Page 46 - Handbook of Battery Materials

12 1 Thermodynamics and Mechanistics

The Faraday constant is the product of the elementary charge e (1.602 × 10 −19 C)
−1
23
and the Avogadro constant N A (6.023 × 10 mol ).
I · t Q
F = = = N A · e (1.3)
n n
Q = quantity of electricity, electric charge
n = number of moles of electrons exchanged.

For the Daniell-element the electron-donating reaction is the oxidation of zinc.
In the following the active mass m which is necessary to deliver a capacity of
1 Ah, is calculated.
Zn → Zn 2+ + 2e −
−1
−1
M = 65.4 g mol , z = 2, F = 26.8 Ah mol , Q = 1Ah
M
m = · Q
z · F
m = 1.22 g

Of course, Faraday’s first law applies to cathodic processes. Therefore, the deposi-
tion of 1 Ah copper ions results in an increase in the electrode mass of m = 1.18 g.
In addition, Faraday recognized that for different electrode reactions and the
same amount of charge the proportion of the reacting masses is equal to the
proportion of the equivalent masses
m A M A · |z B |
= (1.4)
m B M B · |z A |
Equation 1.4 expresses the fact that 1 mol electrons discharges

• 1 mol monovalent ions,
• 1/2 mol bivalent ions, or
• 1/z mol z-valent ions.

1.2.4
Charging

The charging process can only be applied to secondary cells, because, in contrast
to primary cells, the electrochemical reactions are reversible. If primary cells
are charged, this may lead to electrochemical side reactions, for example, the
decomposition of the electrolyte solution with dangerous follow-up reactions
leading to explosions [8].
While charging, ions are generally reduced at the negative electrode and an
oxidation process takes place at the positive electrode. The voltage source must be
at least equivalent to the difference ε 00 between the equilibrium potentials of the
two half cells. Generally the charge voltage is higher.