Redox flow batteries (RFBs) represent a promising technology as, differently from other energy storage devices, they allow energy and power densities to be independently scaled, thus providing great versatility. RFBs have already been commercialized in the form of vanadium redox flow batteries (VRFBs) with commercial and demonstration projects) installed across the globe. However, the high cost of vanadium, combined with the corrosive nature of the electrolyte support, hinders the widespread deployment of VRFBs.

Aqueous organic redox flow batteries (AORFBs) have recently raised great interest because water-soluble organic molecules combine the advantages of high flexibility in molecular engineering of the organic species with low costs.

Water-soluble organic compounds such as viologen, ferrocene, and metal porphyrin (M= VO2+, Ni2+, Cu2+, Zn2+, Co2+) derivatives were synthesized as possible electrolytes for neutral or nearly neutral pH AORFBs. The active species were modified by adding functional groups to increase their water solubility and enhance their electrochemical properties. Redox couples’ electrochemical properties were first studied in a half-cell configuration; then, they were tested as electrolytes inside an operating AORFB.