Aqueous redox flow batteries using iron complex and oxygen as redox couple with anthraquinone-2,7-disulfonate redox mediator

When oxygen (O₂) is used as active material for catholyte of aqueous redox flow batteries (ARFBs), a low capacity issue of ARFBs occurring due to the limited solubility of conventional active materials is alleviated by a large solubility of O₂ into supporting electrolyte, let alone the cost saving for active material. However, a large overpotential is accompanied with direct electrochemical reaction of O₂, and this prevents desirable redox reaction of O₂ included in catholyte and stable operation of ARFBs. To overcome the difficulties, in this study, iron-air ARFBs are suggested. In these ARFBs, a large overpotential occurring for oxygen reduction reaction (ORR) is mitigated by using anthraquinone-2,7-disulfonate (AQDS) mediator. More specifically, redox reaction of iron based complex occurs for the cycling at anolyte. Now, in catholyte, oxygen evolution reaction and ORR of AQDS occur for charging and discharging step at ARFB cell. For discharging step, the reduced AQDS (AQDS²⁻) further reacts with O₂ at catholyte tank, indicating that (i) self-charge of AQDS²⁻ to AQDS and (ii) ORR occur with the production of HO₂⁻ and OH⁻. This demonstrates that ORR occurs chemically in catholyte tank, not electrochemically in ARFB cell. As a result, the electrochemical overpotential occurring during ORR is considerably reduced. Based on that, ARFBs using O₂ and AQDS are stably operated with a low capacity decay rate of 0.09 % per cycle over for 140 cycles.