New Organic Flow Battery Brings Decomposing Molecules Back to Life

After years of advancing the technology for an organic aqueous flow battery, Harvard University researchers encountered a problem. The organic anthraquinone molecules that powered their innovative battery were slowly decomposing, reducing the long-term usefulness of the battery.

Now, the researchers have figured out both how the molecules decompose and how to mitigate and even reverse the decomposition process.

The team is being led by Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science and

Michael Aziz, the Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS).

The molecule, named DHAQ in their paper but referred to as the "zombie quinone" in the lab, is reportedly among the cheapest to produce at large scale. The team's battery revitalization method cuts the rate of capacity fade by at least a factor of 40, while enabling the battery to be composed entirely of low-cost chemicals.

The research was published in the Journal of the American Chemical Society.

"Low mass-production cost is really important if organic flow batteries are going to gain wide market penetration," said Aziz. "So, if we can use these techniques to extend the DHAQ lifetime to decades, then we have a winning chemistry."

"This is a major step forward in enabling us to replace fossil fuels with intermittent renewable electricity," said Gordon.

Since 2014, Aziz, Gordon and their team have been attempting to advance the development of safe and cost-effective organic aqueous flow batteries intended for storing electricity from intermittent renewable sources like wind and solar. Their batteries utilize molecules known as anthraquinones, which are composed of naturally abundant elements including carbon, hydrogen, and oxygen, to store and release energy.

Initially, the researchers believed that the lifetime of the molecules depended on the number of charge and discharge cycles, like in solid-electrode batteries such as lithium-ion. However, as a result of reconciling inconsistent results, the researchers discovered that these anthraquinones are decomposing slowly, regardless of how many times the battery was used. They found that the amount of decomposition was based on the molecule's calendar age, not how often they've been charged and discharged.

That discovery led the them to study the mechanisms resulting in the molecules decomposing.

"We found that these anthraquinone molecules, which have two oxygen atoms built into a carbon ring, have a slight tendency to lose one of their oxygen atoms when they're charged up, becoming a different molecule," said Gordon. "Once that happens, it starts of a chain reaction of events that leads to irreversible loss of energy storage material."

The team found two techniques to avoid the chain reaction.

The first was to expose the molecule to oxygen. They found that if the molecule is exposed to air at just the right portion of its charge-discharge cycle, it takes the oxygen from the air and turns back into the original anthraquinone molecule, as if it is resurrected from the dead. A single experiment recovered 70% of the lost capacity using this method.

Second, the team discovered that overcharging the battery creates conditions that accelerate decomposition. According to their findings, avoiding overcharging extends the lifetime by a factor of 40.

"In future work, we need to determine just how much the combination of these approaches can extend the lifetime of the battery if we engineer them right," said Aziz.

"The decomposition and rebirth mechanisms are likely to be relevant for all anthraquinones, and anthraquinones have been the best-recognized and most promising organic molecules for flow batteries," said Gordon.

"This important work represents a significant advance toward low-cost, long-life flow batteries," said Imre Gyuk, Director of the Department of Energy's Office of Electricity Storage program. "Such devices are needed to allow the electric grid to absorb increasing amounts of green but variable renewable generation."

Support for the research came from the Energy Storage program of the U.S. Department of Energy, the Advanced Research Projects Agency - Energy, the Massachusetts Clean Energy Technology Center, Harvard SEAS, and the Innovation Fund Denmark.

With help from Harvard's Office of Technology Development (OTD), the researchers are looking for commercial partners to scale up the technology for industrial applications. Harvard OTD has filed a portfolio of pending patents related to their innovations in flow battery technology.