Zinc-air batteries operate on electrochemical principles that involve the oxidation of zinc at the anode and the reduction of oxygen at the cathode. These batteries are classified as metal-air systems, where oxygen from the ambient air serves as the active cathode material, eliminating the need for heavy internal oxidizers. The electrochemical reactions in zinc-air batteries are facilitated by an electrolyte, typically aqueous alkaline solutions such as potassium hydroxide (KOH) or neutral electrolytes like saline or mild acidic solutions. The fundamental advantage of this chemistry lies in its high theoretical energy density, stemming from the lightweight nature of oxygen and the high charge capacity of zinc.

The anode in a zinc-air battery consists of metallic zinc, which undergoes oxidation during discharge. The half-reaction at the anode in an alkaline electrolyte can be written as:
Zn + 4OH⁻ → Zn(OH)₄²⁻ + 2e⁻
The zincate ion (Zn(OH)₄²⁻) formed in this reaction is soluble in the electrolyte but tends to decompose into zinc oxide (ZnO) and water when saturation occurs:
Zn(OH)₄²⁻ → ZnO + H₂O + 2OH⁻
This precipitation of ZnO can lead to passivation of the anode if not managed properly, reducing battery performance over time. In neutral or mildly acidic electrolytes, the anode reaction may proceed differently, with zinc dissolving directly as Zn²⁺ ions.

The cathode in a zinc-air battery is responsible for the oxygen reduction reaction (ORR), a critical process that determines the efficiency of the battery. The ORR occurs at the triple-phase boundary where the electrolyte, oxygen, and conductive catalyst meet. In alkaline media, the ORR proceeds through two possible pathways: the four-electron pathway, which is more efficient, or the two-electron pathway, which produces peroxide intermediates. The preferred four-electron pathway can be represented as:
O₂ + 2H₂O + 4e⁻ → 4OH⁻
This reaction is highly dependent on the electrocatalyst used, with materials like manganese oxides, cobalt oxides, or precious metals such as platinum improving reaction kinetics. The two-electron pathway, which is less desirable due to its lower efficiency, proceeds as:
O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻
The peroxide ion (HO₂⁻) can further decompose or participate in side reactions that degrade the electrolyte or electrode materials.

The overall cell reaction during discharge combines the anode and cathode half-reactions. In an alkaline electrolyte, the net reaction is:
2Zn + O₂ → 2ZnO
This reaction is highly exergonic, contributing to the battery's high theoretical energy density. The thermodynamic voltage of a zinc-air cell under standard conditions is approximately 1.65 V, though practical operating voltages are often lower due to overpotentials at both electrodes.

The air cathode is a complex component that must facilitate oxygen diffusion, electron transfer, and ion conduction simultaneously. It typically consists of a porous carbon-based substrate coated with an ORR catalyst and a hydrophobic binder to prevent electrolyte flooding. The porosity of the cathode allows oxygen from the air to reach the reaction sites while maintaining sufficient hydrophobicity to avoid pore clogging by the liquid electrolyte. The electrolyte itself plays a crucial role in ion transport between the electrodes. Alkaline electrolytes, usually 6-8 M KOH, offer high ionic conductivity but are susceptible to carbonation from atmospheric CO₂, which forms carbonate salts that degrade performance over time. Neutral electrolytes avoid this issue but suffer from lower ionic conductivity and slower ORR kinetics.

During charging, the reactions reverse in a rechargeable zinc-air battery. The zinc oxide is reduced back to metallic zinc at the anode:
ZnO + H₂O + 2e⁻ → Zn + 2OH⁻
At the cathode, the oxygen evolution reaction (OER) occurs:
4OH⁻ → O₂ + 2H₂O + 4e⁻
The OER is more energetically demanding than the ORR, leading to higher overpotentials during charging. This asymmetry between discharge and charge reactions contributes to the round-trip inefficiency of rechargeable zinc-air batteries. The reversibility of these reactions is limited by factors such as zinc dendrite formation, shape change of the anode, and degradation of the air cathode.

The theoretical energy density of zinc-air batteries is among the highest of all battery systems, reaching approximately 1086 Wh/kg when considering only the mass of zinc, as oxygen is drawn from the air. In practice, accounting for the electrolyte, casing, and other components reduces this value to around 300-400 Wh/kg, still significantly higher than conventional lithium-ion batteries. The specific energy is particularly advantageous for applications where weight is a critical factor, such as in hearing aids or electric vehicles.

The discharge characteristics of zinc-air batteries are influenced by several factors, including oxygen availability, electrolyte composition, and temperature. Since oxygen must diffuse from the surrounding environment, the cathode design must balance sufficient air access with protection against electrolyte drying. High humidity can dilute the electrolyte, while low humidity may cause evaporation. Temperature affects both the ORR kinetics and the solubility of zincate species in the electrolyte, with performance generally peaking in moderate temperature ranges.

One of the key challenges in zinc-air battery technology is the limited cycle life of rechargeable versions. The irreversible formation of ZnO, zinc dendrite growth during recharging, and degradation of the air cathode materials all contribute to capacity fade over repeated cycles. Research efforts focus on improving anode morphology control, developing bifunctional catalysts for both ORR and OER, and optimizing electrolyte formulations to mitigate these issues. Another challenge is the relatively low power density compared to other battery types, stemming from the sluggish ORR kinetics and mass transport limitations of oxygen.

The electrolyte plays a multifaceted role beyond simple ion conduction. In alkaline zinc-air batteries, the concentration of KOH affects zincate solubility, ionic conductivity, and reaction rates. Too high a concentration may accelerate anode corrosion, while too low a concentration reduces conductivity and promotes ZnO precipitation. Additives such as corrosion inhibitors or zincate-complexing agents are sometimes incorporated to improve performance. In neutral or acidic electrolytes, different challenges arise, including hydrogen evolution at the anode and reduced ORR activity.

From a thermodynamic perspective, the zinc-air system benefits from the strong reducing power of zinc and the high oxidizing potential of oxygen. The Gibbs free energy change for the overall reaction is approximately -318 kJ/mol, corresponding to the high theoretical voltage. However, kinetic limitations, particularly at the air cathode, result in practical operating voltages closer to 1.2-1.4 V under load. The Nernst equation describes how the cell voltage varies with reactant activities, showing dependence on oxygen partial pressure and zincate concentration.

Future developments in zinc-air battery technology may focus on advanced cathode architectures with hierarchical porosity for improved oxygen transport, novel catalysts to enhance both ORR and OER kinetics, and engineered zinc anodes that resist shape change and dendrite formation. The fundamental electrochemistry of the zinc-air system provides a strong foundation for these improvements, with its combination of high energy density, environmental friendliness, and safety advantages over some other battery chemistries. Understanding the intricate balance between the electrochemical reactions, material properties, and system design is essential for advancing this technology toward broader commercial applications.