Multi-redox Molecule for High-Energy Redox Flow Batteries

Abstract

Summary Redox flow batteries (RFBs) are some of the most promising energy storage systems because of their design flexibility; however, their low energy density is a major drawback limiting widespread application. Most conventional approaches to increase the energy density have involved exploiting high-concentration electrolytes. However, this approach results in many technical issues, such as sluggish kinetics. We propose a strategy of boosting the energy density by exploiting a multi-redox phenazine molecule (5,10-dihydro-5,10-dimethyl phenazine [DMPZ]). DMPZ exhibits double-redox activity at −0.15 and 0.61 V versus Ag/Ag+ with remarkable kinetics and chemical stability. Coupled with 9-fluorenone (FL), the DMPZ/FL flow cell can provide the highest energy density per mole (≈85 W hr mol−1) ever reported for RFBs. Furthermore, the marked color change of DMPZ enables the state of charge to be precisely visualized. This novel strategy for a multi-redox material can provide a potential pathway toward high-energy-density RFBs.