Rechargeable organic–air redox flow batteries

Abstract

A rechargeable organic–air flow battery based on aqueous electrolytes is proposed and tests are conducted in a divided cell with a three-electrode configuration. Quinoxaline is used as the negative redox couple due to its low electrode potential of c.a. −0.9 V vs. Hg|HgO in aqueous electrolytes. High-surface-area nickel mesh and manganese-dioxide electrodes were employed for oxygen evolution and reduction, respectively, together with a low-cost hydroxide doped polybenzimidazole (m-PBI) separator (c.a. 20 μm). In typical alkaline electrolytes (2 M NaOH), the open-circuit voltage of the flow battery was c.a. 0.95 V, which is comparable to existing organic-based batteries. The average charge and discharge cell voltage ranges at 5–10 mA cm⁻² were 1.7–1.95 V and 0.4–0.7 V, respectively. Despite using low-cost materials, average coulombic and energy efficiencies of the batteries were c.a. 81 and 25%, respectively, at 7.5 mA cm⁻² over 20 cycles.