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7.2 Possibilities for Bromine Storage 199

6) long-term stability against oxidation or bromination of organic substituents by
bromine;
7) minimization of health and safety risks and environmental impact in the case
of battery failure (electrolyte leakage) or damage.
The ionic conductivities of the polybromide complexes are considerable, ap-
proaching the values of ordinary aqueous salt solutions, due to their ‘fused-salt’
nature. This property means an additional advantage since the electrolyte resistivity,
the ohmic voltage drop, and polarization effects remain low.
The properties and behavior of the bromine-storing polybromide complexes
will now be treated in detail from the fundamental and technological viewpoints,
including economic and ecological (safety risks, recyclability, disposal, etc.) aspects.

7.2
Possibilities for Bromine Storage

7.2.1
General Aspects
Previously studied possibilities for bromine storage systems are listed in Table 7.1.
The widely known reduction of the Br 2 vapor pressure by formation of adducts
with various carbon materials results from strong chemisorption interactions and
has been investigated in the case of activated carbon [4]. Adducts containing up
to ∼85 wt% Br 2 were reported to be stable at ambient temperature; however,
considerable equilibrium bromine vapor pressures were found.
Intercalation of molecular Br 2 into graphite yields compounds with higher
thermal stabilities [6, 7] forming so-called residual compounds which do not
◦
quantitatively release all the included Br 2 even at temperatures of ∼2000 C
[26, 31]. Bromine is fixed in the graphitic lattice, bound in two different ways:
(i) chemisorption on structural crystalline defects and layer edges and (ii) inter-
calation, thus producing separated islands without contacting the crystal edges.
Repeated intercalation and extraction of intercalant species leads to exfoliation and
destruction of the graphite material. Formation of bromine adducts with zeolites
has been reported [8]. A considerable number of crystalline organic compounds
containing polyiodide and polybromide anions such as TMA·0.7H 2 O·xHal 5 H
(TMA = trimesic acid, x = 0.09 for Hal = I, and x = 0.103 for Hal = Br) [32] or
M(dpq) 2 Hal (M = Ni; Pd; dpg = diphenylglyoxime) [33] have been tested with the
intention of modeling new conducting materials. Electrical conductance was re-
ported to depend only negligibly on the nature of the anions. Infinite linear chains
−
of essentially Hal units were found in XRD analyses [32].
5
As early as 1888 Horton reported the formation of stable urotropin-bromide
adducts [23]. The storage of bromine in organic solids and particularly as a
liquid complex phase stabilized by quaternary ammonium salts is of outstanding
importance for modern battery applications. In conjunction with polymeric and
porous carbon matrices, these species were used in investigations aiming at the