238
J. Zhu et al.
In alkaline aqueous electrolyte, the discharge reaction of zinc-air battery is:
2Zn + O 2 + 4O H
−
+ 2H 2 O = 2[Zn(O H) 4 ]
2−
(E
θ
= 1.60 V )
Theoretically, O 2 is reduced on the air electrode to generate OH
− . In neutral
electrolyte, OH
− is transferred to the negative electrode and combines with Zn
2+
to generate Zn (OH) 2 ; In alkaline electrolyte, OH
− combines with Zn
2+ generated
by negative electrode to generate [Zn(OH) 4 ]
2− . In fact, zinc-air battery has similar
problems with aluminum-air battery: first, the working voltage and current density
of neutral zinc-air battery are lower than those of strong alkaline battery; hydrogen
evolution corrosion of strong alkaline batteries is serious. Second, the zinc electrode
is easy to form “dendrites” after being corroded to damage the separator and make the
battery ineffective. Third, when the battery is discharged for a long time, the contact
of reaction products or alkaline electrolyte with external CO 2 will generate a large
amount of carbonate to precipitate on the surface of the air electrode and block the
air electrode gas channel, resulting in the performance and energy attenuation of the
battery. The carbonation of electrolyte will also enhance the acidity of electrolyte and
aggravate the hydrogen evolution corrosion of zinc electrode. Fourth, too high or too
low humidity in the external environment will cause “flooding” or “drying up” of the
semiopen air electrodes, and even “alkali climbing” or “liquid leakage” problems.
Unlike aluminum-air batteries, which improve aluminum alloy composition and heat
treatment process to solve hydrogen evolution, zinc-air batteries mainly achieve
their goals by adding oxides in the negative reaction zone and improving electrolyte
properties.
(2) Zinc cathode and electrolyte of zinc-air battery.
Metal oxides or hydroxides with high hydrogen overpotential are added to the reaction zone of the zinc electrode. The equilibrium potential of these metals in alkaline
solution is generally higher than that of zinc, which is preferentially deposited when
the electrode is charged and generally insoluble when the electrode is discharged.
Due to the high hydrogen evolution overpotential of these added metals, hydrogen
evolution corrosion reaction of Zn reducing H
+ in H 2 O to generate H 2 can be inhibited
while discharging metal zinc, and corrosion of zinc in electrolyte can be effectively
slowed down. Some scholars have also increased hydrogen evolution overpotential by adding high concentration of zinc ions to alkaline electrolyte to inhibit H 2
evolution corrosion.
In order to overcome the problem of carbonation of electrolyte due to long-term
use and absorption of CO 2 , neutral and slightly acidic electrolyte is generally adopted.
Although the working voltage and discharge current density of neutral or slightly
acidic zinc-air batteries are not as high as those of alkaline zinc-air batteries, they
can meet the discharge requirements of medium and small current densities and can
replace alkaline zinc-air batteries in low-power discharge sites. Adding a complexing
agent to the electrolyte to remove the dense zinc oxide layer will greatly improve
J. Zhu et al.
In alkaline aqueous electrolyte, the discharge reaction of zinc-air battery is:
2Zn + O 2 + 4O H
−
+ 2H 2 O = 2[Zn(O H) 4 ]
2−
(E
θ
= 1.60 V )
Theoretically, O 2 is reduced on the air electrode to generate OH
− . In neutral
electrolyte, OH
− is transferred to the negative electrode and combines with Zn
2+
to generate Zn (OH) 2 ; In alkaline electrolyte, OH
− combines with Zn
2+ generated
by negative electrode to generate [Zn(OH) 4 ]
2− . In fact, zinc-air battery has similar
problems with aluminum-air battery: first, the working voltage and current density
of neutral zinc-air battery are lower than those of strong alkaline battery; hydrogen
evolution corrosion of strong alkaline batteries is serious. Second, the zinc electrode
is easy to form “dendrites” after being corroded to damage the separator and make the
battery ineffective. Third, when the battery is discharged for a long time, the contact
of reaction products or alkaline electrolyte with external CO 2 will generate a large
amount of carbonate to precipitate on the surface of the air electrode and block the
air electrode gas channel, resulting in the performance and energy attenuation of the
battery. The carbonation of electrolyte will also enhance the acidity of electrolyte and
aggravate the hydrogen evolution corrosion of zinc electrode. Fourth, too high or too
low humidity in the external environment will cause “flooding” or “drying up” of the
semiopen air electrodes, and even “alkali climbing” or “liquid leakage” problems.
Unlike aluminum-air batteries, which improve aluminum alloy composition and heat
treatment process to solve hydrogen evolution, zinc-air batteries mainly achieve
their goals by adding oxides in the negative reaction zone and improving electrolyte
properties.
(2) Zinc cathode and electrolyte of zinc-air battery.
Metal oxides or hydroxides with high hydrogen overpotential are added to the reaction zone of the zinc electrode. The equilibrium potential of these metals in alkaline
solution is generally higher than that of zinc, which is preferentially deposited when
the electrode is charged and generally insoluble when the electrode is discharged.
Due to the high hydrogen evolution overpotential of these added metals, hydrogen
evolution corrosion reaction of Zn reducing H
+ in H 2 O to generate H 2 can be inhibited
while discharging metal zinc, and corrosion of zinc in electrolyte can be effectively
slowed down. Some scholars have also increased hydrogen evolution overpotential by adding high concentration of zinc ions to alkaline electrolyte to inhibit H 2
evolution corrosion.
In order to overcome the problem of carbonation of electrolyte due to long-term
use and absorption of CO 2 , neutral and slightly acidic electrolyte is generally adopted.
Although the working voltage and discharge current density of neutral or slightly
acidic zinc-air batteries are not as high as those of alkaline zinc-air batteries, they
can meet the discharge requirements of medium and small current densities and can
replace alkaline zinc-air batteries in low-power discharge sites. Adding a complexing
agent to the electrolyte to remove the dense zinc oxide layer will greatly improve