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Energy Storage

Battery electrolytes made from chemical waste

A new use for the tonnes of triphenylphosphine oxide discarded each year?

by Prachi Patel
January 15, 2025

 

Chemical structure of a triphenylphosphine oxide derivative.
Cyclic triphenylphosphine oxide (CPO), derived from chemical waste, can be used to make an organic electrolyte for flow batteries.

With simple molecular tweaking, researchers have converted a common chemical waste product, triphenylphosphine oxide (TPPO), into an electrolyte material suitable for use in batteries connected to the power grid (J. Am. Chem. Soc. 2024, DOI: 10.1021/jacs.4c07750).

Flow batteries rely on reversible redox reactions between two liquid electrolytes, one positively charged and the other negative, stored in large tanks. These batteries promise a cheap way to store renewable energy for the grid. To avoid supply chain issues and boost energy density, researchers seek to replace the vanadium-based aqueous electrolytes used today with those made using organic compounds.

“People have designed very complex molecules for stable electrolytes,” says Northwestern University chemist Christian Malapit. For a sustainable solution, he and his colleagues chose TPPO, a by-product of pharmaceutical and agrochemical production that is discarded in vast quantities every year. While TPPO is known to have high redox activity, it decomposes in just a few reaction cycles.

So the team used a one-step process to form a carbon-carbon bond between phenyl groups in the TPPO, giving a stable derivative they call cyclic triphenylphosphine oxide (CPO). They dissolved the molecule in a solvent mix of acetonitrile and dimethylformamide to give a positively charged electrolyte, or anolyte, that can be cycled over 350 times. They haven’t tested it in a full battery cell yet.

Organic electrolytes have inherent challenges related to flammability and high costs, says Wei Wang, a flow battery expert at Pacific Northwest National Laboratory. Nevertheless, he calls this work “a noteworthy and welcome contribution to the field mainly due to the use of industrial waste products.”

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