Page 229 - Handbook of Battery Materials

198 7 Bromine-Storage Materials

+ -
CHARGER
BIPOLAR
CARBON LOAD
PLASTIC ENDPLATE
ELECTRODE
ZINC DEPOSIT

ZnBr 2
ZnBr 2
CIRCU-
LATION

SEPARATOR
PUMP BROMINE
PUMP BROMINE ON C COMPLEX
ENDPLATE STORAGE
Figure 7.1 Operating principle of a zinc-flow battery.

The system consists of three essential components.
1) The cell stack with the bipolar electrodes.
2) The electrolyte flowing in two independent streams through the electrochem-
ical module and the individual cell compartments, which are divided by
microporous separators. In the discharged state the electrolyte is a homo-
geneous aqueous solution, whereas during charge a two-phase electrolyte is
produced in the bromine loop.
3) Auxiliary equipment, including two pumps, tubing valves, and electrolyte
reservoirs is also necessary.
The bromine-storing equipment, as an essential component of the system, has
to achieve various objectives to ensure the practical success of the battery:
1) a decrease in the concentration of active bromine in the aqueous solution,
which is in equilibrium with the organic bromine-rich phase, to very low
−1
values (0.01 mol L ) to minimize se1f-discharge;
2) a decisive reduction in bromine vapor pressure to a small percentage of
the values obtained with elemental bromine over the full range of operating
◦
temperatures (from approximately −10 to 50 C);
3) provision of the liquid state under the conditions described above;
4) satisfactory electrochemical reactivity in the emulsion with the aqueous elec-
trolyte phase, to minimize polarization and voltage losses;
5) higher specific weight than that of the aqueous solution to enable fast separation
of the two phases;