The
vanadium redox battery is a redox flow battery (a type of
fuel cell) which uses vanadium sulphate as its only reactant.
That's right, the same species is used for both
anode and
cathode reactions, but at different oxidation states.
The VRB was invented by Professor Maria Skyllas-Kazacos and her team at the University of New South Wales (Australia).
Under development since 1985, it has attracted much attention
as an energy storage solution in renewable energy systems,
which suffer from highly variable output. The VRB is also being studied for peak-leveling applications
in conventional power generation, due to its excellent scalability.
The possibility of quick recharge and relative lack of waste materials
makes the VRB a possible contender for powering electric vehicles too,
although whether the energy density can be brought up enough is debatable.
How It Works
A single redox flow cell consists of a positive and negative half cell separated by a permeable membrane.
Each half cell has an inert electrode with electrolyte flowing over it.
A reduction reaction, involving in this case V5+ + e- -> V4+ takes place at the positive electrode (cathode), accepting electrons,
while an oxidation reaction, V2+ -> V3+ + e- goes on at the negative electrode (anode), emitting electrons.
These reactions are reversed when the battery is charged.
Each cell has an output of around 1.3 volts (depending on electrolyte concentration, temperature etc).
In practice, larger voltages are generated by a stack of such cells in series,
with the two electrolytes being pumped continuously through them in parallel from separate storage tanks.
Advantages
The use of solutions to store the energy means that
system power and storage capacity are independent,
which makes vanadium batteries scalable to a wide range of voltages, currents and capacities,
allowing them to be tailored to diverse applications.
VRB's have been used for storage in solar-powered dwellings,
as peak-levelers in power stations, and as battery backup for submarines.
Vanadium sulphate solution is easy to produce, store and handle in comparison with many other fuel cell options.
The use of the same metal species on both sides of the membrane
means that the two solutions do not contaminate one another, and can be used indefinitely,
making mechanical replacements the only waste output from the system.
The concentration of the positive and negative electrolytes may gradually become imbalanced,
but this problem is easily solved by occasionally mixing the contents of the storage tanks
when the battery is fully discharged.
Quick recharge is possible by emptying discharged electrolyte from the tanks and refilling with pre-charged solution,
which would make recharging an electric car as convenient as refueling at a petrol station.
Electrolyte stations would recycle the solution, using electricity straight off the grid,
doing away with expensive fuel transport networks.
The constant flow of fluid through the battery stack helps greatly in dealing with waste heat,
another reason why VRB's are suitable for high current applications.
The capacity of the whole system can be monitored in line
by monitoring the state of charge of the electrolytes,
and the battery can be fully discharged without any detrimental effects.
If you would like more details, check out
www.ceic.unsw.edu.au/centers/vrb/
www.pinnaclevrb.com.au
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