Researchers Strive to Make Renewable Energy More Affordable: Boosting Charge Storage in Redox Flow Batteries

"One of the biggest challenges with renewable energy is that we cannot always control when electricity is generated," said Professor Nav Nidhi of the Department of Materials Science and Chemical Engineering at Stony Brook University. "Solar panels produce more power during sunny hours, while wind turbines generate electricity when the wind is available. But electricity demand continues even when the sun is not shining or the wind is not blowing. Redox flow batteries can help bridge this gap by storing excess renewable electricity and releasing it later when it is needed."

Nidhi, in collaboration with Kuldeepsinh Raj, a PhD candidate at Stony Brook University and Professor Xiaoliang Wei's team at Purdue University, recently released a study that explores possibly a more affordable alternative to the vanadium-based redox flow batteries.

Mahla Sarfaraz Khabbaz at Purdue led the experimental work, while Raj led the computational modeling.

Redox flow batteries store their energy in two separate tanks filled with liquid electrolytes that are pumped through a central cell where the electrochemical reactions take place. A membrane keeps the two liquids from mixing while allowing ions to pass between them.

Unlike a traditional lithium ion battery which has solid electrodes, a redox flow battery dissolves active materials such as metal ions in its external tanks. The power of a redox flow battery is set by the size of that cell, while the energy is set by the size of the tanks.

"Conventional lithium-ion batteries store energy within solid electrode materials, so the amount of energy and the amount of power are locked together, if you want to store twice as much energy, you build twice as much battery," Nidhi said.

"A flow battery separates the two. The energy is stored in two liquids, held in two external tanks. One major advantage is that we can increase the energy storage capacity simply by using larger tanks or more electrolyte, without necessarily increasing the size of the electrochemical cell," Nidhi said.

But a drawback to redox flow batteries is that many of the most reliable ones use expensive active materials like vanadium. Cheaper alternatives called ferrocyanide exist. They use iron paired with either sodium or potassium, but these materials have less potential to dissolve, meaning it has less potential to store energy.

For their study, the researchers investigated how replacing sodium or potassium ions with lithium ions in ferri/ferrocyanide-based electrolytes could improve the solubility of the active materials and increase the potential energy storage capacity of the batteries.

"The most significant improvement was how much active material could dissolve in the electrolyte," Raj said. "Lithium-based ferrocyanide was approximately six to eight times more soluble than its sodium- and potassium-based counterparts under strongly alkaline conditions, potentially allowing more energy to be stored in the same volume of liquid. In strongly alkaline conditions, the lithium version dissolved about six to eight times better than the sodium and potassium ones."

To understand why lithium improves solubility, molecular dynamics simulations and quantum chemistry calculations were performed to investigate how ions and water molecules interact in the electrolyte. "Our simulations revealed that lithium ions interact strongly with both water and ferrocyanide, forming connected, water-containing structures that remain dissolved even at high concentrations. These molecular-level findings helped us understand the mechanism behind the enhanced solubility and delayed saturation observed in lithium-based electrolytes." Raj said.

Higher electrolyte concentrations, however, also present a trade-off. As the electrolyte becomes more concentrated, its viscosity increases, making it harder to pump and potentially reducing battery efficiency.

The team believes the type of energy storage that redox flow batteries can provide will be especially useful for renewable energy.

"Solar farms, for example, could store some of the electricity generated during the day and use it during the evening when demand is high," Raj said. "Ultimately, making flow batteries more affordable, energy-dense, and durable could help renewable energy become a more reliable part of our electricity supply."

- Mattia Stradella

Images courtesy of Kuldeepsinh Raj and Nav Nidhi