Sulfonated Benzo[c]cinnolines for Alkaline Redox-Flow Batteries

Aqueous organic redox-flow batteries (RFBs) are promising for grid-scale energy storage, but developing stable, low-cost electrolytes remains challenging. Herein, azobenzenes were investigated as negative electrolytes because of their low molecular weight and commercial availability. The electrochemical properties and cycling performance of methyl red, an azobenzene dye, were evaluated under alkaline conditions. Analyzing the decomposition products revealed degradation through a hydrazo bond cleavage. To attenuate degradation, a more structurally rigid cis-azobenzene with connected aromatic rings, known as benzo[c]cinnoline (BC), was tested. The sulfonation of BC enabled a higher aqueous solubility, with the di-sulfonated product (ds-BC) exhibiting a reduction potential of (−0.84 V vs Ag/AgCl). In full cells, ds-BC was paired with ferrocyanide-ferricyanide, producing a 1.11 V battery with a capacity fade of 0.77%/day. Density functional theory calculations also predict that reduced BCs exhibit greater thermodynamic stability than azobenzenes. This work introduces benzo[c]cinnoline as a promising molecular scaffold for RFBs.