Environmental Engineering Reference
In-Depth Information
5.3.2.3 Nickel-Cadmium Batteries
Nickel-cadmium batteries use a hydrate nickel oxide as positive electrode, a
metallic cadmium as negative electrode and an aqueous solution of potassium
hydroxide as electrolyte [
17
]. The production of electrons at the negative plate
during discharge occurs via the following semi-reaction:
Cd
þ
2OH
!
Cd O
ð
2
þ
2e
ð
5
:
13
Þ
while at the nickel positive plate the discharge reaction involves a mechanism of
proton transfer between nickelic and nickelous hydroxide within the homogeneous
solid phase:
NiOOH
þ
H
þ
þ
e
!
Ni O
ð
2
ð
5
:
14
Þ
As a consequence the overall discharge reaction is the following:
2NiOOH
þ
Cd
þ
H
2
O
!
2Ni O
ð
2
þ
Cd O
ð
2
ð
5
:
15
Þ
with a cell voltage of 1.30 V at room temperature. During charge cadmium
hydroxide at the negative electrode is re-converted to metallic cadmium, while at
the positive electrode a proton is lost by nickelous hydroxide to re-form nickelic
hydroxide.
Nickel-cadmium batteries represent a good compromise between specific
energy, specific power and reliability, but they suffer from the memory effect, due
to the growth of cadmium crystalline particle size. In particular, they have a
specific energy of 50 Wh/kg, a specific power of 200 W/kg and can accept high
charge and discharge current rate, and this last characteristic makes them inter-
esting for electric vehicle manufacturers. Moreover nickel-cadmium batteries can
store slightly more energy per unit in weight than lead-acid batteries, and have a
fairly high rate of self-discharge at high temperature. Nevertheless a very
important issue to take into consideration is the toxicity of cadmium and the
danger of an environment contamination, because the recycling of nickel-cad-
mium batteries is complex. Those reasons have arrested the use of these systems
and made more attractive other types of batteries.
5.3.2.4 Nickel-Metal Hydride Batteries
The Ni-MH batteries use the same positive electrode and electrolyte as the Ni-Cd
systems, while hydrogen storage alloys are adopted as the negative electrode in
substitution of toxic cadmium. The metal alloys in which hydrogen is stored can be
based on mixtures of nickel with rare earth, or blends of nickel with titanium,
vanadium and zirconium. During discharge the cathodic semi-reactions are the
same as the Ni-Cd batteries, while at the anode the electron production is given by
the following semi-reaction:
