Redox flow battery retains more than 90% of capacity over 6,000 cycles via new catholyte

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Researchers at the University of Akron in the United States have developed a water-soluble positive electrolyte – a, so-called catholyte – that is claimed to increase the energy density of aqueous organic redox flow batteries (AORFBs).

The scientists shaped the catholyte by using two ligands: 2,2′-bipyridine-4,4′-dicarboxylic (Dcbpy) acid, and cyanide. Cyanide was previously used in other research and was found to be able to improve the voltage and solubility of redox batteries compared to the commonly used ferrocyanide ligands.

The academics built the battery through what they called a symmetry-breaking strategy, which consists of changing the symmetry of the redox-active organic molecules, instead of using the common approach of attaching a hydrophilic functional group. Hydrophilic functional groups are membranes used to achieve fast ion transport combined with high molecular selectivity, and to provide aqueous organic flow batteries with energy efficiency and capacity retention.

The research group said its approach results in a considerable improvement of the molecules’ solubility. The battery is claimed to have retained more than 90% of capacity over 6,000 cycles, which the scientists said projects “more than 16 years of uninterrupted service [at] a pace of one cycle per day.” The capacity fading rate was 0.00158% per cycle, or 0.217% per day.

The scientists are currently planning to design new materials for this battery technology and have already submitted a patent application to the relevant U.S. authorities. “To significantly improve the performance of aqueous organic RFBs, the urgency of developing new catholyte[s] is crucial,” said researcher Yu Zhu.

The device was described in the paper Symmetry-breaking design of an organic iron complex catholyte for a long cyclability aqueous organic redox flow battery, published in nature energy.

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