Designing for conjugate addition: an amine functionalised quinone anolyte for redox flow batteries

Abstract

Redox flow batteries (RFBs) are promising grid-level electrical storage systems. The key to this emerging technology is the development of cheap, highly soluble, and high energy-density inorganic and organic electrolytes. Although quinone-based electrolytes have been investigated previously as two-electron anolytes, their susceptibility to undergo conjugate addition is generally viewed as a limiting factor in their application in RFBs. The study of the amine functionalised benzoquinone, 2,5-(4-carboxy-anilino)-1,4-benzoquinone (CABQ), presented here shows, however, that the resulting dihydroxylation leads to stabilisation of the anolyte at a lower potential, bringing with it increased energy density. A combination of ex situ and in situ techniques is used both to identify the anolyte formed after nucleophilic attack and to help explain the cyclic voltammetry and galvanostatic cycling data. A proof-of-concept, lab-scale RFB cell was run against potassium ferrocyanide (voltage of approximately 1.1 V) in basic aqueous solution which showed high coulombic efficiencies (>99.5%).