Polybromide ions

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

Fewer studies of polybromide ions have been carried out. Many salts involving [Br3] are known, and the association in the solid state of [Br3] and Br has been observed to give rise to the linear species 16.19. The [Br ] ion is structurally analogous to [Ig] (Figure 16.7) with Br—Br bond distances that indicate association between Br2 and [Br3] units in the crystal. 

The chemical species present in the electrolyte are actually more complex than that described. In solution, elemental bromine exists in equilibrium with bromide ions to form polybromide ions, Br, where = 3, 5, 7. Aqueous zinc bromide is ionized, and zinc ions exist as various complex ions and ion pairs. The electrolyte also contains complexing agents which associate with polybromide ions to form a low-solubility second liquid phase. The complex reduces the amount of bromine contained in the aqueous phase 10 to 100-fold, which, in addition to the separator, also reduces the amount of bromine available in the eeU for the self-discharge reaction. The complex also provides a way to store bromine at a site remote from the zinc deposits and is discussed further in the next section. Salts with organic cations such as iV-methyl-iV-ethylmorpholinium bromide (MEMBr) are commonly used as the complexing agents. One researcher has proposed a mixture of four quaternary ammonium salts for use in zinc/bromine batteries. The proposed electrolyte has favorable properties with regard to aqueous bromine concentration, resistivity, and bromine diffusion and does not form solid complexes at low temperatures (5°C and above). Complexes with quaternary ammonium ions are reversible and also have an added safety benefit due to a much reduced bromine vapor pressure (see Sec. 35.6). 

Even though zinc-bromine batteries operate with a slightly acidic electrolyte (pH 3), they are discussed here briefly, because they offer another way of escaping the problems of zinc deposition. At this pH value both zinc corrosion as well as the tendency toward dendrite formation are low the latter, furthermore, is prevented by electrolyte circulation [121]. The separator, besides meeting the usual requirements, has to perform an additional duty although it must permit the charge transfer of zinc and bromide ions, it should suppress the transfer of dissolved bromine, of polybromide ions, or of the complex phase. Due to mechanical and chemical susceptibility, ion-selective membranes did not prove effective. Microporous polyethylene separators are usually used in their manufacture and properties they are quite similar to those described in Section 11.2.3.1. 

Safety risks and the environmental impact are of major importance for the practical success of bromine storage system. The nonaqueous polybromide complexes in general show excellent physical properties, such as good ionic conductivity (0.1-0.05 Qcirf1), oxidation stability (depending on the nature of the ammonium ion), and a low bromine vapor pressure. The concentration of active bromine in the aqueous solution is reduced by formation of the complex phase up to 0.01-0.05 mol/L, hence ensuring a decisive decrease of selfdischarge. 

ORD curves of the polyiodide and the polythiocanate are concentration-dependent in opposition to these of polychoride and polybromide [29]. The given interpretation uses a j5-structure-random coil conformational transition due to the site-binding of iodide and thiocyanate ions. We must note that this site-binding could be sufficient to explain the differences of ORD curves of the salts if the side chain chromophores appear optically active which is not shown in the paper. 

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