Zinc-air batteries (ZABs) are a specific type of metal-air battery that uses oxygen from the surrounding air to generate electrical power. Drawing one reactant from the atmosphere allows them to achieve a very high energy density, as they do not need to store an oxidant internally like conventional batteries. Their high energy density and relatively low cost make them highly suitable for small, long-lasting power sources, which is why they are commonly used in devices like hearing aids. The zinc-air battery functions more like a hybrid between a conventional battery and a fuel cell, where the zinc metal acts as the fuel. The controlled reaction between the zinc and the atmospheric oxygen produces electricity, setting them apart from other battery chemistries.
Essential Components and Structure
A zinc-air battery has a relatively simple architecture, consisting of a zinc anode, an alkaline electrolyte, and a unique air cathode. The anode, which is the negative electrode, is typically made from zinc powder mixed into a gel or paste with the electrolyte. This powdered form of zinc provides a large surface area for the chemical reaction, maximizing the energy output.
The electrolyte is usually a highly conductive aqueous solution of potassium hydroxide (KOH). A porous separator is situated between the anode and the cathode to prevent a short circuit while still permitting ionic movement.
The positive electrode, known as the air cathode, is the defining component that enables the battery’s function. This electrode is a porous structure, often made of carbon, that is permeable to air. The air cathode contains specialized catalysts embedded within its structure to facilitate the chemical reaction with oxygen. There must be a small hole or port in the casing that allows oxygen to diffuse into this porous cathode.
The Core Electrochemical Process
The generation of electricity in a zinc-air battery begins when the air seal is removed, allowing atmospheric oxygen to enter the air cathode. This process is essentially a controlled form of oxidation, or rusting, of the zinc metal.
At the air cathode, the oxygen, along with water and electrons returning from the external circuit, is reduced to form hydroxide ions. This reaction, known as the oxygen reduction reaction (ORR), is accelerated by the catalysts within the cathode. The resulting hydroxide ions then migrate through the potassium hydroxide electrolyte toward the zinc anode.
Upon reaching the anode, the hydroxide ions react with the zinc metal, which causes the zinc to oxidize and release electrons. This reaction forms a soluble intermediate product called zincate. The electrons liberated by the zinc oxidation flow through the external circuit to power a device before returning to the air cathode.
The soluble zincate eventually decays into solid zinc oxide and water. The overall chemical process effectively converts zinc and oxygen into zinc oxide and electricity, with the water and hydroxide ions being recycled within the cell. This continuous consumption of zinc and intake of oxygen sustains the current flow until the zinc material is fully consumed.
Primary and Secondary Battery Variations
The most common zinc-air batteries are primary cells, meaning they are non-rechargeable. These primary ZABs offer high energy density and are inexpensive to manufacture, making them the standard choice for button-cell applications like hearing aids. The process of converting zinc metal to zinc oxide during discharge is chemically irreversible, making recharging impractical.
Secondary (Rechargeable) ZABs
Secondary, or electrically rechargeable, zinc-air batteries have been developed for reusable energy storage. Reversing the chemical reaction during charging presents significant technical difficulties.
One major challenge is designing a bifunctional air cathode that can efficiently facilitate both the oxygen reduction reaction during discharge and the oxygen evolution reaction during recharge.
Managing the zinc anode during charging is another significant hurdle. When the reaction is reversed, zinc metal is replated onto the anode from the electrolyte. This can lead to the formation of zinc dendrites. These dendrites can grow through the separator and cause an internal short circuit, prematurely ending the battery’s life. Advancements are being made to overcome these issues, positioning rechargeable ZABs as a potential solution for large-scale applications such as grid energy storage.