Saving sunny days for a rainy day: a new molecule for storing green energy

Redox flow batteries operate on a different principle than conventional batteries.

Traditional batteries, such as the alkaline cells in household devices and the lithium-ion batteries in electric vehicles, store the charge in electrodes housed inside the battery. The active materials in these technologies contain metals — and scaling them up is complex.

“The difference with a redox flow battery is that we use an active material made of potentially renewable organic molecules dissolved in an aqueous solution and stored outside the battery,” Rochefort said.

The system uses two separate tanks containing an electrolyte solution—water, acid and organic molecules—linked by tubes to a central cell. The larger the tanks, the greater the storage capacity. Inside the cell is a membrane against which the liquids from the two tanks flow without ever mixing.

“The molecules do not pass through the membrane into the other tank; they only exchange electrons through the external circuit by transferring them via their own electrodes,” Rochefort explained.

It is at this point of contact that oxidation-reduction, or redox, occurs. This is the process by which the battery is charged and discharged. The separation between the energy stored in the tanks and the power generated in the cell means these two parameters can be scaled independently as needed.

Commercial redox flow batteries are already on the market. They generally use vanadium on both sides of the system. Vanadium is a metal with attractive electrochemical properties but it is not renewable—hence the search for organic molecules to replace at least one side of the battery.

“The organic molecule we have developed contains carbon, hydrogen, nitrogen and oxygen: it blends into water and acid and is oxidized to drive the energy storage reaction,” said Lebel, an expert on organic synthesis.

She and her team tested different molecular groups to identify the most effective ones for energy storage. This work led to the creation of AzoBiPy, a member of the pyridinium family of molecules, which contain positively charged heteroaromatic rings that facilitate electron exchange.

“For now, we are buying some basic molecules from specialized companies," said Lebel, "but we are also exploring bio-based molecules derived from wood or food residues." This approach, she added, could make it possible to extract the required organic molecules from renewable materials.

The challenge of stability

The main advantage of AzoBiPy is that it can exchange two electrons rather than just one. This means each molecule can store twice as much energy as a single-electron molecule, doubling the system’s capacity.

“But the biggest challenge with these organic molecules is stability,” said Lebel. “It must be possible for the charge-discharge cycle to run for a long time without the molecule breaking down.”

This is where AzoBiPy shines. The team tested a flow battery based on this molecule by operating it for 70 consecutive days, completing 192 full charge-discharge cycles. At the end of the trial, the molecule retained nearly 99 per cent of its initial capacity—a performance the researchers describe as exceptional for an organic molecule.

In a festive demonstration at the Department’ of Chemistry's holiday party in December 2024, the prototype flow battery powered a set of Christmas tree lights for eight hours with tanks containing only about two tablespoons of aqueous solution each.

This demonstration also highlighted another major advantage of the system: it is water-based and therefore non-flammable, unlike lithium-ion batteries, which present a fire risk. “This feature is especially important for large-scale, stationary energy storage facilities,” said Rochefort.

Flow batteries powered by molecules such as AzoBiPy could be used to store electricity generated by solar or wind farms. Long-term storage of intermittently generated electricity would make it possible to use it at a later date to meet peak demand.

There could also be residential applications. “It may be possible to develop smaller-scale systems with greener, safer batteries for home use,” Lebel suggested.

The research team is drafting a patent application and is already working on the next stages. 

“We're preparing a scientific article that describes a family of molecules with properties similar to AzoBiPy,” said Lebel. “An entire class of compounds with potential for renewable energy storage is opening up to exploration. We expect this technology to be in wider use within 10 to 15 years.”