Go With the Flow: Diaminoanthraquinones for Use in Symmetric Redox Flow Batteries

Abstract

Redox Flow Batteries (RFBs) have rapidly gained interest as possible electrochemical energy storage systems for large-scale storage. RFBs are appealing as they lack phase change upon cycling, have modular interchangeable components and decouple power output from energy storage capacity. Even with all of these benefits RFBs are just being implemented commercially in large-scale installations as there are still some limitations. Current RFBs have low voltages (leading to low energy densities compared to traditional batteries), cyclability issues and problems with electrolyte crossover the membrane. We propose a new class of RFBs designated as symmetric redox flow batteries (SRFBs) where the same parent electrolyte makes up both the anolyte and catholyte. The assignment of anolyte and catholyte is simply an electrochemical construct and is independent of the solution chemistry. SRFBs are less sensitive to membrane crossover effects. Diaminoanthraquinones (DAAQs) have been targeted as excellent SRFB charge storage compounds. After fundamental electrochemical screening several DAAQs 1,4-bis(isopropylamino)anthraquinone commonly known as disperse blue 134 was the most successful candidate. With 2 reduction and 2 oxidation states, and 2.7 V between them a disperse blue 134 SRFB has a theoretical energy density of 61 Wh/mole as opposed to 17 Wh/mole for the most widely researched all vanadium RFB. A 0.81 Wh/L disperse blue 134 flow cell with > 80% coulombic efficiency and ~ 40% energy efficiency at 100% state of charge was successfully cycled with the identity of each electrolyte altered through charging current sign thus proving that not only an SRFB is possible but that disperse blue 134 is well paired to this technology.