Page 46 - Lindens Handbook of Batteries
P. 46
ELECTROCHEMICAL PRINCIPLES AND REACTIONS 2.3
TABLE 2.1 Conductivity Ranges of Various Electrolytes at
Ambient Temperature
Electrolyte system Conductivity/S cm -1
Aqueous electrolytes 0.1–0.55
Molten salts ~10 -1
-2
Inorganic electrolytes 10 –10 -1
Organic electrolytes 10 –10 -2
-1
-4
Ionic liquids 10 –10 -2
-7
Polymer electrolytes 10 –10 -3
-8
Inorganic solid electrolytes 10 –10 -5
4. In most battery and fuel cell systems, part or all of the reactants are supplied from the electrode
phase, and part or all of the reaction products must diffuse or be transported away from the elec-
trode surface. The cell should have adequate electrolyte transport to facilitate the mass transfer
to avoid building up excessive concentration polarization. Proper porosity and pore size of the
electrode, adequate thickness and structure of the separator, and sufficient concentration of the
reactants in the electrolyte are very important to ensure functionality of the cell. Mass-transfer
limitations should be avoided for normal operation of the cell.
5. The material of the current collector or substrate should be compatible with the electrode mate-
rial and the electrolyte without causing corrosion problems. The design of the current collector
should provide a uniform current distribution and low contact resistance to min imize electrode
polarization during operation.
6. For rechargeable cells, it is preferable to have the reaction products remain at the electrode
surface to facilitate the reversible reactions during charge and discharge. The reaction products
should be stable mechanically as well as chemically with the electrolyte.
In general, the principles and various electrochemical techniques described in this chapter can
be used to study all the important electrochemical aspects of a battery or fuel cell. These include
the rate of electrode reaction, the existence of intermediate reaction steps, the stability of the
electrolyte, the current collector, the electrode materials, the mass-transfer conditions, the value
of the limiting current, the formation of resistive films on the electrode surface, the impedance
characteristics of the electrode or cell, and the existence of the rate-limiting species.
2.2 THERMODYNAMIC BACKGROUND
In a cell, reactions essentially take place at two areas or sites in the device. These reaction sites are
the electrode interfaces. In generalized terms, the reaction at one electrode (reduction in forward
direction) can be represented by
aA+ ne cC (2.2)
where a molecules of A take up n electrons e to form c molecules of C. At the other electrode, the
reaction (oxidation in forward direction) can be represented by
bB d D+ ne (2.3)
The overall reaction in the cell is given by addition of these two half-cell reactions
aA + bB cC + dD (2.4)
