Air-Stable Ferrocene-Based Catholytes for Improved Performance in pH-Neutral Aqueous Organic Redox Flow Batteries

Aqueous pH-neutral organic redox flow batteries are emerging as viable solutions for sustainable grid-scale energy storage. However, their advancement is hindered by the limited availability of catholytes that maintain stable cyclability in both ambient air and inert environments, as well as challenges related to low-cost synthesis and achieving high volumetric capacity. Here, we present the design and synthesis of a series of imidazolium-decorated ferrocene derivatives, culminating in the identification of an ionic liquid-like catholyte that sustains cycling stability in air for over 2000 cycles. At a concentration of 0.5 M, this catholyte exhibits remarkable cycling stability, with no capacity loss observed over a 68-day period in an inert atomosphere. Theoretical calculations and spectroscopic analyses reveal that the methyl imidazolium pendant group enhances compound stability and reduces hydrophilic attacks on ferrocenium through hydrogen-bonding interactions, thereby improving cycling stability. Notably, we have successfully constructed a pH-neutral stacked flow battery that achieves a peak power of 140 W and maintains an unprecedented capacity retention of 95% at 60 mA cm-2 after 20,000 cycles. These findings not only introduce a novel pathway for developing highly stable catholytes but also facilitate assembling high-power-density stacked pH-neutral aqueous organic redox flow batteries.