Ammonium polybromide complexes

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. 

In the zinc/bromine cell, the halogen is stored in the electrolyte as a polybromide (mainly Bra ) and hence a separator is essential Nafion cation exchange membrane has again been used. The electrolyte is aqueous zinc bromide with tetraalkyl-ammonium bromide to aid the bromine complex formation. 

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). 

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