New technology promises to dramatically improve the performance of batteries, fuel cells, and the electrolyzers that make green hydrogen and other fuels from electricity. The advance—used in a type of “flow battery” that’s becoming common for storing renewable energy—boosted the speed at which the battery could provide power fivefold. That jump in performance could sharply reduce the cost of storing green energy for use on the grid, making it easier for societies to completely shift from fossil fuels to renewables.
“This is really an exciting development,” says Michael Aziz, a flow battery expert at Harvard University. Young Moo Lee, an electrochemist at Hanyang University, agrees and says the benefits could be widespread. “It could apply for other devices as well.” Neither was involved in the new work.
At their core, batteries, fuel cells, and other electrochemical devices look similar. They typically harbor two electrodes separated from one another by a membrane that regulates the flow of charge-carrying ions back and forth through a fluid electrolyte. When these devices charge or discharge, electrons travel through an external wire, and charge-carrying ions pass through the membrane from one electrode to the other to balance out the electrical charges. The membrane plays a critical role, acting as a molecular gatekeeper to allow only certain ions through and block all others. But in practice, these gatekeepers are often too zealous, slowing the passage of ions they are meant to let sail through, which saps device performance.
In a version of flow batteries called aqueous organic redox flow batteries, for example, the membranes must allow positively charged potassium ions to pass back and forth between the two sides of the membrane, while blocking the passage of organic compounds that could kill the battery’s operation. Traditional membranes made from organic polymers do a decent job of ensuring that only potassium ions move back and forth. But the polymers in these membranes tend to continuously jiggle around, bumping into the ions and slowing their passage.
To get around this, researchers led by Zhengjin Yang, a chemist at the University of Science and Technology of China, manufactured a series of membranes from a polymer known as a triazine framework. The polymer is able to assemble into a rigid scaffold riddled with tiny pores that are small enough to exclude all but water molecules and the smallest ions from passing through. Having the water molecules around is good, as they help charged ions slip through the pores.
To speed this transport even more, Yang and his team also modified their triazine starting materials so that the rigid pores were lined with negatively charged sulfonate groups. These groups act like a molecular bucket brigade to grab positively charged potassium ions and quickly pass them to the next tethered sulfonate in line, helping the ions zip through the membrane virtually unimpeded.
When the researchers, including colleagues from the United Kingdom and Germany, then used the best iteration of their new membrane to make an aqueous organic redox flow battery, the more slippery ion flow enabled the batteries to discharge and charge five times faster than similar batteries with a traditional membrane, they report today in Nature.
“We’ve been hoping for a significant improvement in membranes for flow batteries,” says Aziz, who previously served as Yang’s postdoctoral adviser. “This looks like it could be it.”
However, the new membranes still have a way to go to prove themselves durable and reliable enough for industrial use, says Michael Guiver, a chemist at Tianjin University. And Lee notes that although the chemistry of the triazine compounds makes them ideally suited for working in water, they may not hold up to acidic or alkaline electrolytes used in other electrochemical devices. Nevertheless, he says, other researchers should be able to adopt the same principles to design membranes for other uses, and thereby improve the performance of a wide array of green energy technologies.
That means the new membranes likely won’t show up first in consumer products, such as cellphone batteries. But the advance could help tackle one of the biggest concerns about society’s switch to renewable energy, namely providing energy when the Sun isn’t shining, and the winds are calm. Cheaper, more efficient membranes mean smaller, cheaper batteries can store the same amount of power to supply consumers overnight. It could also reduce the cost of electrolyzers, which can convert renewable electricity into hydrogen and other fuels that can be stored for months or years, and drop the cost of fuel cells that convert those fuels back into electricity when needed.
Not bad for a piece of technology that most of us will never see.
