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Flow Battery

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Flow batteries allow for the storage of the active materials external to the battery and allows these reactants to be circulated through the cell stack as needed. The first flow battery was the Zinc/Chlorine battery in which the chlorine was stored in a separate cylinder than the cylinder containing the Zinc. It was first used in 1884 by Charles Reynard to power his airship, La France, which contained its own on board chlorine generator. This technology became popular again in the late 1970's, and is continued to be in use as of today.

Modern flow batteries are ordinarily two electrolyte systems in which the two electrolytes are pumped through the cell. This system's advantage is that it provides a nearly unlimited electrical storage capacity, a property not shared by many other batteries,  its only limitation is the capacity of the electrolyte storage reservoirs. Opportunities for thermal management are also facilitated by using the electrolyte as the thermal working fluid as it is pumped through the cells. High power batteries are constructed using a multiple stack of cells in a bipolar  arrangement.


"The Zinc-Bromine battery is a modern example of a flow battery. It is based on the reaction between two commonly available chemicals, Zinc and Bromine." The battery is made up of a Zinc negative electrode and a Bromine positive electrode separated by a micro-porous separator. An aqueous solution of Zinc Bromide is circulated through the two compartments of the cell from two separate reservoirs. The other electrolyte stream in contact with the positive electrode contains Bromine. The Bromine storage medium is immiscible with the aqueous solution containing Zinc Bromide.

The battery uses electrodes that can't and don't take part in the reactions but merely serve as substrates for the reactions. Therefore there is no loss of performance, as in most rechargeable batteries, from repeated cycling causing electrode material deterioration. "When the Zinc-Bromine battery is completely discharged, all the metal Zinc plated on the negative electrodes is dissolved in the electrolyte and reproduced the next time the battery is charged. In the fully discharged state the Zinc-Bromine battery can be left indefinitely." 

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