Page 44 - Lindens Handbook of Batteries
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CHAPTER 2
ELECTROCHEMICAL PRINCIPLES
AND REACTIONS
Mark Salomon
2.1 INTRODUCTION
Batteries and fuel cells are electrochemical devices that convert chemical energy into electrical
energy by electrochemical oxidation and reduction reactions, which occur at the electrodes. A cell
consists of an anode where oxidation takes place during discharge, a cathode where reduction takes
place, and an electrolyte which conducts the current (via ions) within the cell.
The maximum electric energy that can be delivered by the chemicals that are stored within or
supplied to the electrodes in the cell depends on the change in Gibbs energy ∆G of the electrochemical
couple, as shown in Eq. (2.5) and discussed in Sec. 2.2.
It would be desirable if during the discharge all of this energy could be converted to useful
electric energy. However, losses due to polarization occur when a load current i passes through the
electrodes, accompanying the electrochemical reactions. These losses include: (1) activation polar-
ization, which drives the electrochemical reaction at the electrode surface, and (2) concentration
polarization, which arises from the concentration differences of the reactants and products at the
electrode surface and in the bulk as a result of mass transfer.
These polarization effects consume part of the energy, which is given off as waste heat, and thus not all
of the theoretically available energy stored in electrodes is fully converted into useful electrical energy.
In principle, activation polarization and concentration polarization can be calculated from
several theoretical equations, as described in later sections of this chapter, if some electro chemical
parameters and the mass-transfer condition are available. However, in practice it is difficult to deter-
mine the values for both because of the complicated physical structure of the electrodes. As covered
in Sec. 2.5, most battery and fuel cells electrodes are composite bodies made of active material,
binder, performance enhancing additives, and conductive filler. They usually have a porous structure
of finite thickness. It requires complex mathe matical modeling with computer calculations to esti-
mate the polarization components.
There is another important factor that strongly affects the performance or rate capability of a cell,
the internal impedance of the cell. It causes a voltage drop during operation, which also consumes
part of the useful energy as waste heat. The voltage drop due to internal impedance is usually referred
to as “ohmic polarization” or IR drop and is proportional to the current drawn from the system.
The total internal impedance of a cell is the sum of the ionic resistance of the electrolyte (within
the separator and the porous electrodes), the electronic resistances of the active mass, the current
collectors and electrical tabs of both electrodes, and the contact resistance between the active mass
and the current collector. These resistances are ohmic in nature, and follow Ohm’s law, with a linear
relationship between current and voltage drop.
2.1