Anthraquinone-2,6-disulfamidic acid: an anolyte with low decomposition rates at elevated temperatures

Abstract

A new sulfamidic acid anthraquinone derivative was synthesized from 2,6-diaminoanthraquinone with high yields, designed for utilization in redox flow batteries. The active material was investigated with cyclic voltammetry, revealing a reversible redox reaction at approximately -0.65 V vs. Ag/AgCl at pH-values above 12. A stress test in a redox flow battery was applied with hold times at critical states of charge and at 32 °C as well as at 60 °C. Furthermore, the stability was investigated at the maximum concentration of the anolyte. All in all, the material showed the lowest decomposition rates at 60 °C reported so far for an organic anolyte in a redox flow battery.

Scheme 1. Schematic representation of the electrochemistry of anthraquinone.

Scheme 2. Schematic representation of the synthesis of LiAQS. (a) Dichloromethane, pyridine, 0 °C to room temperature, 16 h. (b) Water, lithium hydroxide, room temperature, 30 min, 87%.

Fig. 1. Schematic representation of an unbalanced compositionally symmetric redox flow cell setup utilizing anthraquinone (AQ) as well as anthrahydroquinone (AHQ) and the representative half-cell reactions in the capacity limiting side (CLS) and non-limiting side (CLNS).

Fig. 2. Top: schematic representation of the redox chemistry of the LiAQS. Bottom: cyclovoltammetry of a 50 mM LiAQS in 0.1 M lithium chloride solution. Scan direction: 0 V to −1.2 V and back to 0 V. For each condition five scans were carried out and the third is shown. (left) pH-dependent investigation utilizing lithium hydroxide for adjusting the pH-value with a scan speed of 100 mVs⁻¹, (middle) CV at different scan speeds at a pH-value of 14, (right) plot of the peak current for the oxidation (black) and reduction process (red) vs. the square root of the scan speed at a pH-value of 14.

Fig. 3. (Top left) Long-term stability test using 15 mL of an aqueous 0.1 M LiAQS, 1 M LiCl and 0.25 M LiOH solution vs. 35 mL of the same solution. The values below the red and black squares indicate the decay rate during the representative 50 cyles in % d⁻¹. After every 50^th cycle, the electrolyte is kept at maximum SOC for 24 h. The values above the squares represent the deacy rates during this hold times. After 250 cycles, the temperature of the electrolyte is raised to 60 °C (indicated by the dashed line). The values below the temperature represent the decay rates over the whole temperature phases. (Top right) long-term test using 10 mL of a 0.75 M LiAQS, 1 M LiCl and 0.25 M LiOH solution vs. 25 mL of the same solution. Bottom: current capacity tests using 11.5 mL of a 0.1 M LiAQS, 1 M LiCl and 0.25 M LiOH solution vs. 35 mL of the same solution. Either the charge (left) or the discharge (right) current density is varied. The current density of the counter process is kept at 10 mA cm⁻².