Redox-Flow Battery Operating in Neutral and Acidic Environments with Multielectron-Transfer-Type Viologen Molecular Assembly

Recently, redox flow batteries (RFBs) have attracted attention as a large-scale energy storage technology. To improve their energy density, we investigated organic-based active materials with high water solubility, synthesized regular dendritic structures comprising viologen molecular assemblies, and applied them to RFBs. The compounds containing 3, 5, and 13 viologen molecular units showed electrolysis activity corresponding to the number of units, and it was found that the redox reaction progressed quantitatively. Furthermore, the charge and discharge characteristics confirmed that the energy efficiency was improved compared to methyl viologen batteries. These compounds have high solubility, and the viologen derivatives can function in acidic conditions, in which methyl viologen does not work as a redox active material, due to reduced interaction between molecules and the cation exchange membrane. Finally, molecules having 5 and 13 viologen units can be prepared to concentrations of 1 and 0.5 M, theoretically reaching a capacity of 134 and 174 Ah/L, respectively.