Environmental Engineering Reference
In-Depth Information
mechanical replacement of the spent anode, for remote recharging. Because of this
type of recharging the zinc-air batteries can be also regarded as zinc-air fuel cells,
where the zinc is the replaceable fuel and the air flow, supplied from outside of the
battery system to the cathode side, controls the reaction rate. Because of their
specific operation zinc-air batteries require a proper air-management system to
ensure the right amount of air into the battery for the required power. These
batteries require particular attention during their operation, because quick charge
rates and deep discharges below the minimum voltage could permanently
damage the battery. High value of energy (200 Wh/kg) can be stored in zinc-air
batteries with a low specific power (80 W/kg) (Fig.
5.14
; Table
1.8
), for that
reason zinc-air batteries could be conveniently connected to high power energy
storage systems (supercapacitors,
Sect. 5.4.2
).
5.3.2.8 Aluminum-Air Batteries
The study of aluminum anodes for high specific power Al-air batteries is gaining
in recent years an increasing attention [
35
] due to the very promising theoretical
performance of this type of batteries (theoretical specific energy higher than 8000
Wh/kg, Table
1.8
). These systems generally use alkaline electrolytes and are based
on the following discharge semi-reactions:at the anode:
Al
þ
3OH
!
Al O
ð
3
þ
3e
ð
5
:
26
Þ
at the cathode:
O
2
þ
2H
2
O
þ
4e
!
4OH
ð
5
:
27
Þ
Then the overall energy productive cell reaction is:
4Al
þ
3O
2
þ
6H
2
O
!
4Al O
ð
3
ð
5
:
28
Þ
The main practical problem of this type of battery is the tendency of the anode
to corrosion, according the following parasitic reaction:
Al
þ
3H
2
O
!
Al O
ð
3
þ
3
=
2H
2
ð
5
:
29
Þ
This phenomenon degrades the Coulombic efficiency of the anode, then it has
to be minimized for practical applications where high capacity is required. This
could be overcome by particular aluminum alloys more resistant to corrosion, but
the competing requirement of fast anodic dissolution makes very difficult the
research of the suitable material [
36
,
37
]. Another possibility to improve the anode
performance is to modify the electrolyte composition by adding corrosion inhib-
itors [
38
]. The difficulties met up today in this field leave the possibility to use the
aluminum-air batteries only as mechanically rechargeable systems, with practical
performance (300-500 Wh/kg) very far from the theoretical values (Table
1.8
;
Fig.
5.14
).
