Page 32 - Lindens Handbook of Batteries

BASIC CONCEPTS 1.9

At the positive electrode, the reaction is:
+
Ni(OH) + OH → − NiOOH HO+ e
2 2
After recharge, the secondary battery reverts to its original chemical state and is ready for further
discharge. These are the fundamental principles involved in the charge–discharge mechanisms of a
typical secondary battery.

1.3.4 Fuel Cell
A typical fuel cell reaction is illustrated by the hydrogen/oxygen fuel cell. In this device, hydrogen is
oxidized at the anode, electrocatalyzed by platinum or platinum alloys, while at the cathode, oxygen is
reduced, again with platinum or platinum alloys as electrocatalysts. The simplified anodic reaction is:

2H → 4H + + 4e
2
while the cathodic reaction is:

O + 4 H + + 4 → 2 HO
e
2 2
The overall reaction is the oxidation of hydrogen by oxygen, with water as the reaction product:
2H + O → 2HO
2 2 2

1.4 THEORETICAL CELL VOLTAGE, CAPACITY, AND ENERGY

The theoretical voltage and capacity of a cell are a function of the anode and cathode materials. (See
Chap. 2 for detailed electrochemical theory.)

1.4.1 Free Energy

Whenever a reaction occurs, there is a decrease in the free energy of the system, which is expressed as:

∆ 0 =G −nFE 0

where F = constant known as the Faraday (≈96,500 C or 26.8 Ah)
n = number of electrons involved in stoichiometric reaction

E = standard potential, V
0

1.4.2 Theoretical Voltage
The standard potential of the cell is determined by the type of active materials contained in the cell.
It can be calculated from free-energy data or obtained experimentally. A listing of electrode poten-
tials (reduction potentials) under standard conditions is given in Table 1.1. A more complete list is
presented in Appendix B.
The standard potential of a cell can be calculated from the standard electrode potentials as follows
(the oxidation potential is the negative value of the reduction potential):