions including corrosion are hopefully absent or severely kinetically suppressed. This desired selectivity demands careful selection of the chemical components including their morphology and structure. Nanosize is not necessarily good, and in present commercial lithium batteries, particle sizes are intentionally large. All batteries and fuel cells contain an electropositive electrode (the anode or fuel) and an electronegative electrode (the cathode or oxidant) between which resides the electrolyte. To ensure that the anode and cathode do not contact each other and short out the cell, a separator is placed between the two electrodes. Most of these critical components are discussed in this thematic issue. 

Electrochemical energy conversion devices are pervasive in our daily lives. Batteries, fuel cells and supercapacitors belong to the same family of energy conversion devices. They are all based on the fundamentals of electrochemical thermodynamics and kinetics. All three are needed to service the wide energy requirements of various devices and systems. Neither... 

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