Effect of Molecular Structures on the Charge-Discharge Performance for Organic Redox Flow Battery
Authors: Ohira Akihiro , Funaki Takashi , Ishida Erika , Kim Je-Deok , Sato Yukari 
Published: December 2020

Large-scale storage batteries include lithium-ion batteries, sodium-sulfur batteries, lead-acid batteries, and redox flow batteries (RFBs),1-3 and these storage batteries have been used for distributed power systems in various places depending on the size and application. Above all, although RFBs4,5 have technical issues, including energy density lower than that of other storage batteries, and their vanadium content makes them expensive, they have long life and high design flexibility. In addition, they operate at normal temperature and pose no danger of thermal runaway or explosion. Therefore, the advantages of RFBs have been attracting attention as one way to achieve power leveling for renewable energy.
More recently, research on alternative technology, such as organic, Ti-Mn,6 or hybrid RFBs,7-9 has become active, and in terms of performance, some alternative RFBs, especially organic RFBs, have become comparable to high-cost vanadium RFBs. One of the greatest features of organic RFBs is that it is possible to increase the energy density by controlling the solubility and redox potential based on molecular design, and pioneering and unique research has been reported so far.10-14 The electron transfer reaction rate of the active material in an organic RFB is larger than that in a vanadium RFB, and the reactivity with the carbon electrode is relatively good. For this reason, continued development of RFBs is expected to provide a new power storage technology that can flexibly cope with being combined with other secondary batteries and hydrogen production technologies. Currently, we are focusing on viologen units and have newly synthesized an assemblage of viologen molecules with relatively high symmetry and a regular structure. We are adopting that concept for active material design and aim at improving RFB performance by increasing both the solubility of the designed viologen assembly and the redox response of the individual introduced viologen molecules. Here, we report the application of this idea to aqueous RFBs using newly synthesized viologen molecular units for an anolyte with a relatively high symmetry and regular structure.
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