Positive Electrodes in Other Aqueous Systems
This chapter discusses three topics relating to positive electrodes in aqueous electrolyte battery systems, the manganese dioxide electrode, the nickel electrode and the so-called memory effect that is found in batteries that have “nickel” positive electr
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Positive Electrodes in Other Aqueous Systems
19.1
Introduction
This chapter discusses three topics relating to positive electrodes in aqueous electrolyte battery systems, the manganese dioxide electrode, the nickel electrode and the so-called memory effect that is found in batteries that have “nickel” positive electrodes. The first of these deals with a very common material, MnO2, that is used in the familiar “alkaline” cells that are found in a very large number of small portable electronic devices. This electrode operates by a simple proton insertion reaction. MnO2 can have a number of different crystal structures, and it has been known for many years that they exhibit very different electrochemical behavior. It is now recognized that the properties of the most useful version can be explained by the presence of excess protons in the structure, whose charge compensates for that of the Mn4+ cation vacancies that result from the electrolytic synthesis method. The “nickel” electrode is discussed in the following section. This electrode is also ubiquitous, as it is used in several types of common batteries. Actually, this electrode is not metallic nickel at all, but a two-phase mixture of nickel hydroxide and nickel oxy-hydroxide. It is reversible, and also operates by the insertion and deletion of protons. The mechanism involves proton transport through one of the phases that acts as a solid electrolyte. The result is the translation of a two-phase interface at essentially constant potential. The third topic in this group is a discussion of what has been a vexing problem for consumers. It occurs in batteries that have nickel positive electrodes. The mechanism that results in the appearance of this problem is now understood. In addition, the reason for the success of the commonly used solution to it can be understood.
© Springer International Publishing Switzerland 2016 R.A. Huggins, Energy Storage, DOI 10.1007/978-3-319-21239-5_19
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19.2
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Positive Electrodes in Other Aqueous Systems
Manganese Dioxide Electrodes in Aqueous Systems
19.2.1 Introduction Manganese dioxide, MnO2, is the reactant that is used on the positive side of the very common alkaline cells that have zinc as the negative electrode material. There are several versions of MnO2, some of which are much better for this purpose than others. Thus this matter is more complicated than it might seem at first. MnO2 is polymorphic, with several different crystal structures. The form found in mineral deposits has the rutile (beta) structure, and is called pyrolusite. It is relatively inactive as a positive electrode reactant in KOH electrolytes. It can be given various chemical treatments to make it more reactive, however. One of these produces a modification containing some additional cations that is called birnessite. Manganese dioxide can also be produced chemically, and then generally has the delta structure. The material that is currently much more widely used in batteries is produced electrolytically, and is called EMD. It has the gamma (
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