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A cell is an appliance that converts chemical energy into electrical energy and acts as a power supply of electric current. It can be divided into two types – primary and secondary. A primary cell is a type of cell that can only be used once and then discarded. It cannot be recharged for use again. This happens because the chemical reaction taking place inside the cell is irreversible, i.e., it can only move in the forward reaction and cannot be reversed even after applying external energy. One of the most notable primary cells is the silver oxide cell.
Silver Oxide Cell – Meaning
It is a primary cell where zinc is used as the anode material, and silver oxide is used as the cathode. In an electrochemical cell, the anode material has to be such that it can be a good source of electrons. Metals are mostly used as the anode.
Zinc is a good reducing agent, as it can easily get converted from Zn(0) to Zn(II). Zinc is highly stable when it exists in the form of the Zn2+ cation. It is because the electronic configuration of Zn2+ is [Ar] 3d10. The d-orbital is fully filled, making the cation highly stable.
A cathode is where the electrons released by the anode get deposited and reduce the cathode material. Silver oxide here gets reduced to silver, and hydroxide ions are formed as the by-product of this chemical reaction.
Silver oxide cell is a small and primary cell that keeps a stable discharge with a high capacity. The first silver oxide battery (which is a combination of multiple silver oxide cells) was developed by Andre in the 1930s. The original zinc/silver voltaic combination was developed by Volta in the 1800s.
Silver Oxide Cell Chemistry
Apart from using silver oxide as cathode and zinc as the anode, we use an alkaline electrolyte like potassium hydroxide (KOH) or sodium hydroxide (NaOH) in the cell.
The following half cell reactions take place at the respective electrodes:
Zn + 2OH- → Zn(OH)2 + 2e- (EoZn/Zn2+> 1V)
The above reaction takes place at the anode. In the presence of hydroxide ions, zinc gets oxidised and releases 2 electrons. The electrode reduction potential is highly positive, making this reaction spontaneous and feasible.
Ag2O + 2e- + H2O → 2Ag↓ + 2OH- (EoAg+/Ag> 1V)
The above reaction takes place at the cathode. The electrons released from anode deposit at the silver oxide cathode. Due to the highly positive electrode reduction potential of this reaction, silver(I) ions are readily reduced to silver(0). Silver precipitates out.
The two individual half-cell reactions can be combined to get the overall cell reaction.
Ag2O + Zn + H2O → 2Ag↓ + Zn(OH)2
The cell gives the open-circuit voltage equal to around 1.55V.
Properties and Applications
Silver oxide batteries provide a high energy-to-weight ratio, i.e., high energy can be produced by a relatively lower weight. It is highly durable. It has a high tolerance capacity for the current load. It comes in a variety of sizes, from a small button to a large battery.
It has endless applications in the electronic industry. It is most notably used in calculators and watches as these appliances require a steady discharge of current and a small cell size. The cell employing potassium hydroxide as the electrolyte material is used in LCD watches. If the cell has sodium chloride as the electrolyte, then it would be preferably used in digital watches.
They are used in medical appliances, like hearing aids, and other equipment like pagers and cameras.
Large-sized silver oxide cells are not really used for day-to-day life applications. They are used for military purposes, for example, in submarines and torpedoes. It has a range of applications in aerospace engineering. One interesting fact is it was used to supply power to the moon buggy on Apollo space missions.
Drawbacks
Even though silver oxide cells have several advantages, one major drawback is that the anode material, i.e., zinc, can get corroded due to the high basicity of the electrolyte solution. Corrosion is a highly undesirable state as it becomes increasingly difficult to maintain the capacity of the cell. Electrolysis of the electrolyte takes place, which results in the formation of hydrogen gas. It causes the cell to expand, and the pressure inside it increases.
To counter this corrosion, mercury is employed in small amounts. But, the use of mercury is discouraged. Efforts are underway to produce silver oxide batteries without using mercury.
Another drawback is that it is expensive. Due to this reason, only small-sized silver oxide cells are used for commercial purposes. Large-sized cells are not used commercially.
Conclusion
Silver oxide cell notes are meant to help you understand the chemistry, history, and application of this cell. It is one of the most widely used primary cells. The chemical reaction happening inside the cell is irreversible. It is also called a silver-zinc cell, for its electrode materials are composed of these two metals. This cell has a flatter discharge curve when compared to an alkaline battery. The run-time is greater when compared to a lithium-ion battery. They do not pose thermal runaway problems (a problem very commonly seen in lithium-ion batteries).
