6 Application of Oxygen Reduction Catalysts
233
hydrogen evolution reaction with the electrolyte. When the electrolyte is strongly
alkaline, Al(OH) 3 attached to the surface of the aluminum cathode combines with
OH
− in the electrolyte to generate soluble [A1(OH) 4 ]
− so that the metal aluminum is
continuously exposed to the electrolyte to ensure continuous and rapid discharge of
the aluminum metal. When the negative electrode material of the battery is discharged
to the limit, the purpose of fast charging can be achieved by renewing the negative
electrode metal plate and electrolyte.
In fact, in the neutral electrolyte, due to the adhesion of Al(OH) 3 on the surface
of the negative electrode, the discharge speed of metallic aluminum is slowed down,
resulting in that the neutral aluminum-air battery cannot discharge at large current.
However, in the strong alkaline electrolyte, the exposed metal aluminum will have a
side reaction of hydrogen evolution with the alkaline electrolyte.
2Al + 6H 2 O + 2OH − = 2[A1(OH) 4 ] − + 3H 2 .
At the same time of high-power discharge, there is the problem of low energy
utilization rate (also called battery efficiency) of aluminum cathode due to selfcorrosion phenomenon. Therefore, it is necessary to solve the above problems from
the aspects of aluminum negative electrode, electrolyte, etc.
(2) Aluminum cathode of aluminum-air battery
From the standard electrode potentials E
8 (Al(OH) 3 /Al) = −2.31 V and
E
8 ([Al(OH) 4 ]
− /Al) = −2.328 V of aluminum, it can be seen that aluminum has
a strong ability to lose electrons in neutral and alkaline solutions. However, when the
aluminum electrode is actually discharged, the electrode potential will move forward.
For example, even when it is discharged at a current density of 100 mA/cm
2 in strong
alkaline electrolyte, the electrode potential will even move forward to about − 1.2 V,
showing a serious polarization phenomenon, which is even more serious in neutral
electrolyte [50–53]. The reasons for this are: (1) aluminum oxide passivation film
exists on the surface of aluminum, which inhibits the electrochemical activity of
aluminum; (2) in strong alkaline electrolyte, the amphoteric nature of aluminum
will cause serious hydrogen evolution corrosion of aluminum, resulting in positive
electrode potential shift and reduction of battery current efficiency. This corrosion
will not stop even in nonworking state. (3) Al(OH) 3 colloid generated by corrosion
reaction will reduce the conductivity of electrolyte. In order to solve the problems of
aluminum surface passivation and hydrogen evolution corrosion, trace alloy elements
are usually added to metal aluminum and the heat treatment process of aluminum
alloy is improved at the same time [49, 53]. In order to solve the problem that the
conductivity of electrolyte drops due to Al(OH) 3 colloid, electrolyte is usually filtered
or other additives are added for improvement.
Trace alloying elements added to aluminum anodes can be divided into three
categories according to their functions in aluminum anodes: (1) metals used to destroy
passivation films and reduce oxide film resistance, such as Sn, Ga, In, etc. (2) metals
233
hydrogen evolution reaction with the electrolyte. When the electrolyte is strongly
alkaline, Al(OH) 3 attached to the surface of the aluminum cathode combines with
OH
− in the electrolyte to generate soluble [A1(OH) 4 ]
− so that the metal aluminum is
continuously exposed to the electrolyte to ensure continuous and rapid discharge of
the aluminum metal. When the negative electrode material of the battery is discharged
to the limit, the purpose of fast charging can be achieved by renewing the negative
electrode metal plate and electrolyte.
In fact, in the neutral electrolyte, due to the adhesion of Al(OH) 3 on the surface
of the negative electrode, the discharge speed of metallic aluminum is slowed down,
resulting in that the neutral aluminum-air battery cannot discharge at large current.
However, in the strong alkaline electrolyte, the exposed metal aluminum will have a
side reaction of hydrogen evolution with the alkaline electrolyte.
2Al + 6H 2 O + 2OH − = 2[A1(OH) 4 ] − + 3H 2 .
At the same time of high-power discharge, there is the problem of low energy
utilization rate (also called battery efficiency) of aluminum cathode due to selfcorrosion phenomenon. Therefore, it is necessary to solve the above problems from
the aspects of aluminum negative electrode, electrolyte, etc.
(2) Aluminum cathode of aluminum-air battery
From the standard electrode potentials E
8 (Al(OH) 3 /Al) = −2.31 V and
E
8 ([Al(OH) 4 ]
− /Al) = −2.328 V of aluminum, it can be seen that aluminum has
a strong ability to lose electrons in neutral and alkaline solutions. However, when the
aluminum electrode is actually discharged, the electrode potential will move forward.
For example, even when it is discharged at a current density of 100 mA/cm
2 in strong
alkaline electrolyte, the electrode potential will even move forward to about − 1.2 V,
showing a serious polarization phenomenon, which is even more serious in neutral
electrolyte [50–53]. The reasons for this are: (1) aluminum oxide passivation film
exists on the surface of aluminum, which inhibits the electrochemical activity of
aluminum; (2) in strong alkaline electrolyte, the amphoteric nature of aluminum
will cause serious hydrogen evolution corrosion of aluminum, resulting in positive
electrode potential shift and reduction of battery current efficiency. This corrosion
will not stop even in nonworking state. (3) Al(OH) 3 colloid generated by corrosion
reaction will reduce the conductivity of electrolyte. In order to solve the problems of
aluminum surface passivation and hydrogen evolution corrosion, trace alloy elements
are usually added to metal aluminum and the heat treatment process of aluminum
alloy is improved at the same time [49, 53]. In order to solve the problem that the
conductivity of electrolyte drops due to Al(OH) 3 colloid, electrolyte is usually filtered
or other additives are added for improvement.
Trace alloying elements added to aluminum anodes can be divided into three
categories according to their functions in aluminum anodes: (1) metals used to destroy
passivation films and reduce oxide film resistance, such as Sn, Ga, In, etc. (2) metals