Researchers from the University of Cambridge and Harvard University have developed a method to dramatically extend the lifetime of organic aqueous flow batteries. The team believes this could mark the start of practical commercial aqueous flow batteries.
Flow batteries using a water-based electrolyte are cheaper and safer than those with heavy metals such as vanadium but struggle with the lifetime as the sales in the electrolyte decompose.
The researchers were able to revive the decomposed anthraquinone molecules in the flow battery using a deep discharge, extending the lifetime of the electrolyte by 17 compared to previous research.
“Organic aqueous redox flow batteries promise to significantly lower the costs of electricity storage from intermittent energy sources, but the instability of the organic molecules has hindered their commercialisation,” said co-author Michael Aziz from Harvard. “Now, we have a truly practical solution to extend the lifetime of these molecules, which is an enormous step to making these batteries competitive.”
In previous work, the researchers found that they could extend the lifetime of one of these molecules, named DHAQ but dubbed the ‘zombie quinone’ in the lab, by exposing the molecule to air. The team found that if the molecule is exposed to air at just the right part of its charge-discharge cycle, it grabs oxygen from the air and turns back into the original anthraquinone molecule — as if returning from the dead.
But regularly exposing a battery’s electrolyte to air isn’t exactly practical, as it drives the two sides of the battery out of balance — both sides of the battery can no longer be fully charged at the same time.
To find a more practical approach, the researchers developed a better understanding of how the molecules decompose and invented an electrical method of reversing the process.
Researchers from Professor Clare Grey’s group in Cambridge’s Yusuf Hamied Department of Chemistry, carried out in situ nuclear magnetic resonance (NMR) measurements and discovered the recomposition of active materials by an electric method, deep discharge.
The team found that if they performed a deep discharge, in which the positive and negative terminals of the battery get drained so that the voltage difference between the two becomes zero, and then flipped the polarity of battery, forcing the positive side negative and the negative side positive, it created a voltage pulse that could reset the decomposing molecules back to their original form.
“Usually, in running batteries, you want to avoid draining the battery completely because it tends to degrade its components,” said co-first author Yan Jing from Harvard. “But we’ve found that this extreme discharge where we actually reverse the polarity can recompose these molecules — which was a surprise.”
“Getting to a single-digit percentage of loss per year is really enabling for widespread commercialisation because it’s not a major financial burden to top off your tanks by a few percent each year,” said Aziz.
The research team also demonstrated that this approach works for a range of organic molecules. Next, they aim to explore how much further they can extend the lifetime of DHAQ and other inexpensive anthraquinones that have been used in these systems.
“The most surprising and beautiful thing to me is that this organic molecule can transform in such a complex way, with multiple chemical and electrochemical reactions occurring simultaneously or sequentially,” said co-first author Dr Evan Wenbo Zhao, who carried out the work while he was based at Cambridge, and is now based at Radboud University Nijmegen in the Netherlands. “Yet, we are able to unpick many of these reactions and let them happen in a controlled fashion that favours the operation of a redox flow battery.”
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