The batteries used in our phones, devices and even cars are based on metals such as lithium and cobalt, which are obtained through intensive and invasive mining. As more products rely on battery-based energy storage systems, moving away from metal-based solutions will be critical to ease the transition to green energy.
Now a team at Northwestern University has converted an organic industrial waste product into an efficient storage material for sustainable energy solutions that can one day be used on much larger scales. This is the first time that a waste molecule – more specifically triphenylphosphine oxide (TPPO) – has been used to power a redox flow battery.
“Battery research has traditionally been dominated by engineers and materials scientists,” said Northwestern chemist and lead author Christian Malapit. “Synthetic chemists can contribute to this field by molecularly converting an organic waste product into an energy-storing molecule. Our discovery demonstrates the potential of converting waste compounds into valuable resources and provides a sustainable path for innovation in battery technology.”
Malapit is an assistant professor in the Department of Chemistry at Northwestern's Weinberg College of Arts and Sciences.
What are redox flow batteries?
Unlike lithium and other solid-state batteries that store energy in electrodes, redox flow batteries use a chemical reaction to pump energy back and forth between electrolytes, where their energy is stored. Although redox flow batteries are not as efficient at energy storage, they are considered much better solutions for grid-scale energy storage. The market for redox flow batteries, which currently makes up a small part of the battery market, is expected to grow by 15% to a value of €700 million worldwide between 2023 and 2030.
Electricity from a common waste product
Thousands of tons of TPPO are produced every year through many organic industrial synthesis processes – including the production of some vitamins – but it becomes unusable and must be carefully disposed of after production.
According to the new study published in the Journal of the American Chemical Society, a “one-pot” reaction allows chemists to convert TPPO into a usable product with strong energy storage potential.
“Not only can an organic molecule be used, but it can also achieve high energy density – bringing it closer to its metal-based competitors – as well as high stability,” said graduate student Emily Mahoney. candidate in the Malapit laboratory and first author of the paper. “It is traditionally a challenge to optimize these two parameters together. “It is therefore particularly exciting to be able to show this for a molecule that comes from waste.”
To achieve both energy density and stability, the team had to find a strategy that would allow the electrons to pack tightly together in the solution without losing storage capacity over time. They looked back in time and found a 1968 paper describing the electrochemistry of phosphine oxides, and according to Mahoney, “they ran with it.”
To assess the molecule's resilience as a potential energy storage device, the team then conducted tests using static electrochemical charging and discharging experiments, similar to the process of charging a battery, using the battery, and then charging it again, over and over again. After 350 cycles, the battery remained remarkably healthy and lost little capacity over time.
What's next?
“This is the first example of using phosphine oxides – a functional group in organic chemistry – as a redox-active component in battery research,” Malapit said. “Traditionally, reduced phosphine oxides are extremely unstable. Our molecular engineering approach addresses this instability and paves the way for its application in energy storage.”
In the meantime, the group hopes that other researchers will take over and begin working with TPPO to further optimize and improve its potential.