Physical Properties of the Bromine Storage Phase

The conductivity of a number of bromine containing complexes with different quaternary ammonium cations was studied by Gerold (see Ref. [56]) with respect to the dependence on temperature and bromine 

as long as equal amounts of Bfj are added. According to these investigations the conductivities of the fused polybromide salts increase exponentially with the concentration of Brj reaching values typical for the aqueous electrolyte phases (11-20 Q/cm) at very high bromine contents such as 3 mol Br2/mol complexing agent. 

The dependence on the temperature of the specific resistance (Q/cm) of the pure MEPBr and MEMBr complexes, and a 1 1 mixture there of, as obtained in Ref. [73], is listed in Table 4. It is remarkable that within the complex phases consisting of Bf2 and either pure MEP or MEM the change of specific resistance at the liquid — solid phase transition amounts to about one order of magnitude, where as the value is only doubled in the 1 1 mixture. The table also indicates that MEMBr complexes possess higher melting temperatures. 

Electrical conductivities of polybromide complexes containing MEP and MEM were studied by Arbes [73]. Pure MEPBr complexes always show higher conductivity than those containing only 

Eustace [75] studied the specific resistance of samples of bromine-fused salt phase produced by electrolysis of 3.0 mol L ZnBr2 and 1.0 mol L MEM at 23 °C. As is shown in Fig. 4, a considerable resistance is observed in the initial phase of the charge process, dropping to approximately one-third at 30% Zn utilization. At higher states of charge the increase in the conductivity is significantly slower. 

The dependence on the temperature of the specific resistance (tl/cm) of the pure MEPBr and MEMBr complexes, and a 1 1 mixture thereof as obtained in Ref. [71], is Hsted in Table 7.4. It is remarkable that within the complex phases consisting of 

Bt2 and either pure MEP or MEM the change of specific resistance at the hquid soHd phase transition amounts to about 1 order of magnitude, whereas the value is only doubled in the 1 1 mixture. The table also indicates that MEMBr complexes possess higher melting temperatures. 

30% Zn utilization. At higher states of charge the increase in the conductivity is significantly slower. 

Niepraschk [63] found 23.4 kj mol for pure MEMBr complex and 16.5 kj mol for pure MEPBr complexes at 3 mol Br2/mol complexing agent. These values increase slightly with decreasing concentration of Br2. A value of of 11.5 kJ mol for a complex phase containing MEP MEM in the ratio 3 1 at comparable contents of Bt2 was reported by Hauser [70]. 

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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. 

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