Chemistry
A silver oxide battery uses silver oxide as the positive electrode (cathode), zinc as the negative electrode (anode) plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag(I) to Ag and the zinc is oxidized from Zn to Zn(II). The chemical reaction that takes place inside the battery is the following:
Zinc is the activator in the negative electrode and corrodes in alkaline solution. When this happens, it becomes difficult to maintain the capacity of the unused battery. The zinc corrosion causes electrolysis in the electrolyte, resulting in the production of hydrogen gas, a rise of inner pressure and expansion of the cell. Mercury has been used in the past to suppress the corrosion, despite its harmful effects on the environment.
The silver–zinc battery, on the other hand, uses the opposite electrode composition, the cathode being made of pure silver, while the anode is made from a mixture of zinc oxide and pure zinc powders. The electrolyte used is pure potassium hydroxide solution without any added sodium hydroxide. Chemical processes during the discharge are similar to the silver oxide cell, while the different starting electrode composition makes it possible to recharge such a cell.
During the charge process, silver is first oxidized to silver(I) oxide: 2Ag + 2OH– → Ag2O + H2O + 2e– and then to silver(II) oxide: Ag + 2OH– → AgO + H2O + 2e–, while the zinc oxide is reduced to metallic zinc: 2Zn(OH)2 + 4e– = 2Zn + 4OH–. This process continues until the cell potential reaches the level where the electrolysis of the hydroxide ion is possible, about 1.55 V. This is usually taken as the sign of the end of the charge, as at this point no other charge is taken, and the oxygen generated poses a mechanical and fire hazard for the cell.
Read more about this topic: Silver-oxide Battery
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