1.5 General Equivalent Circuit of an Electrochemical Cell
It is useful to devise electrical circuits whose electrical behavior is analogous to important
phenomena in physical systems. By examination of the influence of changes
in parameters in such equivalent circuits, they can be used as ‘thinking tools’ to
obtain insight into the significance of particular phenomena to the observable properties
of complex physical systems. By use of this approach, the techniques of circuit
analysis that have been developed for use in various branches of electrical engineering
can be used in the analysis of interdependent physical phenomena.
This procedure has proved useful in some areas of electrochemistry, and will
be detailed later in this text. At this point, however, it is utilized to study an ideal
electrochemical cell; i.e. what happens if the electrolyte is not a perfect filter, but
allows flow of some electronic current in addition to the expected ionic current.
An electrochemical cell can be simply modeled as shown in Fig. 1.9, and its basic
equivalent circuit is shown in Fig. 1.10.
The value of electrical equivalent of the theoretical chemical driving force is Eth,
given by
Eth = −ΔGr
◦
/zF (1.17)
as the result of the balance between the chemical and electrical forces acting upon
ionic species in the electrolyte, as mentioned earlier. If there are no impedances
or other loss mechanisms, the externally measurable cell voltage Eout is simply
equal to Eth.
1.5 General Equivalent Circuit of an Electrochemical CellIt is useful to devise electrical circuits whose electrical behavior is analogous to importantphenomena in physical systems. By examination of the influence of changesin parameters in such equivalent circuits, they can be used as ‘thinking tools’ toobtain insight into the significance of particular phenomena to the observable propertiesof complex physical systems. By use of this approach, the techniques of circuitanalysis that have been developed for use in various branches of electrical engineeringcan be used in the analysis of interdependent physical phenomena.This procedure has proved useful in some areas of electrochemistry, and willbe detailed later in this text. At this point, however, it is utilized to study an idealelectrochemical cell; i.e. what happens if the electrolyte is not a perfect filter, butallows flow of some electronic current in addition to the expected ionic current.An electrochemical cell can be simply modeled as shown in Fig. 1.9, and its basicequivalent circuit is shown in Fig. 1.10.The value of electrical equivalent of the theoretical chemical driving force is Eth,given byEth = −ΔGr◦/zF (1.17)as the result of the balance between the chemical and electrical forces acting uponionic species in the electrolyte, as mentioned earlier. If there are no impedancesor other loss mechanisms, the externally measurable cell voltage Eout is simplyequal to Eth.
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