Redox flow battery (RFB) employing oxygen as active material is a promising system because this oxygen can replace 

expensive conventional active materials. However, its practical application is limited due to the high overpotential of 

reactions involving oxygen and the necessity of continuous oxygen supply. To overcome the difficulties, in this study, 

new RFBs using redox reaction of mediator and electrochemical oxygen evolution reaction (OER) are suggested. Here, 

oxygen acts as a self-sustaining active material for catholyte operating without its external supply and anthraquinone2,7-disulfonate

(AQDS) is employed as a redox mediator to reduce the high overpotential of oxygen reduction reaction, 

while iron-2,2-bis(hydroxymethyl)-2,2’,2’-nitrilotriethanol complex (Fe(BIS-TRIS)) is considered as the active material 

for anolyte. A nickel-electrodeposited heat-treated graphite felt (Ni-HGF) electrode is further introduced to simultaneously 

facilitate the OER and redox reaction of AQDS mediator in cathode. Electrochemical analysis indicates that the Ni-HGF 

electrode lowers the potential required for oxygen evolution and provides suitable conditions for AQDS-mediated redox 

reactions, enabling continuous in situ generation and utilization of oxygen during cycling. As a result, in RFB single cell 

employing Ni-HGF electrode, its discharge capacity after 200 cycles (746 h) (2.34 Ah L−1

) is well preserved compared to 

its initial capacity of 2.55 Ah L−1

. Furthermore, additional oxygen injection does not affect the discharge capacity, indicating

that a sufficient amount of oxygen is continuously generated via OER during charging step and such generated oxygen 

is subsequently utilized during discharging step. This work proves a viability of a self-sustaining oxygen-based RFB that 

can operate without external oxygen supply, providing a new strategy for establishing a low-cost energy storage system.