Because of the depletion of finite resources and the extensive
growth in the demand for alternative energy worldwide, metal–air
batteries have been proposed as alternative energy storage
devices. Metal–air batteries have received particular attention
because of their high energy density and capacity, the lack of
dependence of their capacity on load and temperature, their flat
discharge voltage and their low fabrication cost (depends on the
metal used) [1–6]. Lithium–air (Li–air) batteries have been
aggressively studied because of their broad potential for highperformance
applications [7–10]. Such batteries can also operate
as rechargeable batteries [11,12]. Unfortunately, however, during
battery fabrication, the lithium must be handled under inert
conditions because it is very sensitive to ambient conditions and
poses an explosion hazard [13–15]. This is the greatest challenge
for the Li–air battery. As alternatives, other active metal elements
such as aluminum have been recommended.
Aluminum (Al) is an attractive candidate anode material for
metal–air batteries because it has a high theoretical electrochemi-
cal equivalent value, 2.98 A h g1, which is the second highest after
that of lithium (3.86 A h g1) and higher than those of other active
metals, such as magnesium (2.20 A h g1) and zinc (0.82 A h g1)
[16–18]. Aluminum is also an inexpensive metal, as it is the second
most abundant metallic element after silicon, and is characterized
by its environmental friendliness, non-toxicity and high recyclability
[19]. The theoretical specific energy of an Al–air battery with
an alkaline electrolyte can be as high as 200 W h kg1, and with a
neutral salt solution, it is between 300 W h kg1 and 500 W h kg1
[18]. In this paper, we will provide an overview of recent material
developments for various elements of aluminum–air batteries,
including the anode, air cathode and electrolyte. Each component
and material has its own strengths and challenges.
This type of battery comprises three main components: an
anode, a cathode and an electrolyte. The discharging battery
serves as a galvanic cell that drives the electrical current in
an external circuit. The electrolyte plays an important role in
such a battery because it is the conducting medium through
which the two-way charge transfer proceeds between the
electrodes [18]. The electrolyte also separates the anode and
the cathode to avoid a short circuit and simultaneously provides
hydroxide ions to maintain the electrochemical reactions [20].
The oxidation reaction at the anode depends on the type of
electrolyte that participates in the reaction:
Anode : Al ! Al3 þ 3e (1.1)
Cathode : O2 þ 2H2O þ 4e ! 4OH (1.2)
Overall : 4Al þ 3O2 þ 6H2O ! 4AlðOHÞ3 (1.3)
Another undesired (parasitic) reaction occurs at the anode
because of the water reduction reaction. The parasitic hydrogengenerating
reaction can be expressed as follows:
Side reaction : Al þ 3H2O ! AlðOHÞ3 þ
3
2
H2 (1.4)
One major obstacle that hinders the deployment of the Al–air
battery on a commercial scale is the self-corrosion rate of
aluminum [21,22]. There are three main processes that occur on
the surface of aluminum that hinder further oxidation at the anode
in an aqueous-based cell: the formation of an oxide film of Al2O3 or
Al(OH)3; the formation of corrosion products, Al(OH)3 and
Al(OH)4
; and parasitic hydrogen evolution, which lowers the
potential of the battery [23,24]. Because of this limitation, further
development effort is needed to reduce the corrosion rate.