Galvanic cells discharge

As a galvanic cell discharges, the cell potential, E, decreases and reactants form products. -> When = 0, the cell is completely discharged and the cell reaction is at equilibrium. 

The lUPAC convention is consistent with the signs that the electrodes actually develop in a galvanic cell. That is, in the Cu/Ag cell shown in Figure 18-4, the Cu electrode becomes electron rich (negative) owing to the tendency of Cu to be oxidized to Cu while the Ag electrode becomes electron deficient (positive) because of the tendency for Ag" to be reduced to Ag. As the galvanic cell discharges spontaneously, the silver electrode is the cathode, while the copper electrode is the anode. 

Electrons participating in the intercalation/deintercalation reaction (Equation (5.1)) can be represented by a current-producing system. Second, it is characteristic that the current-producing system reversibly operated by a self-driven (galvanic) cell (discharging the battery) performs the electrical useful work AG = —zFE (where E is the EMF of the cell), because electrical potential difference is spontaneously developed between two electrodes. By contrast, when the cell is short-circuited - that is, when the two electrodes are not separated from each other but are directly in electrical contact - electrons do not appear explicitly but rather participate in corrosion (or permeation in the case of solid electrolyte cells). They perform no electrical useful work because the two electrodes have the same electrical potential. 

Since the final and initial states are identical in both experiences, the free enthalpy change AGgyst is the same in the two processes. This result is general and applies to all redox reactions. A galvanic cell discharge is equivalent to a chemical process that can be decomposed into two electrochemical half-reactions that take place simultaneously but separately, the thermodynamic evaluations of both processes being identical. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

For instance, when current flow is from the right to the left in galvanic cell (1.19), the zinc electrode will be the cathode, and its surface is the site of the cathodic reaction involving the deposition of zinc by discharge of zinc ions from the solution ... 

OCV and Discharge Voltage The OCV, of a galvanic cell depends on the electrochemical system selected for it and is somewhat affected by the electrolyte... 

Another kinetic aspect is observed if a component other than the electroactive species is predominantly mobile. The electroactive species are in this case made available to the electrolyte by the motion of the other components in the opposite direction. In a binary compound this does not make a difference to the electrode performance. But in the case of a compound with more than two components the composition is changed to values which are not expected from a thermodynamic point of view for the variation of the concentration of the electroactive species. Other phases are formed which may provide a lower or higher activity of the electroactive species than that expected thermodynamically. This has an influence both on the current and the cell voltage. Upon discharging and charging a galvanic cell, the composition of the electrode at the interface with the electrolyte may follow very different compositional pathways (Weppner, 1985). 

Even more important than the cell voltage is the energy that may be obtained from a galvanic cell as it discharges and the composition of the electrode changes. Integration of Eqn (8.41) provides the following expression... 

In-situ electrochemical techniques may be conveniently used to analyse the fundamental thermodynamic and kinetic parameters which are responsible for the performance of electrodes. The methods are nondestructive and may be applied to the actual galvanic cell. The data may be easily determined as a function of the discharge state. 

GITT also provides very comprehensive information about the kinetic parameters of the electrode by analysis of the electrical current. The current 1, which is driven through the galvanic cell by an external current or voltage source, determines the number of electroactive species added to (or taken away from) the electrode and discharged at the electrode/ electrolyte interface. A chemical diffusion process occurs within the electrode and the current corresponds to the motion of mobile ionic species within the electrode just inside the phase boundary with the electrolyte (at x = 0)... 

Zinc-Manganese Dioxide. In 1866 Leclanche invented a galvanic cell in which the reduction of Mn02 is the cathodic reaction in the cell s discharge. The corresponding anodic dissolution reaction is the oxidation of zinc. The Leclanche cell is a (so-called) dry cell, i.e., the ammonium electrolyte is immobilized in the form of a paste. There are three forms of the zinc-manganese dioxide batteries ... 

The capacity K of a galvanic cell is simply based on the electric charge, Qdisch, which can be restored in the course of the discharge process ... 

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