Effect of Molecular Structures on the Charge-Discharge Performance for Organic Redox Flow Battery

Abstract

Power generation systems using natural energy such as solar and wind power that does not generate greenhouse gases such as carbon dioxide have been actively introduced all over the world. In order to use such renewable energy efficiently, an energy storage system is required with introducing the power generation system. Large scale storage batteries are promising as energy storage system and are famous for lithium ion batteries, sodium-sulfur batteries, lead storage batteries and redox flow batteries (RFB).1-3 RFB can be enlarged relatively easily because its capacity (tank) and output (cell) can be designed independently, and has features not found in other secondary batteries. In recent years, development of new electrolyte to replace vanadium RFB has been vigorously promoted. For example, a RFB using titanium-manganese (Ti-Mn) as an electrolyte is expected to have lower cost and higher performance than a vanadium RFB.4,5 In addition, in the case of a RFB using an organic molecule exhibiting redox activity as an electrolyte, because the adjustment of the multi-electron transfer reaction and the redox potential can be achieved by complex formation with metal ions, introduction of various functional groups, and polymerization, in recent years, batteries having high energy density comparable to vanadium RFB have been developed.6-8 In RFB, improvement of the energy density of the electrolyte is one of the most important issues. Here, we designed organic-based active materials with high solubility, and studied their application to RFB for the purpose of improvement of energy density. The viologen units exhibiting high solubility were arranged in a tree-like manner, and water-soluble model compounds containing 3, 5, and 13 molecules of viologen units, respectively, were synthesized. These compounds were confirmed to improve energy efficiency as compared to methyl viologen alone in charge-discharge test. In addition, in the case of compound containing 5 molecules of viologen unit, it has been found that it dissolves to about 1 M equivalent, and the theoretical capacity reaches 134 Ah/L. It is thought that the solubility can be improved by arranging the molecules regularly, which may lead to the improvement of energy density and the advance of the battery performance.