Skyllas-Kazacos’s solution to this problem was to use the same chemical element for both electrolytes. She could still provide the required difference in redox potential by ensuring that the element was in different "oxidation states" in the two solutions – in other words its atoms carried different electrical charges. The element she eventually decided on was the metal vanadium, which can exist in four different charge states – from V(ii), in which each vanadium atom has two positive charges, to V(v), with five. Dissolving vanadium pentoxide in dilute sulphuric acid creates a sulphate solution containing almost equal numbers of V(iii) and V(iv) ions.
When Skyllas-Kazacos added the solution to the two chambers of her flow battery and connected an outside power supply to the electrodes, she found that the vanadium at the positive electrode changed into the V(v) form while at the negative electrode it all converted to the V(ii) form. With the external battery disconnected, electrons flowed spontaneously from the V(ii) ions to the V(v) ions and the flow battery generated a current (see Graphic). Best of all, it didn’t matter too much if a few vanadium ions on one side of the membrane leaked across to the other: this slightly discharged the battery, but after a recharge the electrolyte on each side was as good as new.
After more than a decade of development, Skyllas-Kazacos’s technology was licensed to a Melbourne-based company called Pinnacle VRB, which installed the vanadium flow battery on King Island. With 70,000 litres of vanadium sulphate solution stored in large metal tanks, the battery can deliver 400 kilowatts for 2 hours at a stretch. It has increased the average proportion of wind-derived electricity in the island’s grid from about 12 per cent to more than 40 per cent.