Scientists from the University of Southern California (USC) claim to have developed an inexpensive, safe redox flow battery.

The device incorporates iron sulfate – a cheap waste product of the steel industry – alongside the anthraquinone disulfonic acid (AQDS) organic material already used to improve the stability and solubility of redox flow batteries.

“While the two compounds are well known individually, it’s the first time they’ve been combined to prove potential for large scale energy storage,” the researchers claimed.

The scientists estimated they could produce a device based on the two compounds at a commercial scale for $66/kWh based on iron sulfate costing around $0.10 per kilo and AQDS $3. “If manufactured at scale, electricity would cost less than half the energy derived from the redox batteries that use vanadium, which is more expensive and toxic,” said the USC group.

The scientists found no measurable change in device capacity over 500 cycles, in symmetric cells, and no deterioration of active material across long periods of cycling. “Although the thermodynamic cell voltage of 0.62 V is not as high as that for the vanadium system, we project an active material cost (including acid), to be about $54/kWh for the symmetric cell, using acidic solutions of iron sulfate and AQDS,” the team wrote in the study A Durable, Inexpensive and Scalable Redox Flow Battery Based on Iron Sulfate and Anthraquinone Disulfonic Acid, published in the Journal of The Electrochemical Society.

Voltage losses

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The researchers also ascertained principal voltage losses arose from the ohmic resistance of the electrode and electrolyte – the opposition of the materials to the flow of electric current.

The USC researchers said they reduced ohmic resistance by almost 40% by altering the composition of the carbon-nanotube-modified electrodes. That, they said, resulted in a rise in discharge voltage, leading to an increase in the power density and energy efficiency of the system. “With these characteristics, the iron/AQDS battery overcomes the challenges encountered with other iron-based flow batteries,” said the developers of the device.

Although the iron-AQDS battery offers lower cell voltage than vanadium redox flow alternatives, that would be more than compensated by higher durability and lower material costs, according to the Southern California group.

In January, an international group led by Spanish research center CIC EnergiGune said it was designing novel redox organic flow batteries. The researchers said their devices would offer longer duration and higher power and energy densities in a more environmentally friendly format. The European Commission provided €3.8 million to that initiative. “The project aims to demonstrate that organic flow batteries can be a sustainable alternative to vanadium batteries, a material included in the list of critical raw materials by the European Commission,” the CIC scientists said.