Flow Batteries: Improving Battery Technology for Electric Vehicles

One interesting approach has been developed by Professor Maria Skylass-Kazacos and her team at the University of New South Wales, Australia. Skylass-Kazacos has developed a vanadium-flow battery. A schematic of the vanadium-flow battery is shown in Figure 7. In a flow battery, the reactants are in solution instead of in solid plates (as in the lead-acid battery). On one side of the battery is a solution of vanadium (V) ions dissolved in sulfuric acid. The solution on the other side is vanadium (II) ions in sulfuric acid. (Note: Vanadium (V) ions produce yellow solutions and solutions of vanadium (II) ions are violet.) Just as in the lead-acid battery, electron flow produces a current.

Oxidation half-reaction:

V²⁺ → V³⁺ + e-

(19)

Reduction half-reaction:

V⁵⁺ + e- → V⁴⁺

(20)

The solutions are separated by a sheet of graphite. The graphite is chemically inert and conducts electrons well. (You used graphite rods as electrodes for the same reasons in Experiment 5.) A semi-permeable membrane serves to complete the circuit and functions as a salt bridge. Without the membrane, as electrons flow from the anode (V²⁺solution) to the cathode (V⁵⁺ solution), excess positive spectator ions would build up in the anode solution and the reaction would stop prematurely. Hydrogen ions may be the species that crosses the membrane to maintain charge balance, but this has not been experimentally verified. The cell has a potential of 1.6 volts when fully charged (assuming an electrolyte concentration of 2 M vanadium at 25°C).

Vanadium Flow Battery diagram

Figure 7

This schematic of a vanadium-flow battery shows a violet solution of vanadium (II) being pumped into a chamber on one side of a membrane and a yellow solution of vanadium (V) being pumped into the other side. The vanadium (II) is oxidized to a bluegreen solution of vandium (III) and returns to the anode reservoir. The vanadium (V) is reduced to a blue solution of vanadium (IV) and returns to the cathode reservoir.

The vanadium battery is not the only flow-redox battery known. A flow-redox battery that uses iron and chromium ions was developed by NASA, for instance. However, the all-vanadium battery has a significant advantage over other flow-redox cells because mixing the electrolyte solutions does not damage the battery. If some of the vanadium "leaks" through the membrane, the only disadvantage is energy that would have been generated from the transfer of electrons through the graphite is squandered. Once the battery is recharged, the vanadium ions will return to their charged oxidation states. The electrolyte solution is not permanently contaminated.

The vanadium-flow-redox battery may be an excellent alternative to lead-acid batteries for electric vehicles. The vanadium battery should be lighter and have a longer lifetime, and the vanadium compounds in the battery are less toxic than lead compounds. (The volume of solution required, although lighter than lead plates, may not result in significant space savings over lead-acid batteries.) Most importantly, though, for electric-vehicle applications, is that the vanadium-flow cell can be recharged in two ways. Just like lead-acid batteries, a vanadium battery can be plugged into a charger and recharged over a period of hours. However, if one doesn't want to wait several hours for the battery to recharge, the discharged electrolyte solutions can simply be drained and replaced with fully charged electrolyte solutions. This process would not be much more time consuming than refilling a standard car with gasoline. Will urban areas of the future be dotted with "vanadium-electrolyte filling stations?" If electric-vehicle use is coupled with electricity production from renewable energy sources, the vanadium battery may help us all breathe easier.