Chemists at Zhengzhou University have developed a flow battery anolyte from thioctic acid — a natural sulfur compound — that retained 98.1% of its capacity over 2,200 charge cycles, with independent reviewers calling it a step beyond the vanadium benchmark.
The chemistry
The work by Wei Guo and Yongzhu Fu was published in Nature Sustainability on 23 October 2026. The anolyte is a short chain of thioctic acid units formed by opening the molecule's five-membered sulfur ring so that units link up; sulfur-sulfur bonds break and re-form to store charge. The team says the chain-building step offers a scalable route to high-concentration redox-active species. Flow batteries store electricity in tanks of liquid rather than sealed cells, making them a natural fit for multi-hour grid storage.
Performance
Paired with a cobalt-based catalyst, the anolyte reached 128 amp-hours per litre capacity and used 95.5% of its active material at a current density of 50 mA/cm². In a separate long test, a 63 Wh stack using an iron-based electrolyte on the positive side ran for more than 20 days, after which the team recovered 75.9% of the thioctic acid from its anolyte. Chemical analysis showed the recovered material matched the original, and cells built with recycled thioctic acid worked.
Recycling economics
Used anolyte is diluted with water and acidified, causing thioctic acid to come out as a solid; without the dilution step the recovered material stayed in chain form. Recycling 100 litres of concentrated anolyte would require roughly US$0.6 of acid while recovering US$82.7 of thioctic acid, according to the authors' estimates.
Independent verdict and open questions
"The molecule not only outperforms the industry-standard vanadium benchmark but also offers exceptional reusability." — Sungmin Park and Yongchai Kwon, Seoul National University of Science and Technology, writing in Nature Sustainability's News & Views
Park and Kwon argue that making water-based flow batteries truly sustainable hinges on recycling the organic molecules inside them. Still, caveats remain. The headline 2,200-cycle result was measured at a sulfur-sulfur bond concentration of 0.25 molar — a dilute mix — and the abstract gives no sign that the high-capacity cell was run that long. Because grid energy is stored in volume, whether the concentrated version can hold up for years of daily use is unresolved. Recycling cost estimates are the authors' own and deliberately partial, omitting stirring, filtration, labour and plant costs.
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