Current producing reaction

It is evident that E is known as soon as K is known for the current-producing reaction at various temperatures. 

The overall current-producing reaction can be obtained by combining the cathodic reaction occurring at one electrode (index 1 ) with the anodic reaction occurring at the other electrode (index 2 ), while the equations for these reactions must be written so that the values of n in these equations are identical (the reactions must be coupled) ... 

This equation links the EMF of a galvanic cell to the Gibbs energy change of the overall current-producing reaction. It is one of the most important equations in the thermodynamics of electrochemical systems. It follows directly from the first law of thermodynamics, since nF% is the maximum value of useful (electrical) work of the system in which the reaction considered takes place. According to the basic laws of thermodynamics, this work is equal to -AG . 

It is typical that in Eq. (3.23) for the EMF, all terms containing the chemical potential of electrons in the electrodes cancel in pairs, since they are contained in the expressions for the Galvani potentials, both at the interface with the electrolyte and at the interface with the other electrode. This is due to the fact that the overall current-producing reaction comprises the transfer of electrons across the interface between two metals in addition to the electrode reactions. 

This gives rise to an important conclusion. For nonconsumable electrodes that are not involved in the current-producing reaction, and for which the chemical potential of the electrode material is not contained in the equation for electrode potential, the latter (in contrast to a Galvani potential) depends only on the type of reaction taking place it does not depend on the nature of the electrode itself. 

The Gibbs-Helmholtz equation also links the temperature coefficient of Galvani potential for individual electrodes to energy effects or entropy changes of the electrode reactions occurring at these electrodes. However, since these parameters cannot be determined experimentally for an isolated electrode reaction (this is possible only for the full current-producing reaction), this equation cannot be used to calculate this temperature coefficient. 

Thus, the electrode processes occurring in zinc-carbon batteries with salt electrolytes are complicated, and their thermodynamic analysis is difficult. In a rough approximation disregarding secondary processes, the current-producing reaction can be described as... 

Electrochemical Processes The charged positive electrodes of these batteries contain NiOOH, an oxide hydroxide of trivalent nickel, and the negative electrodes contain metallic cadmium or iron (M). As a rule, KOH solution serves as the electrolyte. The main current-producing reactions on the electrodes and in the cell in general can be written as... 

Whereas current-producing reactions occur at the electrode surface, they also occur at considerable depth below the surface in porous electrodes. Porous electrodes offer enhanced performance through increased surface area for the electrode reacdon and through increased mass-transfer rates from shorter diffusion path lengths. The key parameters in determining the reaction distribution include the ratio of the volume conductivity of the electrolyte to the volume conductivity of the electrode matrix, the exchange current, the diffusion characteristics of reactants and products, and the total current flow. The porosity, pore size, and tortuosity of the electrode all play a role. 

In contrast to what occurred in the jar, in the batteries, the overall chemical reaction occurs in the form of two spatially separated partial electrochemical reactions. Electric current is generated because the random transfer of electrons is replaced by a spatially ordered overall process (current-producing reaction). 

The main current-producing reactions on the negative electrodes of alkaline nickel storage batteries can be written as... 

The battery design must ensure the conditions required for the current-producing reaction and for practical utilization of the electrical energy released. Among these conditions are ... 

A problem that appears immediately when a specific single-cell battery is designed is the required ratio of the reactants for the positive and negative electrodes and for the electrolyte. An unnecessary excess in one of the components is undesirable. It is therefore typical for the reactants to be loaded in the stoichiometric ratio given by the current-producing reaction with the actual utilization coefficient of each of... 

The main current-producing reaction is described by Equation (11.11). The product of this reaction is lithium dithionite that is insoluble in the electrolyte. Therefore, a reasonable porous cathode structure is of great importance for provision of high capacity and power characteristics of sulfur dioxide-lithium cells. The cathodes in such cells are similar to cathodes of thionyl chloride-lithium cells. The cells of the "lithium-sulfur dioxide" system are also produced using the rammed and wound coil designs. Porous polypropylene is applied as a separator. 

The current-producing reaction in manganese dioxide-lithium cells is described by Equation (11.2) here, indicator x is generally close to unity. In this case, the theoretical specific capacity of such a cell is 285 Ah/kg, which corresponds to the theoretical energy density of 998 Wh/kg at OCV of 3.5 V. The actual energy density for disk and cylindrical cells of a not too low capacity (above 0.1 Ah) is 200-350 Wh/kg and strongly depends on the discharge mode. 

The overall current-producing reaction (11.8) describes the process of cell discharge only approximately. The intermediate product of cathodic polyfluorocarbon reduction is assumed to be a certain solvated intercalation compound C-F-Li decomposed in the course of discharge. 

In the course of discharge, sodium is anodically oxidized to sodium ions Na" ", which penetrate the solid electrolyte and act as current carriers in it. Sulfur is reduced on the positive electrode and reacts with sodium ions from the electrolyte giving rise to various sodium polysulfides, NajS. The overall current-producing reaction can be divided into two stages ... 

A convenient measure for the relative rates of current-producing reactions of fuel cells of a given type but differing in size is by using the current density, that is, the current per unit surface area S of the electrodes) i = I/S, with the units in milliampere per square centimeter. The power density Ps = P/S (the units in milliwatt per square centimeter) is a convenient measure of relative efficiency of different varieties of fuel cells. 

The electrochemical reactions occurring at the electrodes of polymer electrolyte membrane fuel cells, as well as the overall current-producing reaction are the same as in the hydrogen-oxygen fuel cells with liquid acidic electrolyte discussed in Section 16.4 (Eqs. 16.2 and 16.3). 

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