Scientists have developed a vanadium redox flow battery that could improve large-scale renewable and grid energy storage in the future. With large-scale electricity storage being a crucial challenge for the renewable energy sector, this battery upgrade could be a major breakthrough, as it is more effective than previous versions in terms of energy density and the stability of electrolyte solutions used.
The research, by a team from the Department of Energy's Pacific Northwest National Laboratory in Richland Washington, and published in Advanced Energy Materials [Li et al., Adv Energy Mater (2011) doi: 10.1002/aenm.201100008], found that changing the electrolyte solution of the flow battery can substantially improve how well it performs. The team added hydrochloric acid to the sulfuric acid that is normally found in vanadium batteries, to increase the energy storage capacity in the battery by 70 %, and help raise the temperature at which they can operate.
Liyu Li, lead author on the study, said “Our small adjustments greatly improve the vanadium redox battery. And with just a little more work, the battery could potentially increase the use of wind, solar, and other renewable power sources across the electric grid.”
To improve the battery's performance, the scientists found that a mixture of 6 parts hydrochloric acid and 2.5 parts sulfuric acids provided the best balance of electrolytes. Tests showed that the new electrolyte mixture retained 70 % more vanadium ions, meaning that its electricity capacity was also 70 % greater.
A vanadium battery works by moving liquid from two external tanks to its central chamber where the liquids are mixed. The tanks hold electrolytes, with one electrolyte containing V5+ ions, while the other is full of V2+ ions. When activated, the ion-saturated electrolyte from both tanks is pumped into the central chamber, where a chemical reaction causes the ions to change their charge, creating electricity.
To charge the battery, electrical charge is moved to the central chamber, resulting in another reaction that restores the original valence of the vanadium ions, while the electrical energy is converted into chemical energy stored in the vanadium ions, and the electrolytes are pumped back into to their tanks.
It was shown that a vanadium battery at room temperature and containing the new electrolyte mixture was able to maintain an 87 % energy efficiency rate for 20 days. The new mixture meant the battery could still operate at a much wider range of temperatures, so cheaper cooling systems could be used.
As Li points out “Before renewable sources such as wind and solar can contribute significantly to the overall energy mix, large-scale energy storage is needed to smooth out the intermittency of the renewable power and make it dispatchable.” Although more research is needed, this new upgrade could be the answer to these problems.