Synthesis of multifunctional azo dyes based organic electroactive material: Efficient redox couple for aqueous redox flow batteries

The synthesis, electrochemical properties, and application of a novel multifunctional azo dye (MFAD), containing sulfonated and carboxylated naphthalene groups, were explored for aqueous organic redox flow batteries (AORFBs). MFAD was synthesized via a diazo coupling of 3-aminobenzoic acid with 5-amino-2-naphthalenesulfonic acid, resulting in a 97 % yield. Electrochemical testing was carried out using potassium ferrocyanide and potassium permanganate as catholytes with the MFAD as anolyte in different conditions, i.e., MFAD was dissolved in 1M KOH and made two different supporting electrolyte systems: (i) urea with KCl (MFAD1) and (ii) urea, KCl, and Na₃PO₄ (MFAD2). At a lower current (0.01 A), MFAD1 showed stable charging for 143 cycles but suffered discharge instability after 15 cycles, reducing coulombic efficiency from 99 % to 50 %. Higher current (0.03 A) with supporting electrolytes significantly improved charge capacity and stability. MFAD1 demonstrated a higher average volumetric charge capacity (1200.2 mAh L⁻¹) and average discharge capacity (829.3 mAh L⁻¹), whereas MFAD2, although showing a slightly lower discharge capacity (818.6 mAh L⁻¹), delivered superior coulombic efficiency (76.5 %) compared to MFAD1 (68.8 %). To further assess MFAD's full cell performance, MFAD was further paired with KMnO₄ (0.2 M in 1M KOH) without any supporting electrolyte. Under a current of 0.03 A and 10 min cycling, the MFAD/KMnO₄ cell achieved an average discharge capacity of 128.4 mAh L⁻¹, maintaining 80 % capacity retention and a coulombic efficiency of 77 %. Long-term cycling over 47.3 h demonstrated excellent stability and had also retained 97 % of the initial capacity. Compared to MFAD1 and MFAD2, the MFAD/KMnO₄ system outperformed in stability and coulombic efficiency, highlighting MFAD's strong potential as a scalable and efficient anolyte for high-performance AORFBs. Overall, these findings emphasize the promising role of azobenzene-based molecules for advancing next-generation energy storage systems.