Rising sea and atmospheric carbon dioxide levels have made the search for efficient sources of renewable energy more urgent than ever before. While options such as solar and wind power have been gaining momentum, in order for these energy sources to go mainstream, the issue of storage needs to be addressed. That answer might just lie in an unlikely material – a fluorescent dye known as BODIPY.

Researchers at the University at Buffalo say BODIPY – short for boron-dipyrromethene – hides two "unusual" chemical properties that make it a potential candidate for stockpiling energy in a redox flow battery: storing electrons and participating in electron transfer. This discovery is potentially significant because next-gen batteries need to do these two things very well in order to store and deliver energy at grid scale. BODIPY, which is also used as a sensor to identify proteins, is "very good" at performing these functions, say the researchers led by Dr. Timothy Cook and first author Anjula M. Kosswattaarachchi.

"As the world becomes more reliant on alternative energy sources, one of the huge questions we have is, 'How do we store energy?' What happens when the sun goes down at night, or when the wind stops?" says lead researcher Cook. "All these energy sources are intermittent, so we need batteries that can store enough energy to power the average house."

Redox flow batteries could potentially address this problem as their energy tanks can easily be enlarged to store vast amounts of energy without a corresponding exponential increase in cost, which is in contrast to other systems, such as Lithium-ion (Li-ion) technology. Although champions of flow technology say these batteries are safer and more cost-effective compared to the Li-ion variety, which currently dominates the market for battery-based energy storage systems, they have not been able to reach the same level of commercial success despite developments promising to lower costs and double energy density. Can BODIPY give redox flow batteries the jolt they need?

To test how well a BODIPY-based battery would run, the team dissolved PM 567, a powdered BODIPY dye, in a solvent and repeatedly charged and drained it for the duration of 100 cycles. They found that the compound was able to store and transfer electrons for the entire period without degrading, unlike many other chemicals. Although the researchers only tested PM 567, since different BODIPY varieties share chemical properties, based on the results, it is possible other dyes would be able to store energy just as well. Based on the test results, the researchers predict BODIPY batteries would be able to generate an estimated 2.3 volts of electricity.

The researchers mixed PM 567 with a solvent called acetonitrile to create the dye solution for their experiments(Credit: Douglas Levere)

On a separate note, the study's use of a single substance for the redox storage in both the cathodic and the anodic compartment is another notable feature, points out Roland Roesler, an associate professor in inorganic chemistry at the University of Calgary, who was not involved in the research.

"This study adds to the body of knowledge in the field, especially in regard to the non-aqueous systems, and helps bring it closer to applications," says Roesler. "All redox flow batteries produced on applicative scale to date have been water based, and although there is a lot of promise for future development, neither of the existing systems could be called a commercial success. Most notably, this study uses one single substance for the redox storage in both the cathodic and the anodic compartment. This is a major advantage akin to that of the vanadium system, which has been by far the most successful so far."

Elaborating on this, Ted Roberts, a professor of chemical and petroleum engineering at the University of Calgary, who was also not part of the study tells New Atlas that such an arrangement could be advantageous operationally.

"The most highly developed system uses vanadium on both sides, but when discharged the vanadium is in a different oxidation state (i.e. a different species of vanadium) on each side," Roberts says. "Having the same chemical on both sides could give some important operational benefits, since cross-over through the battery membrane shouldn't affect performance, and if the battery gets out of balance this can be resolved simply by mixing the two solutions (for other redox battery chemistries a complex rebalancing system can be required)."

That said, the study of BODIPY's role in energy storage is still in its nascent stage and whether such a system can eventually find its way into a large-scale production is up for debate. This is because it also shares some of the features – such as low solubility and conductivity, which translate into low energy density and performance – that characterize redox flow batteries based on organic solvents, observes Roesler.

"There is a lot of work that needs to be done until then, and the need for such systems suitable for grid energy storage will increase sharply with the proportion of renewable energy delivered to the grid," he says.

The team's findings were published in ChemsusChem.

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