Quaternary ammonium-polybromide complexes

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights 

It is evident that the shapes and relative stabilities of the polybromide anions depend to a large extent on the nature of the countercations. The tendency to form 

An overview of the most important quaternary ammonium salts tested for possible applicability in zinc-bromine batteries is presented in Table 2. A rough classification has been applied according to the substance classes of the substituents attached to the nitrogen. 

Ai-chloromethyl-iV-methyl pyrrolidinium bromide (C-MEP) [48,49,75] A -chloromethyl-A-methyl morpholidinium bromide (C-MEM) [49] 2-Bromo-cyclohexylpyridinium bromide [50, 56, 63, 76] 

The two basic requirements for efficient bromine storage in zinc-bromine batteries, which need to be met in order to ensure low self-discharge and more over a substantial reduction of equilibrium vapor pressure of Brj of the poly bromide phase in association with low solubillity of active bromine in the aqueous phase. As mentioned by Schnittke [4] the use of aromatic A-substituents for battery applications is highly problematic due to their tendency to undergo bromination. Based on Bajpai s 

Br forms V-shaped planar species [42, 43] with relatively strong terminal and weak central bonds. Raman spectra in aqueous and acetonitrile solutions [36] exhibit terminal stretching modes of 250 and 257 cm which are essentially higher than those in Br. Structural data from experimental studies exist only for crystalline compounds in which the Brj ions adopt linear geometry. In a theoretical study using density functional theory [43], a central bond angle of 114.6° and a central terminal bond ratio of were obtained the latter is somewhat 

Eustace noticed the correlation between asymmetric N-substitution and low melting points (at temperatures 15 °C) of the substances [73] observed in this study. The importance of sufficiently hi densities of the fused salts for an efficient separation of the complex phase and the aqueous solution was emphasized. Mixtures of various quaternary ammonium bromides were tested subsequently [48, 49, 66, 75], aiming particularly at the avoidance of crystallization in the 

Benzyldimethyl-(3-isobutoxy-2-hydroxy propyl)-N+Br [60] N-Methyl-N-ethyl pyrrolidinium bromide (MEP) [3,14, 47-49, 63-72] N-Methyl-N-ethyl morpholinium bromide (MEM) [3, 14, 47-49, 53, 56, 63-73] 

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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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