Page 45 - Handbook of Battery Materials

1.2 Electrochemical Fundamentals 11

−
Zn → Zn 2+ + 2e , normal potential ε 00 =−0.76 V NHE
−
Cu → Cu 2+ + 2e , normal potential ε 00 =+0.34 V NHE
For the Daniell element in Figure 1.2 the following potential difference is obtained:

ε 00 = ε 00, Cu/Cu 2+ − ε 00, Zn/Zn 2+ (1.1)

Cu/Cu 2+ = + 0.34 V terminal voltage
= 1.10 V +
+
H /2H = 0.00 V H /2H = 0.00 V
2
2
Zn/Zn 2+ =− 0.76 V
Under equilibrium conditions the potential difference ε 0 corresponds to the
terminal voltage of the cell.
If there are no standard conditions or if it should not be possible to measure the
standard potential, the value can be determined by thermodynamic calculations
(see Section 1.4.1).
For the arrangement of a galvanic cell for use as a power source the half cells
are chosen such that their potentials ε I,II are as far apart as possible. Therefore,
it is obvious why alkaline metals, especially lithium or sodium, are interesting as
new materials for the negative electrode. As they have a strong negative standard
potential and a comparatively low density, a high specific energy can be realized by
combination with a positive electrode.
The following examples, the Daniell element, nickel-cadmium cells, and
lithium-manganese dioxide cells, show the influences of the electrode materials
on different cell parameters.

1.2.3
Discharging

During the discharge process, electrons are released at the anode from the
electrochemically active material, which is oxidized. At the same time, cathodic
substances are reduced by receiving electrons. The transport of the electrons occurs
through an external circuit (the consumer).
Looking at first at the anode, there is a relationship between the electronic current
I and the mass m of the substance which donates electrons, and this is known as
the first Faraday law [7]:
M
m = · I · t (1.2)
z · F
m = active mass
M = molar mass
z = number of electrons exchanged
−1
F = Faraday constant: 96 485 C mol −1 = 26.8 Ah mol .
t = time