Environmental Engineering Reference
In-Depth Information
The electrolyte is composed of vanadium salts dissolved in sulfuric acid,
and if the electrolytes are accidentally mixed, the battery suffers no perma-
nent damage. The standard cell potential E
0
is 1.26 V at concentrations of 1
M
, but under actual cell conditions, the open circuit voltage (OCV) is 1.4 V
at 50% state of charge (SOC) and 1.6 V at 100% SOC. The electrode reactions
occur in solution. The reaction at the negative electrode in discharge is V
2+
→
V
3+
+ e and the reaction at the positive electrode is V
5+
+ e → V
4+
. Both reac-
tions are reversible on the carbon felt electrodes. An ion-selective membrane
is used to separate the electrolytes in the positive and negative compartments
of the cells. Cross-mixing of the reactants would result in a permanent loss
in energy storage capacity for the system because of the resulting dilution
of the active materials. Migration of other ions (mainly H
+
) to maintain elec-
troneutrality, however, must be permitted. Thus, ion-selective membranes
are required.
Because the electrolyte is returned to the same state at the end of every
cycle, it may be reused indefinitely. The negative half-cell uses the V
2+
-V
3+
redox couple, whereas the positive half-cell uses the V
4+
-V
5+
redox couple.
The positive and the negative vanadium redox couples show relatively fast
kinetics that allow high Coulombic and voltage efficiencies to be achieved
without costly catalysts. However only V
5+
, V
4+
, and V
3+
are stable in air; V
2+
is easily oxidized by atmospheric oxygen, which must be taken into account
when servicing the negative electrolytic solution. However, different oxida-
tion states are not sufficient to make an element work in a liquid electrolytic
solution. The element must also be soluble. Although V
2+
, V
3+
, and V
4+
species
are easily soluble in sulfuric acid, the long-term stability of concentrated V
5+
solutions is rather limited due to the formation of insoluble V
2
O
5
precipitates
at elevated electrolyte temperatures. It must be noted that 0.9 molar V
5+
solu-
tions are stable even at elevated temperatures, and increasing the concentra-
tion of sulfuric acid can increase stability of V
5+
solutions.
As the reactions involve only dissolved salts, the electrode acts only as a
site of reaction and does not participate in the chemical process. Therefore,
it does not suffer ill effects due to changes in composition. Because it does
not experience a physical or chemical change, it can carry out a large number
of charge and discharge cycles without a significant decrease in capacity.
The electrodes are located in the reaction cells that are grouped together in
a series of blocks known as stacks. Each stack contains a series of conduct-
ing (bipolar) plates with a positive electrolyte on one side and a negative on
the other. Because they share the same electrolyte, each cell of a flow bat-
tery is practically identical, unlike conventional batteries connected in series
whose available power is limited by the poorest battery in the string. Thus
the user must depend on the manufacturer to produce batteries with little
variability.
The use of solutions to store energy means that system power and stor-
age capacity are independent, making vanadium batteries scalable to a wide
range of voltages, currents, and capacities and allowing them to be tailored
