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7
Bromine-Storage Materials

Christoph Fabjan and Josef Drobits

7.1
Introduction

As a positive active material for rechargeable cells, bromine offers various attractive
properties such as high voltage and specific energy and power. The main difficulty
encountered with the operation of a battery using bromine/bromide electrode is the
necessity to develop a suitable for the storage of this aggressive and toxic elemental
halogen.
Several systems using different negative materials such as Zn [1–3], Li [4], and
A1 [5] were investigated, but at present practical success has been achieved only
with the zinc–bromine battery. A satisfactory approach for bromine storage was
the formation of organic nonaqueous polybromine complex phases by the reaction
of quaternary ammonium salts, dissolved in the aqueous electrolyte solution, with
elemental bromine generated in the charge process, reducing energy losses due to
self-discharge to extraordinarily low values.
A variety of complexes exist in a solid or liquid state at ambient temperatures in
the range required for battery operation. Liquid polybromine phases are preferred
since they enable storage of the active material externally to the electrochemical cell
stack in a tank, hence enhancing the storage capacity of the system and reducing
energy losses in standby periods to very low values. This design requires electrolyte
circulation and application of the flow battery concept, utilizing its characteristic
advantages and accepting the drawbacks associated with this type of storage system.
The operating principle and the conceptual design of a multicell bipolar zinc-flow
battery are presented in Figure 7.1.
From an aqueous ZnBr 2 solution zinc is plated at the cathode during charge
while bromine is generated at the anode, forming the water-immiscible poly-
bromine phase with the complexing agents. In the discharge process zinc metal
is dissolved anodically, and the active bromine is consumed from an emulsion of
the complex phase and aqueous solution pumped over the electrode surface in
the cathodic reaction. Hence the active materials are reconverted to form aqueous
aq
s
ZnBr 2 solution. The net reaction, Zn + Br naq ↔ ZnBr , provides a cell voltage of
2 2
−1
approximately 1.82 V and a theoretical specific energy of 430 Wh kg .
Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.