Alkaline aqueous organic redox flow batteries of high energy and power densities using mixed naphthoquinone derivatives

Mixture of 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQ-S) and 2-hydroxy-1,4-naphthoquinone (Lawsone) is used as negative active species for aqueous organic redox flow battery (AORFB), while ferrocyanide (FeCN) is considered as positive active species in alkaline electrolyte. NQ-S is initially transformed to NQ-OH that has the same chemical structure with Lawsone by the nucleophilic attack under potassium hydroxide (KOH) electrolyte. The mixture of NQ-S and Lawsone (NQ-SO) has a higher solubility in KOH electrolyte (1.26 M in 1 M KOH) than the individual NQ-SO and NQ-S (0.42 and 0.83 M in 1 M KOH, respectively) because of the hydrophilic sulfite (–SO₃²⁻) groups eliminated from NQ-S over the transformation and the polar-polar interaction amid –SO₃²⁻ group, organic species and KOH electrolyte. Lawsone to NQ-S ratio for optimal NQ-SO is 2:1, and this ratio is determined by the hygroscopic properties of –SO₃²⁻ group. The cell voltage of AORFB using 0.6 M NQ-SO and 0.4 M FeCN is 1.01 V and its charge efficiency, discharge capacity, state of charge (SOC) and power density under 100 mA∙cm⁻² are 99%, 23 Ah·L⁻¹, 70% and 90 mW·cm⁻². Furthermore, when the concentration of NQ-SO increases to 1.2 M (AORFB using 1.2 M NQ-SO and 0.4 M FeCN), its discharge capacity considerably increased to 40.3 Ah·L⁻¹ with SOC of 83% and power density of 72 mW·cm⁻².