Aqueous flow battery using iron and oxygen as redox couple and cobalt(triisopropanolamine) as redox mediator

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.