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
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. Other advantages include the possibility of quick recharge and a relative lack of waste materials.
How It Works
A single redox flow cell consists of a positive and negative half cell separated by a proton exchange 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.
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 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. This has led to interest in the VRB's application to electric vehicles, since recharging could be as convenient as refueling at a petrol station. Advocates envision electrolyte stations which would recycle the solution, using electricity straight off the grid, doing away with expensive fuel transport networks.
At its current level of development, however, the VRB seems unsuitable to mobile applications. The primary reason is a relatively low energy density compared to other storage solutions - around 25 to 35 Wh/kg, compared to 30–40 Wh/kg for lead-acid and 80–200 Wh/kg for lithium-ion. In addition, redox flow cells are more complex than these competitors, as they contain pumps and other moving parts. The situation may change in future; improving energy density is the main focus of flow cell research.
It's in high current applications that the VRB shines. The constant flow of fluid through the battery stack helps greatly in dealing with waste heat, and the system is able to adapt rapidly to sudden changes in load. 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.
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