An industrial waste product has been converted into a component for batteries that can stably store large amounts of charge. Such batteries could perform a vital function for power grids by smoothing out the peaks and troughs of renewable energy.

Redox flow batteries (RFBs) store energy as two liquids called an anolyte and a catholyte in a pair of tanks. When these fluids are pumped into a central chamber, separated by a thin membrane, they chemically react and produce electrons to generate energy. The process can be reversed to recharge the battery by placing a current across the membrane.

Such batteries are cheap, but they have downsides. They are bulky, often as big as a shipping container, and require constant maintenance because of their moving parts. They also rely on metals like lithium and cobalt, which are in short supply.

Now, Emily Mahoney at Northwestern University in Evanston, Illinois, and her colleagues have discovered a simple process that can turn a previously useless industrial waste product into a useful anolyte, which could potentially replace these rare metals.

Their process takes triphenylphosphine oxide, created in the manufacture of products including vitamin tablets, and converts it into cyclic triphenylphosphine oxide, which has a high potential for storing negative charge. The battery shows no reduction in effectiveness even after 350 charging and draining cycles.

“Having an anolyte with a highly negative potential increases the overall cell potential and therefore the efficacy of the battery,” says Mahoney. “However, often the increase in potential is coupled with stability issues, so having a compound that can be both stable and highly negative is exciting.”

Mahoney says RFBs are designed to be safe and have large capacities, so they could be used to store energy from wind and solar generation, but they are unlikely to replace lithium-ion batteries in cars or smartphones because of their bulk.