Performance evaluation of aqueous organic redox flow battery using methylene blue and vanadium redox couple

In this study, methylene blue which is one of dye materials was introduced as active material for aqueous redox flow battery. The redox potential of methylene blue was shifted to negative direction as pH increased. The full-cell performance was evaluated by using methylene blue as the negative active material and vanadium as the positive active material with acid supporting electrolytes. The cell voltage of methylene blue/V⁴⁺ is very low (0.45 V). In addition, the maximum solubility of methylene blue in water is only 0.12 M. Therefore, the cell test was performed with very low concentration (0.0015 M methylene blue, 0.15 M V⁴⁺) at first time. Cut-off voltage range was 0 to 0.8 V and 1 mA·cm^-2 current density was adopted during cycling. As a result, current efficiency (CE) was 99.67%, voltage efficiency (VE), 88.83% and energy efficiency (EE) was 85.87% and discharge capacity was (0.0500 Ah·L^-1) at 4 cycle. In addition, the cell test was performed with increased concentration (0.1 M methylene blue, 0.15 M V⁴⁺) with 10 mA·cm^-2 current density, leading to higher discharge capacity (3.8122 Ah·L^-1) with similar efficiency (CE=99%, VE=85%, EE=85% at 4 cycle).