Abstract: Oxygen (O₂) is an abundant material with its highly positive redox potential, making it a cost-effective choice for the cathodic active material of aqueous flow batteries (AFBs). However, utilizing O₂ as an active material may induce a high overpotential issue for oxygen reduction reaction (ORR). To address this problem, this study proposes a new AFB system employing iron-2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol complex (Fe(BIS-TRIS)) and O₂ as redox couple and cobalt(triisopropanolamine) complex (Co(TiPA)) as the redox mediator. Co(TiPA) can mitigate ORR overpotential through a mediated electron transfer (MET) mechanism. More specifically, during the charging step, in the catholyte, Co(II)(TiPA)s are oxidized to Co(III)(TiPA)s at the cathode, while HO₂^-s are oxidized in the electrolyte tank, producing O₂. During the discharging step, Co(III)(TiPA)s are reduced to Co(II)(TiPA)s. The resulting Co(II)(TiPA) then chemically reacts with O₂ in the electrolyte tank, regenerating Co(III)(TiPA). Namely, this cycle ensures that Co(III)(TiPA) is electrochemically reduced to Co(II)(TiPA) at the cathode, while the reduced Co(II)(TiPA) is chemically re-oxidized in the electrolyte tank, effectively mediating electron transfer between electrode and oxygen. This process facilitates ORR without direct electrochemical reaction at the cathode, thereby alleviating its overpotential. UV-Vis spectroscopic analysis verifies that Co(TiPA) spontaneously reacts with O₂ and mediates ORR. Fe(BIS-TRIS)-O₂ AFB maintains 79.1 % of its initial capacity over 170 h, demonstrating the feasibility of Co(TiPA) as the redox mediator. However, its structural degradation under oxygen evolution reaction is observed, limiting the long-term stability of Fe(BIS-TRIS)-O₂ AFB. Thus, its structural modifications or development of alternative redox mediators are required.

Key words: Oxygen, Aqueous flow battery, Redox mediator, Energy storage system, Oxygen reduction reaction, Co(TiPA), Fe(BIS-TRIS)