As the world continues its shift towards renewable forms of energy like wind and solar, scientists see devices known as redox flow batteries as part of the solution to our storage needs. There is still come work to do in bringing current designs up to speed in terms of both performance and sustainability, but a new approach by scientists from the US Department of Energy tackles the problem on both these fronts by drawing on a compound commonly found in candles, which boosts energy density and lifespan compared to current designs.
Redox flow batteries are considered a good candidate for storing intermittent forms of energy like wind and solar, because unlike lithium-ion batteries that store energy in electrode materials, they store this energy in liquid electrolytes inside huge external tanks. This means that the storage potential can be increased simply by increasing the size of the tanks, an attribute that is well suited to renewable energy that isn't generated on an on-demand basis, but is often needed to be stowed away for later use.
But before these devices are scaled up to size needed for common grid applications, researchers are working to iron out a few kinks in the design. Most redox flow batteries rely on the metal vanadium to facilitate the transfer of electrons, which offers great reliability during charging and discharging. But because it is expensive to mine, scientists have been exploring how organic materials can act as cheaper and more environmentally-friendly alternatives.
We have seen some meaningful advances in this area of late, with scientists finding some success in incorporating shrimp shell compounds, organic polymers and saltwater. Now, scientists at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) have found another promising candidate in an organic compound called fluorenone, which is already used in LEDs, solar panels and to help make candles smell nice.
There was work to do in adapting fluorenone for use in redox flow batteries. In its regular form, the molecule isn't water soluble enough for this application, and isn't able to easily accept and donate electrons. But through a complex chemical treatment, the researchers were able to address these shortcomings and equip fluorenone with the characteristics needed for use in an aqueous redox flow battery.
“This is a great demonstration of using molecular engineering to change a material from one widely considered impossible for use into something useful for energy storage,” says study author Wei Wang. “This opens up important new chemical space that we can explore.”
The ability of fluorenone to carry out the necessary reactions in the water-based electrolyte was found to be linked to its concentration, with the team eventually landing on an optimal recipe. The flow battery created by the team is about the size of a postage stamp, and was able to operate continuously for 120 days. This involved 1,111 charging cycles through which it lost less than three percent of its capacity, demonstrating a far superior lifespan to other organic flow batteries. Promisingly, the team says the battery possess an energy density that is more than double that of vanadium-based batteries.
The PNNL researchers have filed a patent for their new battery design, and have published their work in the journal Science.
Source: PNNL
