Zinc-metal oxide cells

Materials of construction for the zinc/mercuric oxide cells are limited not only by their ability to survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility with the electrode materials. As far as the external contacts are concerned, these are decided by corrosion resistance, compatibility with the equipment interface with respect to galvanic corrosion, and, to some degree, cosmetic appearance. Metal parts may be homogeneous, plated metal, or clad metal. Insulating parts may be injection-, compression-, or transfer-molded polymers or rubbers. 

The zinc/silver oxide cell consists of three active components a powdered zinc metal anode, a cathode of compressed silver oxide, and an aqueous electrolyte solution of potassium or sodium hydroxide with dissolved zincates. The active components are contained in an anode top, cathode can, separated hy a harrier and sealed with a gasket. 

Because of the slight solubility of silver oxides in alkaline electrolyte, little work was done with zinc/silver oxide cells until 1941 when Andre suggested the use of a cellophane barrier. Cellophane prevents migrating silver ions from reaching the anode - " by reducing them to insoluble silver metal. The cellophane is oxidized and destroyed in the process, making it less effective for long-life cells. 

The more familiar types of primary alkaline systems are the zinc/manganese dioxide, zinc/ mercuric oxide, and zinc/silver oxide batteries. These, typically, use potassium or sodium hydroxides, in concentrations from 25 to 40% hy weight, as the electrolyte, which functions primarily as an ionic conductor and is not consumed in the discharge process. In simple form, the overall discharge reaction for these metal oxide cells can be stated as... 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

The market for batteries is huge, with new types and applications being developed all the time. For example, a watch battery is a type of silver oxide cell silver in contact with silver oxide forms one half-cell while the other is zinc metal and dications. Conversely, a car battery is constructed with the two couples lead(IV) lead and lead(IV) lead(II). The electrolyte is sulphuric acid, hence this battery s popular name of lead-acid cell (see further discussion on p. 347). 

Zinc is more easily oxidized than iron. Therefore, zinc, not iron, becomes the anode in the galvanic cell. The zinc metal is oxidized to zinc ions. In this situation, zinc is known as a sacrificial anode, because it is destroyed (sacrificed) to protect the iron. Iron acts as the cathode when zinc is present. Thus, iron does not undergo oxidation until all the zinc has reacted. 

Let us continue with the example of copper ions in contact with copper metal and zinc ions in contact with zinc metal. This combination is usually referred to as the Darnell cell or zinc/copper couple(Fig. 6.5a). For this electrochemical cell the reduction and oxidation processes responsible for the overall reaction are separated in space one half reaction taking place in one electrode compartment and the other takes place in the other compartment. 

The standard cell potential for the zinc electrode for the oxidation of zinc metal to zinc ion is 0.76 volt. 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

The manufacture of secondary batteries based on aqueous electrolytes forms a major part of the world electrochemical industry. Of this sector, the lead-acid system (and in particular SLI power sources), as described in the last chapter, is by far the most important component, but secondary alkaline cells form a significant and distinct commercial market. They are more expensive, but are particularly suited for consumer products which have relatively low capacity requirements. They are also used where good low temperature characteristics, robustness and low maintenance are important, such as in aircraft applications. Until recently the secondary alkaline industry has been dominated by the cadmium-nickel oxide ( nickel-cadmium ) cell, but two new systems are making major inroads, and may eventually displace the cadmium-nickel oxide cell - at least in the sealed cell market. These are the so-called nickel-metal hydride cell and the rechargeable zinc-manganese dioxide cell. There are also a group of important but more specialized alkaline cell systems which are in use or are under further development for traction, submarine and other applications. 

The Oxidation-Reduction Reactions Part 2 movie (1eChapter 18.1) and the Galvanic Cells I movie (eChapter 18.1) both illustrate the same reaction, oxidation of zinc metal by copper(II) ions. Explain why this reaction as it is shown in the Oxidation-Reduction Reactions Part 2 movie cannot be used to generate a voltage. 

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